ABG Interpretation Practice
Work through each blood gas one step at a time: pH, primary disorder, compensation, anion gap, and delta ratio. Quick drills generate unlimited values; clinical cases add history and a bedside question.
Reference card
| Value | Normal range used in this widget (Castro & Zubair, 2026) |
|---|---|
| pH | 7.35 to 7.45 |
| PaCO2 | 35 to 45 mmHg |
| HCO3− | 22 to 26 mEq/L. Ranges vary among laboratories; Theodore (2025) lists 21 to 27. |
| PaO2 | No defined normal; a resting PaO2 above 80 mmHg is reasonably considered normal unless it differs substantially from prior values (Theodore, 2025) |
| Anion gap (without K+) | Reference ranges vary by laboratory and analyzer; Lewis (2025a) gives a typical value of 12 mEq/L, and Palmer and Clegg (2025) use 12 to 14. This widget treats >12 as elevated, and quick drills avoid values of 12 to 14. |
| Primary disorder | Expected compensation (Palmer & Clegg, 2025) |
|---|---|
| Metabolic acidosis | PaCO2 = 1.5 × HCO3 + 8 (±2) (Winter’s formula) |
| Metabolic alkalosis | PaCO2 rises 0.7 mmHg per 1 mEq/L rise in HCO3: 40 + 0.7 × (HCO3 − 24). The widget accepts ±2. Compensation should not raise PaCO2 above 55 mmHg (Lewis, 2025a). |
| Respiratory acidosis | HCO3 rises 1 per 10 mmHg rise in PaCO2 (acute); 3.5 per 10 (chronic) |
| Respiratory alkalosis | HCO3 falls 2 per 10 mmHg fall in PaCO2 (acute); 5 per 10 (chronic) |
| Interpretive caution | A pH within 7.35 to 7.45 does not exclude an acid-base disorder. Compensation or opposing mixed disorders can move pH toward normal. If the bicarbonate response to a respiratory disorder falls between the acute and chronic predictions, correlate with the clinical time course and prior values rather than assigning duration from a single ABG (Palmer & Clegg, 2025). |
| Calculation | Formula and interpretation |
|---|---|
| Anion gap | Na+ − (Cl− + HCO3). In clinical practice, calculate the gap from the serum chemistry panel; ABG HCO3− is calculated from pH and PaCO2. This widget uses the displayed HCO3− for simplified, internally consistent practice cases (Lewis, 2025a). |
| Albumin-corrected AG | Add 2.5 for each 1 g/dL that albumin falls below 4.0 (Palmer & Clegg, 2025) |
| Delta ratio | (AG − 12) / (24 − HCO3). Roughly equal changes support a simple gap acidosis; a much larger fall in HCO3 suggests a coexisting non-gap acidosis, and a much smaller fall suggests a coexisting metabolic alkalosis (Palmer & Clegg, 2025). Conventional teaching cutoffs are <1 for combined gap and non-gap acidosis, 1 to 2 for simple gap acidosis, and >2 for gap acidosis plus metabolic alkalosis. These are heuristics and should be interpreted with the clinical picture and other laboratory data (Rastegar, 2007). |
| Venous blood gas | Peripheral venous pH runs about 0.02 to 0.04 lower than arterial, PCO2 about 3 to 8 mmHg higher, and HCO3 about 2 to 3 mEq/L higher. A VBG cannot assess oxygenation or the A-a gradient and is unreliable in shock or extreme acid-base disturbance (pH <7.2 or >7.6) (Theodore, 2026). |
| A-a gradient | PAO2 = FiO2 × (760 − 47) − PaCO2/0.8 at sea level; A-a = PAO2 − PaO2. On room air, a common age-adjusted upper estimate is about age/4 + 4; the expected gradient widens as FiO2 increases. Hypoxemia with a normal gradient points to hypoventilation (or low inspired oxygen); an elevated gradient points to V/Q mismatch, right-to-left shunt, or impaired diffusion (Wood, 2026). |
References
Castro, D., & Zubair, M. (2026, July 5). Arterial blood gas. In StatPearls. StatPearls Publishing. Retrieved September 23, 2026, from https://www.ncbi.nlm.nih.gov/books/NBK536919/
Emmett, M. (2020). Metabolic alkalosis: A brief pathophysiologic review. Clinical Journal of the American Society of Nephrology, 15(12), 1848–1856. https://doi.org/10.2215/CJN.16041219
Lewis, J. L., III. (2025a, April). Acid-base disorders. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/nephrology/acid-base-regulation-and-disorders/acid-base-disorders
Lewis, J. L., III. (2025b, December). Metabolic acidosis. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/nephrology/acid-base-regulation-and-disorders/metabolic-acidosis
Lewis, J. L., III. (2025c, March). Respiratory acidosis. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/nephrology/acid-base-regulation-and-disorders/respiratory-acidosis
Lewis, J. L., III. (2026, July). Respiratory alkalosis. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/nephrology/acid-base-regulation-and-disorders/respiratory-alkalosis
O’Malley, G. F., & O’Malley, R. (2025, April). Aspirin and other salicylate poisoning. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/injuries-poisoning/poisoning/aspirin-and-other-salicylate-poisoning
Palmer, B. F., & Clegg, D. J. (2025). Mixed acid-base disturbances: Core curriculum 2025. American Journal of Kidney Diseases, 86(3), 372–382. https://doi.org/10.1053/j.ajkd.2025.04.014
Rastegar, A. (2007). Use of the ΔAG/ΔHCO3− ratio in the diagnosis of mixed acid-base disorders. Journal of the American Society of Nephrology, 18(9), 2429–2431. https://doi.org/10.1681/ASN.2006121408
Theodore, A. C. (2025). Arterial blood gases. UpToDate. Retrieved September 23, 2026, from https://www.uptodate.com/contents/arterial-blood-gases
Theodore, A. C. (2026). Venous blood gases and alternatives to arterial carbon dioxide measurement in adults. UpToDate. Retrieved September 23, 2026, from https://www.uptodate.com/contents/venous-blood-gases-and-alternatives-to-arterial-carbon-dioxide-measurement-in-adults
Umpierrez, G. E., Davis, G. M., ElSayed, N. A., Fadini, G. P., Galindo, R. J., Hirsch, I. B., Klonoff, D. C., McCoy, R. G., Misra, S., Gabbay, R. A., Bannuru, R. R., & Dhatariya, K. K. (2024). Hyperglycemic crises in adults with diabetes: A consensus report. Diabetes Care, 47(8), 1257–1275. https://doi.org/10.2337/dci24-0032
Uribarri, J., Goldfarb, D. S., Raphael, K. L., Rein, J. L., & Asplin, J. R. (2022). Beyond the urine anion gap: In support of the direct measurement of urinary ammonium. American Journal of Kidney Diseases, 80(5), 667–676. https://doi.org/10.1053/j.ajkd.2022.05.009
Wood, K. L. (2026, April). Measurement of gas exchange. In Merck Manual Professional Version. Merck & Co. Retrieved September 23, 2026, from https://www.merckmanuals.com/professional/pulmonary-disorders/tests-of-pulmonary-function-pft/measurement-of-gas-exchange
For education and exam preparation only. Values are simplified (sea-level barometric pressure, fixed cutoffs) and do not replace clinical judgment.