Exhaled Nitric Oxide (FeNO): Uses, Procedure, and Results

Exhaled nitric oxide is a specialized pulmonary function measurement used to assess airway inflammation, especially in patients with suspected or known asthma.

Unlike spirometry, which measures airflow and lung mechanics, this test provides information about the inflammatory activity occurring inside the airways. The result is commonly reported as FeNO, which stands for fraction of exhaled nitric oxide.

Because eosinophilic airway inflammation often responds to corticosteroid therapy, FeNO can help clinicians evaluate asthma control, identify steroid responsiveness, and monitor treatment over time.

What Is Exhaled Nitric Oxide?

Exhaled nitric oxide refers to the amount of nitric oxide detected in a person’s exhaled breath. Nitric oxide is a gas produced by cells throughout the body, including cells involved in airway inflammation. In the lungs, increased nitric oxide production is often associated with inflammatory activity in the airway lining.

The test is most commonly discussed in relation to asthma because many patients with asthma have eosinophilic airway inflammation. Eosinophils are inflammatory white blood cells that can contribute to airway swelling, mucus production, bronchial hyperresponsiveness, and asthma symptoms. When eosinophilic inflammation is active, nitric oxide levels in the exhaled breath may rise.

This is why exhaled nitric oxide testing can be useful in pulmonary function testing. It does not directly measure airway obstruction, oxygenation, ventilation, or gas exchange. Instead, it provides a noninvasive marker of airway inflammation.

For respiratory therapy students, the key point is that exhaled nitric oxide is an inflammation-related test, not a routine airflow test. It adds information that spirometry may not reveal.

What Does FeNO Mean?

FeNO stands for fraction of exhaled nitric oxide. It is the measured concentration of nitric oxide in exhaled breath.

FeNO values are reported in parts per billion, or ppb. This is important because exhaled nitric oxide testing should not be confused with inhaled nitric oxide therapy. Inhaled nitric oxide is a therapeutic pulmonary vasodilator gas often discussed in neonatal and critical care settings, and it is commonly measured in parts per million, or ppm. Exhaled nitric oxide is a diagnostic test measured in ppb.

A FeNO value reflects the concentration of nitric oxide coming from the airways during a controlled exhalation. The value can help determine whether eosinophilic inflammation is likely, whether a patient may respond to inhaled corticosteroids, and whether anti-inflammatory therapy is working.

However, FeNO should not be interpreted as a stand-alone diagnostic test. A high result may support the presence of eosinophilic airway inflammation, but it does not automatically diagnose asthma by itself. The value must be considered with the patient’s symptoms, medical history, spirometry results, bronchoprovocation testing, medication use, and response to treatment.

Why Nitric Oxide Matters in the Airways

Nitric oxide is produced in the respiratory tract as part of several physiologic and inflammatory processes. In the setting of asthma, airway inflammation can increase nitric oxide production. This is especially true when eosinophilic inflammation is present.

Asthma is a chronic inflammatory airway disease. Patients may experience wheezing, coughing, chest tightness, and shortness of breath. Airway obstruction may be variable, meaning spirometry can sometimes appear normal between episodes. This is one reason exhaled nitric oxide can be useful. It may detect inflammatory activity even when routine lung function is not clearly abnormal.

For example, a patient with chronic cough or intermittent wheezing may have normal baseline spirometry. If the FeNO value is elevated, it may suggest that eosinophilic airway inflammation is contributing to the symptoms. The clinician may then consider additional testing, such as methacholine challenge testing, or may evaluate the patient’s response to anti-inflammatory treatment.

Note: This makes FeNO helpful as part of a broader clinical picture. It gives insight into the inflammatory component of airway disease, while other tests evaluate airflow limitation, lung volumes, diffusion, or airway responsiveness.

How Exhaled Nitric Oxide Differs From Spirometry

Spirometry is one of the most common pulmonary function tests. It measures airflow and volume during forced breathing maneuvers. Common spirometry values include FVC, FEV1, and the FEV1/FVC ratio. These values help identify obstructive and restrictive patterns.

Exhaled nitric oxide testing is different. It does not measure how much air a patient can forcefully exhale. It does not measure the speed of airflow in the same way spirometry does. It also does not directly measure oxygen or carbon dioxide levels.

Instead, FeNO testing measures a trace gas in exhaled breath. This trace gas can serve as a marker of airway inflammation. A patient may have normal spirometry but still have elevated FeNO, especially if airway inflammation is present without obvious obstruction at the time of testing.

This distinction is important in both clinical practice and exam preparation. Spirometry answers questions about airflow and lung mechanics. Exhaled nitric oxide answers questions about airway inflammation, especially eosinophilic inflammation.

Clinical Uses of Exhaled Nitric Oxide Testing

Exhaled nitric oxide testing is most useful in asthma evaluation and management. It can help clinicians determine whether eosinophilic airway inflammation is likely and whether the patient may respond to corticosteroid therapy.

One common use is in patients with suspected asthma. If a patient has symptoms such as wheezing, coughing, chest tightness, or episodic shortness of breath, FeNO may help support the presence of inflammatory airway disease. A high value does not confirm asthma by itself, but it strengthens the case for eosinophilic inflammation when symptoms are consistent with asthma.

Another use is in patients already diagnosed with asthma. FeNO can help assess whether the disease is controlled. If the value is elevated, it may suggest ongoing inflammation despite current therapy. This may lead the clinician to evaluate medication adherence, inhaler technique, allergen exposure, or the need for controller medication adjustment.

FeNO can also help monitor response to inhaled corticosteroids. Since eosinophilic inflammation often improves with corticosteroid therapy, a meaningful decrease in FeNO after treatment may suggest that the therapy is reducing airway inflammation.

In some cases, FeNO can help identify patients who are likely to benefit from inhaled corticosteroids. A high value in a symptomatic patient suggests that corticosteroid responsiveness is more likely. A low value makes eosinophilic inflammation and steroid responsiveness less likely, although it does not rule out asthma completely.

FeNO and Asthma

Asthma is the main condition associated with exhaled nitric oxide testing. This is because asthma often involves eosinophilic airway inflammation, and FeNO is useful for identifying this inflammatory pattern.

When asthma is poorly controlled, airway inflammation may increase. This can lead to higher FeNO values. Clinically, this may be seen in patients with persistent symptoms, frequent rescue inhaler use, nighttime symptoms, or recurrent exacerbations.

An elevated FeNO value may suggest that the patient is not adequately controlled on the current treatment plan. It may also suggest that the patient is not taking inhaled corticosteroids consistently, is using the inhaler incorrectly, or is being exposed to triggers that increase inflammation.

However, asthma is not diagnosed by FeNO alone. A clinician must consider the full clinical picture. This may include symptom patterns, triggers, family history, allergy history, spirometry, bronchodilator response, bronchoprovocation testing, and treatment response.

Note: FeNO is best understood as an additional tool. It helps answer the question: Is eosinophilic airway inflammation likely to be contributing to this patient’s symptoms?

FeNO and COPD

Exhaled nitric oxide testing is mainly associated with asthma, but it may have some usefulness in certain patients with COPD. COPD is not typically thought of as a primarily eosinophilic disease, but some patients with COPD may have eosinophilic inflammation or asthma-COPD overlap features.

During COPD exacerbations, exhaled nitric oxide may increase in some patients. However, the test is generally considered less useful in COPD than in asthma. For exam preparation, asthma remains the most important patient population linked to eNO testing.

If a question asks which patient would benefit most from exhaled nitric oxide testing, the best answer is usually the patient with suspected or uncontrolled asthma, wheezing, chronic cough, or suspected eosinophilic airway inflammation. It would not usually be the best choice for unrelated conditions such as ARDS or asbestos exposure.

Conditions That May Affect FeNO

Although asthma is the main disease associated with elevated FeNO, other conditions may also influence results. FeNO may increase during inflammatory airway or lung conditions, including chronic bronchitis, COPD exacerbation, pneumonia, alveolitis, bronchiolitis obliterans, bronchiectasis, sarcoidosis, and some chronic cough conditions.

This means elevated FeNO is not specific to asthma. It supports the presence of inflammatory activity, but it does not identify the exact disease by itself.

Some conditions and patient factors may lower FeNO. Smoking is one of the most important examples. Smokers may have reduced exhaled nitric oxide values. This matters because a smoker with asthma may still have airway inflammation, but the FeNO value may not be as high as expected.

Cystic fibrosis has also been associated with reduced eNO values. Because multiple factors can influence the measurement, interpretation should always include patient history and clinical context.

How the Test Is Performed

Exhaled nitric oxide testing is noninvasive and usually takes only a few minutes. The patient breathes through a mouthpiece connected to a device that measures nitric oxide in the exhaled breath. Many modern systems are portable and provide visual feedback to help the patient maintain the correct exhalation pattern.

The general process begins with patient preparation. The patient should avoid eating, drinking anything other than water, and smoking for at least 1 hour before the test. This helps reduce factors that could alter the result.

The test should also be performed before other pulmonary function tests, such as spirometry, exercise testing, or methacholine challenge testing. These tests can affect the airways and may change the measured nitric oxide value. For this reason, if FeNO is ordered with other PFTs, FeNO should be done first.

During the test, the patient exhales to residual volume, places the mouthpiece in the mouth, and inhales through the device to total lung capacity. The device removes nitric oxide from the inspired air so the measured value reflects nitric oxide coming from the patient rather than the environment.

The patient then exhales slowly and steadily through the device at a controlled flow rate. The goal is to produce a stable nitric oxide plateau that can be analyzed.

Online and Offline Measurement

Exhaled nitric oxide can be measured using online or offline techniques.

In online measurement, the analyzer samples exhaled gas continuously during the breathing maneuver. This allows the device to measure nitric oxide in real time while the patient exhales.

In offline measurement, the patient’s exhaled breath is collected in a separate container or sampling device. The sample is then analyzed later.

Both methods are designed to capture a sample that reflects lower airway nitric oxide while minimizing contamination from other sources, especially nasal nitric oxide. Nasal nitric oxide concentrations are much higher than lower airway concentrations, so contamination can falsely elevate the result.

Note: Online testing is commonly used with modern commercial devices because it provides immediate feedback and results. Many devices also include built-in flow control, pressure monitoring, and visual coaching features.

Why Flow Control Is Important

Flow control is one of the most important technical requirements in FeNO testing. Nitric oxide concentration varies inversely with exhaled flow. This means the measured concentration changes depending on how fast the patient exhales.

To standardize the test, the recommended exhalation flow is 0.05 L/sec, which is the same as 50 mL/sec. The allowed variation is typically ±10%.

Maintaining this flow allows the test to produce a reliable plateau and makes results comparable between tests. If the patient exhales too fast or too slowly, the value may not be accurate.

Modern devices often provide visual feedback to help patients maintain the correct flow. For example, the device may show an animation, gauge, or target zone that guides the patient through the maneuver. If the flow or pressure is outside the acceptable range, the maneuver may be rejected.

Note: For exam purposes, remember that the standard FeNO exhalation flow is 0.05 L/sec, or 50 mL/sec.

Preventing Nasal Nitric Oxide Contamination

Nasal nitric oxide levels are much higher than lower airway nitric oxide levels. If nasal gas enters the exhaled sample, the measured FeNO value may be falsely elevated.

To prevent this, the patient exhales against resistance while maintaining a positive mouth pressure, often around +5 cm H₂O. This pressure helps close the velum in the posterior pharynx, which separates the nasal cavity from the oral exhaled airflow.

A nose clip is not typically used for standard FeNO testing because the testing method relies on positive mouth pressure to prevent nasal contamination. This is different from many other pulmonary function tests where nose clips are commonly used.

Note: This technical detail matters because accurate FeNO testing depends on measuring nitric oxide from the lower airways, not from the nasal passages.

Patient Preparation for FeNO Testing

Proper patient preparation improves the reliability of the result. Before the test, the patient should avoid eating, drinking anything except water, and smoking for at least 1 hour. The technologist should also record recent respiratory infections, current medications, and other factors that may affect interpretation.

Medication history is especially important. Inhaled corticosteroids can lower FeNO by reducing eosinophilic airway inflammation. If a patient has been taking corticosteroids consistently, the result may be lower than it would be without treatment.

Smoking status should always be documented. Smoking can reduce FeNO values, which may make airway inflammation appear less significant than it really is.

Recent respiratory infections may also affect results. If the patient has an active or recent infection, the FeNO value may not reflect the patient’s usual baseline.

Note: The test should be performed before spirometry, bronchial challenge, or exercise testing because these procedures may alter the measurement. This is a common exam point for respiratory therapy students.

Acceptability and Repeatability

For a FeNO test to be valid, the patient must perform the maneuver correctly. The patient should inhale NO-free air to total lung capacity, then exhale steadily against resistance at the standard flow rate.

In adults, the exhalation should last long enough to produce a stable plateau. The plateau is the portion of the exhalation where the nitric oxide concentration becomes steady enough to measure reliably.

The patient should perform at least two acceptable efforts. These values should agree closely, commonly within 10% of each other. The acceptable values are then averaged to produce the final reported result.

Poor technique can lead to unreliable results. Examples include inconsistent flow, failure to maintain pressure, inadequate exhalation time, leaks around the mouthpiece, or failure to inhale properly through the device.

Note: Because FeNO values are measured in very small concentrations, standardization is essential.

Interpreting FeNO Values in Adults

Adult FeNO interpretation often uses low, intermediate, and high ranges.

• A value less than 25 ppb in adults suggests that eosinophilic inflammation and corticosteroid responsiveness are less likely. This does not absolutely rule out asthma, but it makes eosinophilic inflammation less likely as the main explanation for symptoms.
• A value greater than 50 ppb in adults suggests that eosinophilic airway inflammation is likely. In a symptomatic patient, it also suggests that responsiveness to corticosteroids is likely.
• Values between 25 and 50 ppb are considered intermediate. These results should be interpreted carefully based on the patient’s symptoms, history, medications, and other test results. An intermediate value should not be automatically labeled normal or abnormal without context.

Note: Some educational sources also describe normal adult ranges around 15 to 25 ppb, with an upper limit of normal around 35 ppb. The important concept is that higher values suggest increased eosinophilic activity, especially when symptoms are consistent with asthma.

Interpreting FeNO Values in Children

Children have different interpretive cut points than adults.

• A value less than 20 ppb in children suggests that eosinophilic inflammation and corticosteroid responsiveness are less likely.
• A value greater than 35 ppb in children suggests that eosinophilic inflammation is likely. In symptomatic children, this also suggests that corticosteroid responsiveness is likely.
• Values between 20 and 35 ppb are intermediate and should be interpreted in clinical context.

Note: Some exam review sources describe the pediatric normal range as approximately 5 to 22 ppb, with an upper limit of normal around 25 ppb. This means a value that might not seem extremely high for an adult may still be elevated in a child.

For example, a 10-year-old child with wheezing, chronic cough, and a FeNO value of 35 ppb should be interpreted as having an elevated value. The child’s age matters.

Monitoring Changes Over Time

FeNO can be useful not only for initial assessment but also for follow-up. Changes over time may help clinicians determine whether airway inflammation is improving or worsening.

A significant increase depends on the baseline value. If the baseline FeNO is above 50 ppb, an increase of more than 20% from one visit to the next is considered significant. If the baseline value is below 50 ppb, an increase of more than 10 ppb is considered significant.

A meaningful decrease can suggest response to anti-inflammatory therapy. If the starting value is above 50 ppb, a reduction of at least 20% may indicate a significant response. If the starting value is below 50 ppb, a drop of more than 10 ppb may be meaningful.

Note: This makes FeNO useful when adjusting asthma therapy, evaluating adherence, or tracking disease control over time. However, the value should still be interpreted alongside symptoms and other clinical information.

Role in Corticosteroid Therapy

One of the most useful clinical applications of FeNO is evaluating likely responsiveness to corticosteroids. Inhaled corticosteroids are anti-inflammatory controller medications commonly used in asthma management.

Because high FeNO values suggest eosinophilic airway inflammation, they may indicate that the patient is more likely to respond to corticosteroid therapy. If a symptomatic patient has a high FeNO value, the clinician may consider starting or increasing anti-inflammatory treatment.

If a patient is already prescribed inhaled corticosteroids but still has a high FeNO value, several possibilities should be considered. The patient may not be using the medication consistently. The inhaler technique may be incorrect. The dose may be inadequate. The patient may be exposed to allergens or irritants that continue to drive inflammation.

Note: A decrease in FeNO after corticosteroid therapy can support that the treatment is reducing airway inflammation. This can be helpful when monitoring asthma control.

Limitations of Exhaled Nitric Oxide Testing

FeNO is useful, but it has limitations. It should not be used as the only test to diagnose asthma. A high value supports eosinophilic inflammation, but it does not prove asthma by itself. Other inflammatory conditions can also raise FeNO.

A low value does not completely rule out asthma. Some patients with asthma do not have eosinophilic inflammation. Others may have low values because they are already taking corticosteroids, because they smoke, or because of other patient-specific factors.

FeNO also does not measure airflow obstruction. A patient with elevated FeNO may have normal spirometry, and a patient with abnormal spirometry may not necessarily have elevated FeNO.

The test requires proper technique, controlled flow, and careful interpretation. Factors such as smoking, diet, recent infection, medications, age, and testing sequence can affect the result. Therefore, FeNO should be viewed as one piece of the overall assessment.

Pediatric Considerations

FeNO testing can be useful in children because it does not require the same forceful expiratory effort as spirometry. Some children who struggle with maximal forced spirometry may still be able to complete a controlled exhaled nitric oxide maneuver.

However, pediatric testing still requires cooperation. The child must follow instructions, inhale properly through the device, and exhale steadily at the required flow. Visual feedback and coaching can improve performance.

Pediatric interpretation must also account for age, growth, symptoms, and reference values. A value considered borderline in an adult may be elevated in a child. This is why pediatric cut points differ from adult cut points.

Overinterpreting or underinterpreting pediatric pulmonary function data can lead to problems. Overreading results may lead to unnecessary medication changes. Underreading results may delay needed treatment. FeNO should be interpreted carefully with the child’s clinical presentation.

Equipment Used to Measure FeNO

FeNO can be measured with specialized nitric oxide analyzers. Some systems use chemiluminescence technology, while others use electrochemical sensors.

Chemiluminescence analyzers are sensitive and can measure very small concentrations of nitric oxide. They require calibration gases that cover the expected clinical range. They also require zero gas that is free of nitric oxide. Ambient air may be passed through a nitric oxide scrubber to create this zero gas.

Electrochemical sensors use an amperometric technique, where current is produced when voltage is applied between electrodes. This technology helped make handheld portable FeNO devices possible.

Modern devices often include flow control, pressure monitoring, and patient coaching features. These tools help standardize the maneuver and improve reliability.

For lower airway FeNO measurement, analyzers must be sensitive enough to measure values in parts per billion. Typical clinical analyzer specifications may include a range from 1 to 500 ppb, sensitivity around 1 ppb, fast response time, low drift, and strong reproducibility.

Exam Points for Respiratory Therapy Students

For respiratory therapy students, exhaled nitric oxide has several high-yield points.

• Remember that eNO or FeNO is mainly used to evaluate eosinophilic airway inflammation, especially in asthma.
• The result is reported in parts per billion. Do not confuse this with inhaled nitric oxide therapy, which is measured in parts per million.
• The patient should avoid eating, drinking, and smoking for at least 1 hour before the test.
• FeNO should be performed before spirometry, exercise testing, or methacholine challenge testing.
• The standard exhalation flow is 0.05 L/sec, or 50 mL/sec.
• The patient must exhale steadily enough to produce a stable plateau. At least two acceptable values should agree closely, often within 10%.
• Elevated values suggest increased eosinophilic activity and may indicate poor asthma control or likely corticosteroid responsiveness.
• Smoking may reduce FeNO values, which can complicate interpretation.
• FeNO is not a stand-alone asthma diagnosis. It should be interpreted with symptoms, history, spirometry, bronchoprovocation testing, and response to therapy.

Example Clinical Application

Consider a patient with chronic cough, intermittent wheezing, and normal baseline spirometry. Normal spirometry does not rule out asthma or airway inflammation. If the patient’s FeNO value is markedly elevated, this supports the presence of eosinophilic airway inflammation.

The clinician may then consider whether the patient has asthma, cough-variant asthma, allergic airway disease, or another inflammatory condition. Additional testing, such as methacholine challenge, may be used to assess airway hyperresponsiveness.

Now consider a patient with known asthma who reports worsening symptoms despite prescribed inhaled corticosteroids. If the FeNO value is high, this may suggest ongoing inflammation. The clinician may review adherence, inhaler technique, allergen exposure, and medication dosing.

Note: In both examples, the FeNO result does not replace clinical judgment. It adds useful information about the inflammatory status of the airways.

Exhaled Nitric Oxide Practice Questions

1. What does exhaled nitric oxide testing primarily measure?
It measures the amount of nitric oxide in a patient’s exhaled breath to help assess airway inflammation.

2. What does FeNO stand for?
Fraction of exhaled nitric oxide.

3. Which disease is exhaled nitric oxide testing most commonly associated with?
Asthma

4. Why is FeNO useful in asthma assessment?
It helps detect eosinophilic airway inflammation, which is commonly associated with asthma and often responds to corticosteroid therapy.

5. What type of airway inflammation is most closely linked with elevated FeNO?
Eosinophilic airway inflammation.

6. What unit is used to report FeNO values?
Parts per billion, or ppb.

7. How is FeNO different from spirometry?
FeNO measures airway inflammation, while spirometry measures airflow and lung mechanics.

8. Can FeNO diagnose asthma by itself?
No. It should be interpreted with symptoms, medical history, spirometry, bronchoprovocation testing, and response to therapy.

9. What does a high FeNO value suggest in a symptomatic patient?
It suggests that eosinophilic airway inflammation is likely and that the patient may respond to corticosteroid therapy.

10. What does a low FeNO value suggest?
It suggests that eosinophilic inflammation and corticosteroid responsiveness are less likely.

11. According to the ATS guideline summarized in Ruppel’s, what adult FeNO value is considered low?
Less than 25 ppb.

12. According to the ATS guideline summarized in Ruppel’s, what pediatric FeNO value is considered low?
Less than 20 ppb.

13. According to the ATS guideline summarized in Ruppel’s, what adult FeNO value is considered high?
Greater than 50 ppb.

14. According to the ATS guideline summarized in Ruppel’s, what pediatric FeNO value is considered high?
Greater than 35 ppb.

15. How should an adult FeNO value between 25 and 50 ppb be interpreted?
It is an intermediate value and should be interpreted based on the clinical situation.

16. How should a pediatric FeNO value between 20 and 35 ppb be interpreted?
It is an intermediate value and should be interpreted in clinical context.

17. What is the standard exhalation flow rate for FeNO testing?
0.05 L/sec, or 50 mL/sec

18. What is the allowed variation for the standard FeNO exhalation flow?
±10%

19. Why must exhalation flow be controlled during FeNO testing?
Nitric oxide concentration varies inversely with exhaled flow, so a standardized flow is needed for accurate results.

20. What should a patient avoid for at least 1 hour before FeNO testing?
Eating, drinking anything except water, and smoking.

21. When should FeNO testing be performed if spirometry is also ordered?
FeNO should be performed before spirometry.

22. Why should FeNO be measured before methacholine challenge testing?
Methacholine challenge testing can alter the airway and affect the measured nitric oxide level.

23. Why should FeNO be measured before exercise testing?
Exercise testing can change airway physiology and may affect the FeNO result.

24. What does an elevated FeNO level suggest about asthma control?
It may suggest that airway inflammation is increased and asthma is not well controlled.

25. What type of medication response can FeNO help predict?
It can help predict likely responsiveness to corticosteroid therapy.

26. Why is nitric-oxide-free gas used during FeNO testing?
It helps ensure the analyzer measures nitric oxide coming from the patient’s airways rather than from the environment.

27. How is nitric oxide usually removed from the inspired gas during FeNO testing?
The patient inhales through a nitric oxide scrubber.

28. Why should ambient nitric oxide be recorded during FeNO testing?
Ambient nitric oxide can affect the measurement, so documenting it helps with quality control and interpretation.

29. What lung volume does the patient inhale to during the FeNO maneuver?
Total lung capacity

30. What lung volume does the patient exhale to before inhaling through the FeNO device?
Residual volume

31. Is a breath hold used during standard FeNO testing?
No. The patient inhales to total lung capacity and then exhales slowly without holding the breath.

32. Why does the patient exhale against resistance during FeNO testing?
To create positive mouth pressure and help prevent nasal nitric oxide from contaminating the sample.

33. What positive mouth pressure is commonly used during FeNO testing?
About +5 cm H₂O

34. What structure closes during positive-pressure exhalation to help prevent nasal contamination?
The velum in the posterior pharynx.

35. Why is nasal nitric oxide contamination a concern during FeNO testing?
Nasal nitric oxide levels are much higher than lower-airway levels and could falsely elevate the result.

36. Is a nose clip typically used during standard FeNO testing?
No. The technique uses positive mouth pressure to prevent nasal nitric oxide contamination.

37. How long should an adult exhalation last during acceptable online FeNO testing?
More than 6 seconds.

38. What should the FeNO maneuver produce for an acceptable measurement?
A stable nitric oxide plateau.

39. How many acceptable FeNO efforts are typically needed?
At least two acceptable efforts.

40. How close should repeat acceptable FeNO values be?
They should usually agree within 10% of each other.

41. What is done with two acceptable FeNO values?
They are averaged to produce the final reported result.

42. What technology is commonly used by analyzers to measure exhaled nitric oxide?
Chemiluminescence analysis

43. What type of sensor made handheld portable FeNO devices possible?
Electrochemical sensors

44. What technique do electrochemical sensors use to measure nitric oxide?
An amperometric technique based on current produced when voltage is applied between electrodes.

45. What is the typical clinical measurement range listed for FeNO analyzers?
Approximately 1 to 500 ppb.

46. What sensitivity is listed for nitric oxide analyzers used for FeNO?
About 1 ppb.

47. Why may chemiluminescence analyzers require recalibration?
They can be sensitive to ambient conditions, especially temperature changes.

48. What calibration gas is required for nitric oxide analyzers?
A zero gas that is free of nitric oxide.

49. How can zero gas be created for FeNO equipment?
Ambient air can be passed through a nitric oxide scrubber.

50. Why do some FeNO devices include visual feedback or incentive displays?
They help the patient maintain the correct exhalation flow and pressure during the maneuver.

51. What does online FeNO measurement mean?
Exhaled gas is sampled continuously at the mouth during the breathing maneuver.

52. What does offline FeNO measurement mean?
Exhaled air is collected in a separate sampling device and analyzed later.

53. Why is FeNO considered a specialized pulmonary function test?
It answers a specific clinical question about airway inflammation rather than routine airflow or lung volume measurement.

54. What does a FeNO value of 97 ppb suggest in an adult with normal spirometry and respiratory symptoms?
It strongly suggests increased airway inflammation despite normal baseline lung function.

55. What does a FeNO value of 396 ppb indicate in an adult?
It is far above the high threshold and strongly supports significant airway inflammation.

56. What does a FeNO value of 65 ppb suggest in an adult with normal spirometry?
It suggests eosinophilic inflammation in the airways and/or alveoli.

57. Why can FeNO be abnormal even when spirometry is normal?
FeNO reflects airway inflammation, while spirometry reflects airflow mechanics.

58. What additional test may be used with FeNO to evaluate airway hyperresponsiveness?
Methacholine challenge testing

59. What does methacholine challenge testing help demonstrate?
It helps demonstrate airway hyperreactivity or bronchial hyperresponsiveness.

60. How can FeNO and methacholine challenge testing complement each other?
FeNO helps identify inflammatory likelihood, while methacholine challenge helps assess airway responsiveness.

61. What symptom may be the main complaint in patients with asthmatic responses to inhaled irritants?
Cough

62. Does wheezing always have to be present for FeNO to be clinically useful?
No. Patients may have airway inflammation even when wheezing is absent.

63. What does increased eNO in an asthma patient generally suggest about disease control?
It suggests increased airway inflammation and possible poor asthma control.

64. What treatment change may be considered when FeNO is elevated in a symptomatic asthma patient?
The clinician may consider starting or increasing inhaled corticosteroid-containing therapy.

65. What can a decrease in FeNO after anti-inflammatory treatment suggest?
It can suggest that airway inflammation is improving in response to therapy.

66. What reduction is considered meaningful when the starting FeNO value is above 50 ppb?
A reduction of at least 20%.

67. What reduction is considered meaningful when the starting FeNO value is below 50 ppb?
A drop of more than 10 ppb.

68. What increase is considered significant when the baseline FeNO value is above 50 ppb?
An increase of more than 20% from one visit to the next.

69. What increase is considered significant when the baseline FeNO value is below 50 ppb?
An increase of more than 10 ppb.

70. Why is smoking status important when interpreting FeNO?
Smoking may reduce FeNO values and can make airway inflammation appear less elevated than expected.

71. How might smoking affect a patient with asthma who has airway inflammation?
The patient may still have an elevated FeNO, but the value may be lower than it would be if the patient did not smoke.

72. When is smoking status especially important in FeNO interpretation?
When the FeNO value is near the upper limit of normal.

73. Why should recent respiratory infections be documented before FeNO testing?
Recent infections can affect the result and may influence interpretation.

74. Why should the patient’s medication regimen be recorded before FeNO testing?
Medications, especially inhaled corticosteroids, can affect airway inflammation and FeNO values.

75. What does FeNO add to the information obtained from spirometry?
It adds information about airway inflammation that spirometry does not directly measure.

76. What adult FeNO value makes eosinophilic inflammation less likely?
Less than 25 ppb.

77. What pediatric FeNO value makes eosinophilic inflammation less likely?
Less than 20 ppb.

78. What adult FeNO value makes eosinophilic inflammation more likely?
Greater than 50 ppb.

79. What pediatric FeNO value makes eosinophilic inflammation more likely?
Greater than 35 ppb.

80. What is the adult upper limit of normal for eNO testing?
35 ppb

81. What is the pediatric upper limit of normal for eNO testing?
25 ppb

82. What is the normal adult range for eNO testing?
15 to 25 ppb

83. What is the normal pediatric range for eNO testing?
5 to 22 ppb

84. A 10-year-old patient has wheezing, chronic cough, and an eNO value of 35 ppb. How should this be interpreted?
The value is elevated for a child and suggests increased eosinophilic activity.

85. Why is a child’s eNO value interpreted differently from an adult’s?
Children have different reference ranges and lower upper limits of normal than adults.

86. What NBRC content code is associated with recommending exhaled nitric oxide as a diagnostic procedure?
IE11

87. What difficulty levels are associated with the eNO topic?
Recall, Application, and Analysis.

88. Which patient population is usually the best choice for eNO testing on exam questions?
A patient with asthma attacks, suspected asthma, or suspected eosinophilic airway inflammation.

89. Why is eNO testing not usually the best choice for a patient with ARDS?
ARDS is not the main condition for which eNO testing has been shown to be beneficial.

90. Why is eNO testing not usually the best choice for a patient with asbestos exposure?
Asbestos exposure is not a primary indication for eNO testing.

91. What does increased eNO reflect physiologically?
Increased airway inflammatory activity, especially eosinophilic activity.

92. What does eNO testing help the physician understand about an asthma patient?
It helps show whether airway inflammation is increased and whether asthma may be poorly controlled.

93. What type of therapy might be modified based on an elevated eNO result?
Anti-inflammatory controller therapy, especially corticosteroid therapy.

94. What should the therapist do first if a patient is ordered eNO analysis, spirometry, and residual volume testing?
Perform the eNO analysis first.

95. Why should eNO be separated from inhaled nitric oxide therapy in exam preparation?
Exhaled nitric oxide is a diagnostic PFT measurement in ppb, while inhaled nitric oxide is a therapeutic gas measured in ppm.

96. What does a FeNO value of 100 ppb at 50 mL/sec suggest?
It may suggest asthma-related airway inflammation.

97. What does a normal baseline spirometry result fail to rule out?
It does not rule out airway inflammation or hyperreactive airway disease.

98. What kind of patient effort is required for FeNO compared with forced spirometry?
FeNO requires a controlled steady exhalation rather than a maximal forced expiratory maneuver.

99. Why may FeNO testing be useful in some pediatric patients who struggle with spirometry?
Some children may be able to perform a controlled exhalation even if they have difficulty with maximal forced spirometry.

100. What is the main clinical value of exhaled nitric oxide testing?
It provides noninvasive information about airway inflammation that can support asthma evaluation, treatment decisions, and follow-up.

Final Thoughts

Exhaled nitric oxide testing is a useful noninvasive tool for assessing airway inflammation, especially eosinophilic inflammation associated with asthma. It provides information that routine spirometry may miss because it focuses on inflammation rather than airflow mechanics.

A high FeNO value can support the presence of eosinophilic airway inflammation and may suggest likely responsiveness to corticosteroid therapy, while a low value makes this pattern less likely.

Accurate testing requires proper preparation, correct sequencing before other pulmonary function tests, controlled exhalation flow, and careful interpretation. When used in context, FeNO can help guide asthma evaluation, treatment decisions, and follow-up care.