MedCourseFinder

Arterial blood gas interpretation: a stepwise approach

Source-checked · Content last updated · 4 min read ·

This is the date the draft was written, not a medical review date.

A systematic sequence for reading an arterial blood gas: check validity, classify the pH, identify the primary process, assess compensation, calculate the anion gap and delta ratio, then evaluate oxygenation. The rules of thumb given are approximations and the result must always be read against the clinical picture.

  1. Step 1. 1. Check validity and context

    Confirm that the sample is arterial and properly handled, and note the FiO2 or oxygen device, temperature, and clinical setting.

  2. Step 2. 2. Look at the pH

    A pH below about 7.35 indicates acidaemia and above about 7.45 alkalaemia (usual arterial range 7.35-7.45). A normal pH does not exclude a disorder: a mixed disturbance or full compensation can leave the pH near normal.

  3. Step 3. 3. Identify the primary process

    Compare PaCO2 (normal about 35-45 mmHg, 4.7-6.0 kPa) and HCO3 (normal about 22-26 mmol/L) with the pH direction. Acidaemia with high PaCO2 is respiratory acidosis; acidaemia with low HCO3 is metabolic acidosis; alkalaemia with low PaCO2 is respiratory alkalosis; alkalaemia with high HCO3 is metabolic alkalosis.

  4. Step 4. 4. Assess compensation

    Compare the measured value with the expected one. Metabolic acidosis: expected PaCO2 = 1.5 x HCO3 + 8 (+/- 2) (Winter formula). Metabolic alkalosis: PaCO2 rises roughly 0.7 mmHg per 1 mmol/L rise in HCO3. Respiratory acidosis: HCO3 rises about 1-2 mmol/L per 10 mmHg PaCO2 rise if acute, and about 3-4 if chronic. These rules are approximations; a value outside the expected range suggests a second, mixed disorder.

  5. Step 5. 5. Calculate the anion gap

    In metabolic acidosis, anion gap = Na - (Cl + HCO3); the normal value is about 8-12 mEq/L when potassium is not included, but it is method-dependent, so use the local reference. Correct for hypoalbuminaemia by adding about 2.5 for each 1 g/dL (10 g/L) that albumin is below 4 g/dL (40 g/L). A raised gap suggests added unmeasured acids (lactate, ketones, uraemia, toxins).

  6. Step 6. 6. Use the delta ratio

    In a high-anion-gap acidosis, compare the rise in the gap with the fall in HCO3: delta ratio = (anion gap - 12) / (24 - HCO3). Below about 0.4 suggests a hyperchloraemic non-gap acidosis; 0.4-0.8 a mixed picture; 0.8-2 a pure high-gap acidosis; above 2 a concurrent metabolic alkalosis (or pre-existing high bicarbonate). The cut-offs vary between authors.

  7. Step 7. 7. Assess oxygenation

    Interpret PaO2 against FiO2: calculate PaO2/FiO2 and the alveolar-arterial gradient, where PAO2 = FiO2 x (Patm - 47) - PaCO2 / 0.8 (mmHg, at sea level Patm is 760). The expected A-a gradient is roughly age/4 + 4 mmHg in room air (approximate), and rises with age; a raised gradient indicates V/Q mismatch, shunt or diffusion impairment rather than hypoventilation alone.

  8. Step 8. 8. Integrate with the clinical context

    Interpret the pattern with the history, examination, electrolytes, lactate and trends over time, and decide on the underlying cause. This guide is for teaching; decisions on treatment rest on clinical judgement and local protocols.

Formulas

Winter formula (expected PaCO2 in metabolic acidosis)
Expected PaCO2 (mmHg) = 1.5 x HCO3 (mmol/L) + 8, +/- 2Used to test whether respiratory compensation is appropriate; a measured PaCO2 above or below the range suggests a coexisting respiratory disorder. Approximate.
Anion gap
Anion gap (mEq/L) = Na - (Cl + HCO3); albumin-corrected gap = anion gap + 2.5 x (4.0 - albumin in g/dL)Normal about 8-12 mEq/L without potassium, but method-dependent (some laboratories report 4-12); use the local reference interval.
Delta ratio (delta gap / delta HCO3)
Delta ratio = (anion gap - 12) / (24 - HCO3)Below 0.4: hyperchloraemic non-gap acidosis; 0.4-0.8: mixed; 0.8-2: pure high-gap acidosis; above 2: concurrent metabolic alkalosis. Cut-offs vary by author.
Alveolar gas equation (PAO2)
PAO2 (mmHg) = FiO2 x (Patm - 47) - PaCO2 / 0.8Patm is 760 mmHg at sea level and 47 mmHg is the water vapour pressure at 37 C; the respiratory quotient is taken as 0.8. Adjust Patm for altitude.
Expected alveolar-arterial gradient
Expected A-a gradient (mmHg, room air) = age / 4 + 4; A-a gradient = PAO2 - PaO2Approximate rule of thumb for adults breathing room air; the gradient widens with age and with increasing FiO2.
PaO2/FiO2 ratio
PaO2/FiO2 = PaO2 (mmHg) / FiO2 (fraction, e.g. 0.21 to 1.0)In the Berlin definition of ARDS, PaO2/FiO2 of 200-300, 100-200 and 100 mmHg or below define mild, moderate and severe hypoxaemia respectively (with a minimum PEEP requirement).

Pitfalls

  • Air bubbles in the syringe lower PaCO2 and push PaO2 toward room-air values (about 150 mmHg), so expel them immediately and cap the syringe.
  • Delayed analysis at room temperature changes the result through cellular metabolism (PaO2 can fall substantially within about 20 minutes), so analyse promptly. Excess liquid heparin dilutes the sample and lowers PaCO2 and HCO3.
  • Venous or mixed samples are mistaken for arterial ones: venous pH and bicarbonate agree reasonably with arterial values, but venous PCO2 agrees poorly with PaCO2 and cannot be relied on as an absolute value, and venous PO2 cannot be used to assess oxygenation. Check SaO2 and the clinical picture.
  • Temperature correction and analyser reference ranges differ between laboratories; the analyser usually reports at 37 C, and the acid-base values in hypothermia or fever should be interpreted according to local practice.
  • Lactate can be falsely raised by delayed processing, tourniquet use or prolonged sample storage; interpret a high value with the clinical context, and remember that a normal pH and PaCO2 do not exclude a mixed disorder.

Calculators

References

  1. Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
  2. Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
  3. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)
  4. Murray and Nadel's Textbook of Respiratory Medicine. Elsevier, 7th edition (2021) — Tier 3 (textbook)

References are listed only where the article cites them. See the sources page for the full policy. How sources are chosen and credited

Back to the index