ABG Calculator
Arterial Blood Gas Interpretation
Understanding Arterial Blood Gas Interpretation
An arterial blood gas (ABG) is one of the most informative tests in respiratory and critical care medicine. From a single small sample of arterial blood, you can assess how well a patient is ventilating, how well they are oxygenating, and whether their acid-base status is balanced or disturbed. For respiratory therapists, learning to read an ABG quickly and confidently is a core clinical skill, and it shows up constantly on board exams and at the bedside.
This page explains the physiology behind acid-base balance, walks through a systematic method for interpreting any blood gas, and shows how the calculator above fits into that process. Use the tool to check your reasoning, but build the understanding here so you can interpret a gas even when no calculator is handy.
What an ABG Actually Measures
A standard arterial blood gas reports several values, and each tells a different part of the story.
- The pH reflects the overall acidity or alkalinity of the blood. It is the bottom line of acid-base balance, the single number that tells you whether the blood is too acidic, too alkaline, or in range. The narrow normal window of 7.35 to 7.45 reflects how tightly the body defends this value, because even small deviations affect enzyme function, electrolyte handling, and cardiac performance.
- The PaCO2, or partial pressure of carbon dioxide, is the respiratory component. Carbon dioxide is an acid in solution, so it is controlled by the lungs through ventilation. When a patient breathes more, they blow off CO2 and the PaCO2 falls; when they breathe less, CO2 accumulates and the PaCO2 rises. A normal PaCO2 sits between 35 and 45 mm Hg.
- The HCO3, or bicarbonate, is the metabolic component. Bicarbonate is a base, and it is regulated by the kidneys. The normal range is 22 to 26 mEq/L. Bicarbonate is the body’s primary chemical buffer, neutralizing excess acid and helping to keep pH stable.
- The PaO2 (partial pressure of oxygen, normally 80 to 100 mm Hg), the SaO2 (arterial oxygen saturation, normally 95 to 100 percent), and the base excess (normally −2 to +2 mEq/L). The PaO2 and SaO2 describe oxygenation, which is a separate question from acid-base status and is addressed later on this page.
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The Chemistry of Acid-Base Balance
To interpret a blood gas, it helps to understand what the body is trying to accomplish. Metabolism constantly produces acid. Cellular respiration generates carbon dioxide, and tissues produce fixed acids such as lactic acid and ketoacids. Left unchecked, this acid load would quickly drive the pH down to levels incompatible with life. The body counters this with three overlapping defenses: chemical buffers, the lungs, and the kidneys.
The most important buffer is the bicarbonate buffer system, which can be summarized by a single reversible reaction:
CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−
Read this equation in both directions, because it explains nearly everything about acid-base physiology. When acid (H+) accumulates, it combines with bicarbonate, shifts the reaction to the left, and is eventually exhaled as CO2.
When the blood becomes too alkaline, the reaction shifts to the right, generating more hydrogen ions. The lungs control the left side of the equation by adjusting how much CO2 is exhaled, and the kidneys control the right side by retaining or excreting bicarbonate and hydrogen ions.
This division of labor is the key to the whole subject. The lungs handle the respiratory component (CO2), and the kidneys handle the metabolic component (bicarbonate). When you see an abnormal PaCO2, you are looking at a respiratory problem. When you see an abnormal bicarbonate, you are looking at a metabolic problem.
Note: Almost every interpretation flows from sorting which system is driving the disturbance and which is trying to fix it.
The Four Primary Acid-Base Disorders
There are exactly four primary acid-base disorders. Each one is named by its direction (acidosis or alkalosis) and its source (respiratory or metabolic).
Respiratory Acidosis
Respiratory acidosis occurs when the lungs fail to remove enough carbon dioxide, so PaCO2 rises and pH falls. This is fundamentally a problem of hypoventilation. Common causes include chronic obstructive pulmonary disease, severe asthma, respiratory depression from opioids or sedatives, neuromuscular diseases such as Guillain-Barré or myasthenia gravis, chest wall deformities, and airway obstruction.
Any condition that reduces minute ventilation or impairs gas exchange enough to trap CO2 can produce it. The hallmark is an elevated PaCO2 above 45 mm Hg with an acidic pH below 7.35.
Respiratory Alkalosis
Respiratory alkalosis is the opposite: the patient blows off too much carbon dioxide, PaCO2 falls below 35 mm Hg, and pH rises above 7.45. This is a problem of hyperventilation. Anxiety and pain are classic triggers, but it is important not to assume anxiety first, because hypoxemia, fever, sepsis, pulmonary embolism, early salicylate (aspirin) toxicity, pregnancy, high altitude, and over-aggressive mechanical ventilation all drive the respiratory rate up.
In a hypoxemic patient, the hyperventilation is an appropriate response to low oxygen, and the underlying cause deserves attention rather than reassurance.
Metabolic Acidosis
Metabolic acidosis occurs when bicarbonate is lost or consumed, or when fixed acids accumulate faster than the body can clear them. The bicarbonate falls below 22 mEq/L and the pH drops below 7.35. This is the most clinically varied of the four disorders, and it is usefully divided into two categories based on the anion gap, discussed in detail below.
Causes include diabetic ketoacidosis, lactic acidosis from shock or sepsis, kidney failure, severe diarrhea, and a range of toxic ingestions. Metabolic acidosis is often a marker of serious underlying illness, so identifying it should prompt a search for the cause.
Metabolic Alkalosis
Metabolic alkalosis develops when the body gains bicarbonate or loses acid, raising the bicarbonate above 26 mEq/L and the pH above 7.45. The most common scenarios are loss of stomach acid through prolonged vomiting or nasogastric suction, and loss of volume and potassium from diuretic therapy.
Other causes include excessive bicarbonate administration, hyperaldosteronism, and Cushing syndrome. Metabolic alkalosis is frequently sustained by volume depletion and low chloride or potassium, which is why correcting those electrolytes is often part of the treatment.
Compensation: How the Body Fights Back
The body does not accept an acid-base disturbance passively. When one system fails, the other tries to pull the pH back toward normal. This process is called compensation, and recognizing it is what separates a basic reading from a complete interpretation.
The principle is straightforward. If the lungs cause the problem (an abnormal PaCO2), the kidneys compensate by adjusting bicarbonate. If a metabolic problem causes it (an abnormal bicarbonate), the lungs compensate by adjusting ventilation and therefore PaCO2. The compensating system always moves in the same direction as the primary problem in terms of pH effect, working to offset it.
Consider a patient with chronic respiratory acidosis from COPD. Their PaCO2 is chronically elevated, which would acidify the blood. Over days, the kidneys retain bicarbonate to buffer that acid, raising the HCO3 and nudging the pH back toward normal. The result is a high PaCO2 and a high HCO3 together, with a pH that may be only mildly low or even back within range. That combination of two abnormal values pulling in opposite pH directions is the signature of compensation.
Timing matters because the two systems work at very different speeds. Respiratory compensation is fast, beginning within minutes and reaching its effect within hours, because changing the respiratory rate is immediate. Renal compensation is slow, taking one to several days to develop fully, because the kidneys must adjust bicarbonate handling at the cellular level. This is why an acute respiratory disturbance shows little bicarbonate change, while a chronic one shows a large, established change.
Compensation is described in three stages:
- Uncompensated (acute): The primary system is abnormal, but the compensating system has not yet responded. The pH is clearly abnormal and the second value is still normal. For example, a high PaCO2 with a normal bicarbonate and a low pH is acute, uncompensated respiratory acidosis.
- Partially compensated: Both systems are now abnormal, the compensating system is clearly working, but the pH has not yet returned to the normal range. The body is still catching up.
- Fully compensated: Both systems are abnormal, and the compensation has succeeded in bringing the pH back into the 7.35 to 7.45 range. The pH is normal, but the abnormal PaCO2 and bicarbonate reveal that a real disturbance is present and balanced.
One subtle but important rule: the body does not overcompensate. Compensation can bring the pH back into the normal range, but it will not push it across the midline of 7.40. This gives you a reliable way to identify the primary disorder even in a fully compensated gas.
If the pH sits on the acidic side of 7.40 (between 7.35 and 7.40), the primary problem is an acidosis, and you look at whichever value, PaCO2 or bicarbonate, is causing acidity. If the pH sits on the alkaline side (between 7.41 and 7.45), the primary problem is an alkalosis. The calculator above uses exactly this logic to classify fully compensated gases.
A clear, step-by-step guide that simplifies ABG interpretation, helping respiratory therapy students and clinicians confidently analyze acid–base balance, oxygenation, and compensation for exams and real-world patient care.
A Systematic Method for Interpreting Any ABG
The fastest way to read a blood gas reliably is to follow the same steps every time. With practice this becomes nearly automatic, and it prevents the common mistake of jumping to a conclusion from one value.
- Step 1: Look at the pH. Decide whether the blood is acidic, alkalotic, or normal. Below 7.35 is acidemia. Above 7.45 is alkalemia. Within range is normal, but remember that a normal pH can still hide a fully compensated disorder or a mixed picture, so do not stop here.
- Step 2: Look at the PaCO2. This is the respiratory value. Above 45 is respiratory acidosis territory, below 35 is respiratory alkalosis territory. Ask whether the PaCO2 explains the pH. A high PaCO2 makes blood acidic; a low PaCO2 makes it alkaline.
- Step 3: Look at the HCO3. This is the metabolic value. Below 22 is metabolic acidosis territory, above 26 is metabolic alkalosis territory. Ask the same question: does the bicarbonate explain the pH? A low bicarbonate makes blood acidic; a high bicarbonate makes it alkaline.
- Step 4: Identify the primary disorder. Match the abnormal value that explains the pH to the disturbance. If the pH is acidic and the PaCO2 is high, the primary problem is respiratory acidosis. If the pH is acidic and the bicarbonate is low, it is metabolic acidosis. The value that moves the pH in the same direction as the pH abnormality is the primary driver.
- Step 5: Assess compensation. Look at the other value. If it is normal, the disorder is uncompensated. If it is abnormal in the direction that opposes the primary disturbance, compensation is underway. Then check the pH: still out of range means partial compensation, back in range means full compensation.
A widely used mnemonic captures the relationships in steps 2 and 3: ROME, which stands for Respiratory Opposite, Metabolic Equal. In respiratory disorders, the pH and PaCO2 move in opposite directions, so a high CO2 goes with a low pH.
In metabolic disorders, the pH and bicarbonate move in the same direction, so a low bicarbonate goes with a low pH. If you can remember ROME, you can reconstruct the entire classification table from scratch.
The Anion Gap: Refining Metabolic Acidosis
When you identify a metabolic acidosis, the work is not finished, because the cause matters enormously and the anion gap helps narrow it. The anion gap estimates the unmeasured anions in the blood and is calculated as:
Anion Gap = Na+ − (Cl− + HCO3−)
A normal anion gap is generally considered to be around 8 to 12 mEq/L, though reference ranges vary slightly between laboratories. Dividing metabolic acidosis by the gap points you toward the underlying problem.
A high anion gap metabolic acidosis means unmeasured acids have accumulated in the blood. The classic mnemonic is MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Iron or Isoniazid, Lactic acidosis, Ethylene glycol, and Salicylates. These are added-acid states, and several of them are medical emergencies.
A normal anion gap metabolic acidosis means bicarbonate has been lost directly and is replaced by chloride, keeping the gap unchanged. This is also called hyperchloremic metabolic acidosis. Common causes include severe diarrhea, which dumps bicarbonate from the gut, and renal tubular acidosis, in which the kidneys fail to handle acid or bicarbonate normally.
Calculating the gap takes only a moment, requires the sodium and chloride from a basic metabolic panel alongside the bicarbonate, and frequently changes the differential diagnosis. It is one of the highest-yield extra steps you can take after spotting a metabolic acidosis.
Access our quiz with sample TMC practice questions and in-depth explanations to master essential ABG concepts.
Checking the Adequacy of Compensation
Once you have classified a gas, you can go one step further and ask whether the compensation is appropriate. The body compensates by predictable amounts, and a compensation that is too large or too small suggests a second, hidden disorder. This is where expected-compensation formulas come in.
For metabolic acidosis, Winter’s formula predicts the expected PaCO2:
Expected PaCO2 = (1.5 × HCO3) + 8 ± 2
If the measured PaCO2 matches the prediction, the respiratory compensation is appropriate. If the PaCO2 is higher than expected, there is an additional respiratory acidosis. If it is lower than expected, there is an additional respiratory alkalosis. A patient in diabetic ketoacidosis who is not breathing fast enough to meet Winter’s prediction, for instance, has a concerning second problem layered on top.
For metabolic alkalosis, the expected respiratory compensation is roughly a 0.7 mm Hg rise in PaCO2 for every 1 mEq/L rise in bicarbonate. For respiratory disorders, the expected bicarbonate change differs between acute and chronic states, since the kidneys need days to respond fully. In acute respiratory acidosis, bicarbonate rises about 1 mEq/L for every 10 mm Hg rise in PaCO2, while in the chronic state it rises closer to 3.5 to 4 mEq/L per 10 mm Hg. The difference between those two numbers is precisely how you distinguish an acute from a chronic respiratory problem on paper.
Note: These formulas go beyond what the calculator above performs, but they are worth knowing for board exams and for the bedside, because they catch the mixed disorders that a simple classification can miss.
Mixed Acid-Base Disorders
Not every patient fits neatly into one of the four boxes. A mixed disorder is present when two or more primary disturbances occur at the same time. A patient with both severe vomiting and severe diarrhea might have a metabolic alkalosis and a metabolic acidosis simultaneously, which can leave the pH deceptively normal. A patient in septic shock might develop both a lactic (metabolic) acidosis and a respiratory acidosis from fatigue.
The clues to a mixed disorder include a pH that is more abnormal than either single value would explain, compensation that falls outside the expected range, or two values that point in directions inconsistent with a single primary problem with compensation.
Note: When the calculator above returns a combined or unclassifiable result, treat it as a prompt to slow down, calculate the anion gap, apply the compensation formulas, and consider whether more than one process is at work.
Don’t Forget Oxygenation
Acid-base interpretation focuses on pH, PaCO2, and bicarbonate, but the ABG also reports oxygenation, and a patient can have perfect acid-base balance while being dangerously hypoxemic. Oxygenation is assessed separately.
The PaO2 tells you the partial pressure of oxygen dissolved in arterial blood, normally 80 to 100 mm Hg on room air. The SaO2 tells you what percentage of hemoglobin is carrying oxygen, normally 95 percent or higher. A low PaO2 defines hypoxemia and should always be interpreted in the context of how much supplemental oxygen the patient is receiving, because a PaO2 of 90 on room air is reassuring while the same value on a high-flow device is not.
Two tools refine the picture. The alveolar-arterial (A-a) gradient compares the oxygen in the alveoli to the oxygen that actually reaches the blood, helping distinguish a lung problem from pure hypoventilation.
The P/F ratio, the PaO2 divided by the fraction of inspired oxygen, grades the severity of hypoxemic respiratory failure and is central to the definition of acute respiratory distress syndrome, where a ratio below 300 indicates mild, below 200 moderate, and below 100 severe disease.
Note: For respiratory therapists managing oxygen and ventilator settings, these oxygenation measures are often as important as the acid-base picture.
Putting It Together: Worked Examples
A few quick examples show the method in action.
- A patient presents with a pH of 7.28, a PaCO2 of 60, and a bicarbonate of 26. The pH is acidic, the PaCO2 is high and explains the acidity, and the bicarbonate is still normal. This is acute respiratory acidosis, uncompensated, consistent with an acute hypoventilation event such as an opioid overdose.
- A second patient shows a pH of 7.37, a PaCO2 of 68, and a bicarbonate of 38. The pH is on the acidic side of normal, the PaCO2 is high, and the bicarbonate is also high, working against the acidosis. This is fully compensated respiratory acidosis, the classic picture of stable COPD where the kidneys have had time to retain bicarbonate.
- A third patient has a pH of 7.22, a PaCO2 of 24, and a bicarbonate of 10. The pH is acidic, the bicarbonate is low and explains it, and the PaCO2 is low, indicating the lungs are hyperventilating to compensate. This is a partially compensated metabolic acidosis, the sort of gas seen in diabetic ketoacidosis, and the next step is to calculate the anion gap and check Winter’s formula.
How to Use the Calculator
The tool at the top of this page applies the systematic method automatically. Enter the pH, the PaCO2 in mm Hg, and the bicarbonate in mEq/L, then select Calculate. The calculator evaluates each value against its normal range, identifies the primary disorder, and determines the degree of compensation, returning a plain-language interpretation such as acute respiratory acidosis or fully compensated metabolic alkalosis.
Use it to confirm your own reasoning rather than to replace it. The greatest value comes from working through a gas by hand first, predicting the answer, and then checking it against the tool. Over time you will find that you reach the same conclusion the calculator does before you ever press the button, which is exactly the skill that board exams and bedside practice reward.
A Note on Clinical Judgment
This calculator and the information on this page are educational resources designed to support learning and quick reference. They do not account for the anion gap, expected-compensation formulas, oxygenation, or the full clinical context of a real patient, and they are not a substitute for professional medical evaluation.
A blood gas is always interpreted alongside the patient in front of you: their history, their physical exam, their other laboratory values, and their trajectory over time. Numbers describe a moment; clinical judgment connects that moment to the right decision.
Note: Use this tool to build the foundation, and let careful, patient-centered reasoning carry it the rest of the way.
ABG Calculator
Arterial Blood Gas Interpretation
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Castro D, Patil SM, Zubair M, et al. Arterial Blood Gas. [Updated 2024 Jan 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026.