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How to interpret an ABG, step by step
Written by a physician at GDA Software · last updated 5 November 2026
To interpret an arterial blood gas (ABG): check the pH, find the primary process from the pCO2 and bicarbonate, calculate the anion gap corrected for albumin, use the delta-delta to look for a hidden metabolic disorder, check compensation to look for a hidden respiratory disorder, add the osmolar gap when a toxic alcohol is possible, and then list the causes. The steps below explain each one, followed by a worked triple-disorder example.
Want to check your work? ABG Pro DDx (free) and ABG Pro run every step below on Android and iPhone and give a differential diagnosis for each disorder.
Normal values used here
| Value | Normal |
|---|---|
| pH | 7.35–7.45 |
| pCO2 | 35–45 mmHg (40 for calculations) |
| HCO3 | 23–28 mEq/L (25 for calculations) |
| Anion gap | albumin (g/dL) × 2.5, about 10 when albumin is 4.0 |
| Osmolar gap | under 10 |
These are the defaults in ABG Pro and ABG Pro DDx. Labs differ, so use your own lab’s ranges where you have them.
Step 1: Check the pH
Below 7.35 is acidemia; above 7.45 is alkalemia. A normal pH does not rule out a disorder: two or three opposing disorders can cancel out, which is exactly what happens in the worked example below.
Step 2: Find the primary process
Look at which way the pCO2 and HCO3 have moved. In acidemia, a high pCO2 points to respiratory acidosis and a low HCO3 to metabolic acidosis. In alkalemia, a low pCO2 points to respiratory alkalosis and a high HCO3 to metabolic alkalosis. When both have moved the same way, one is usually compensating for the other; step 5 tells you whether the compensation is the right size.
Step 3: Calculate the anion gap
Anion gap = Na − (Cl + HCO3). Albumin is the main unmeasured anion, so a low albumin hides a high anion gap. Correct for it: the normal anion gap is about albumin (g/dL) × 2.5. Always calculate the anion gap, even when the pH and bicarbonate look normal. A clearly raised anion gap means a metabolic acidosis is present whatever the pH.
Step 4: Use the delta-delta
If the anion gap is high, compare how much it rose with how much the bicarbonate fell: delta-delta = (AG − normal AG) ÷ (25 − HCO3).
- Below 1: the bicarbonate fell more than the anion gap explains, so there is also a normal anion gap (hyperchloremic) acidosis.
- 1 to 2: an anion gap acidosis alone.
- Above 2: the bicarbonate is higher than it should be, so there is also a metabolic alkalosis.
Treat the ratio as one piece of evidence rather than a rule, especially near the cutoffs.
Step 5: Check compensation
Compare the measured value with what compensation predicts. If it falls outside the expected range, a second, respiratory or metabolic, disorder is present.
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | pCO2 = 1.5 × HCO3 + 8 ± 2 (Winters’ formula). Lower means an added respiratory alkalosis; higher, an added respiratory acidosis. |
| Metabolic alkalosis | pCO2 = 40 + 0.7 × (HCO3 − 25) |
| Respiratory acidosis | HCO3 rises 1 (acute) or 3.5 (chronic) for every 10 mmHg rise in pCO2 |
| Respiratory alkalosis | HCO3 falls 2 (acute) or 4.5 (chronic) for every 10 mmHg fall in pCO2 |
Our apps allow a little more room before calling a second disorder: 5 mmHg either side of the predicted pCO2, and 3 mEq/L either side of the predicted bicarbonate. In a chronic setting they predict the pCO2 in metabolic acidosis as HCO3 + 15 instead of using Winters’ formula.
Step 6: Check the osmolar gap when it matters
With a high anion gap acidosis and no clear cause, or a possible ingestion, calculate the osmolar gap: measured osmolality − (2 × Na + glucose ÷ 18 + BUN ÷ 2.8). A gap above about 10 raises concern for a toxic alcohol such as methanol or ethylene glycol.
Step 7: List the causes
- High anion gap metabolic acidosis: ketoacidosis, lactic acidosis, kidney failure, and toxins such as methanol, ethylene glycol and salicylates.
- Normal anion gap metabolic acidosis: diarrhea, renal tubular acidosis and large volumes of saline.
- Metabolic alkalosis: vomiting or gastric suction, and diuretics.
- Respiratory acidosis: COPD, sedative drugs and neuromuscular weakness.
- Respiratory alkalosis: sepsis, pain or anxiety, pneumonia, pulmonary embolism, pregnancy and liver disease.
Worked example: a triple disorder hiding behind a normal pH
A 30-year-old with type 1 diabetes has had three days of vomiting and now has a fever and a cough.
| Test | Result |
|---|---|
| pH | 7.45 |
| pCO2 | 30 mmHg |
| HCO3 | 20 mEq/L |
| Na / Cl | 140 / 90 mEq/L |
| Albumin | 4.0 g/dL |
| Glucose / BUN | 360 / 28 mg/dL |
| Measured osmolality | 315 mOsm/kg |
- pH: 7.45 is at the top of the normal range, so the pH alone suggests nothing is wrong.
- Primary process: pCO2 (30) and HCO3 (20) are both low. The pH can’t settle which came first, so keep going.
- Anion gap: 140 − (90 + 20) = 30. The normal gap for an albumin of 4.0 is 4.0 × 2.5 = 10, so the gap is high by 20: a high anion gap metabolic acidosis is present despite the normal pH.
- Delta-delta: (30 − 10) ÷ (25 − 20) = 20 ÷ 5 = 4. Well above 2, so there is also a metabolic alkalosis. Put another way, without the acidosis the bicarbonate would be about 20 + 20 = 40.
- Compensation: Winters’ formula predicts a pCO2 of 1.5 × 20 + 8 = 38, or 36 to 40. The measured 30 is lower, so there is also a respiratory alkalosis.
- Osmolar gap: calculated osmolality = 2 × 140 + 360 ÷ 18 + 28 ÷ 2.8 = 280 + 20 + 10 = 310. The gap is 315 − 310 = 5, which is normal, so a toxic alcohol is unlikely.
- Causes: diabetic ketoacidosis explains the anion gap acidosis, vomiting the metabolic alkalosis, and fever with a likely pneumonia or sepsis the respiratory alkalosis.
Answer: a triple acid-base disorder — high anion gap metabolic acidosis, metabolic alkalosis and respiratory alkalosis. Enter the same values in ABG Pro DDx to see the app’s interpretation and differential diagnosis.
All the formulas
| Value | Formula |
|---|---|
| Anion gap (AG) | Na − (Cl + HCO3) |
| Normal anion gap, corrected for albumin | albumin (g/dL) × 2.5. A high anion gap is reported when the AG is more than 3 above this. |
| Delta-delta | (AG − normal AG) ÷ (25 − HCO3). Below 1: also a normal anion gap acidosis. 1 to 2: anion gap acidosis alone. Above 2 (or negative, when HCO3 is above 25): also a metabolic alkalosis. |
| Expected pCO2 in metabolic acidosis | Acute setting: 1.5 × HCO3 + 8 (Winters’ formula). Chronic setting: HCO3 + 15. |
| Expected pCO2 in metabolic alkalosis | 40 + 0.7 × (HCO3 − 25) |
| Expected HCO3 in respiratory acidosis | 25, rising 1 (acute) or 3.5 (chronic) for every 10 mmHg of pCO2 above 40 |
| Expected HCO3 in respiratory alkalosis | 25, falling 2 (acute) or 4.5 (chronic) for every 10 mmHg of pCO2 below 40 |
| Calculated osmolality | 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 |
| Osmolar gap | Measured − calculated osmolality. Above 10 is reported as elevated. |
| A-a gradient | FiO2 × (barometric pressure − 47) − pCO2 ÷ 0.8 − PaO2. The normal value for age is (age + 10) ÷ 4, and the gradient is reported as elevated when it is more than 10 above that. |
The app reports an added respiratory disorder when the measured pCO2 is more than 5 mmHg from the predicted value, and an added metabolic disorder when the bicarbonate is more than 3 mEq/L from the predicted value. It uses 40 mmHg and 25 mEq/L as normal values, with default normal ranges of 35–45 mmHg for pCO2 and 23–28 mEq/L for bicarbonate, which you can change in settings. Units: pCO2 and PaO2 in mmHg; Na, Cl and HCO3 in mEq/L; albumin in g/dL; glucose and BUN in mg/dL. The app also accepts SI units.
Sources
- Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57–64. The apps’ calculations are based on this review.
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312–322.
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810.
- Rastegar A. Use of the ΔAG/ΔHCO3− ratio in the diagnosis of mixed acid-base disorders. J Am Soc Nephrol. 2007;18(9):2429–2431.
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174.
- Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011;58(3):480–484.
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434–1445.
- Mehta AN, Emmett JB, Emmett M. GOLD MARK: an anion gap mnemonic for the 21st century. Lancet. 2008;372(9642):892.
This guide is for education. It is not a substitute for clinical judgment, and our apps should not be used to guide actual patient care.