Acid–Base Revision: A Worked ABG Method for EDAIC

A useful ABG method identifies the direction of pH, the process explaining it, whether the other component fits expected compensation, and whether electrolytes reveal an additional metabolic clue. A near-normal pH does not exclude a mixed disorder.

This original teaching case develops exam interpretation rather than a patient-treatment plan. Laboratory reference intervals, sampling conditions and clinical context remain necessary in practice.

Use a sequence that exposes contradictions

Start with the specimen and units. Confirm whether the sample is arterial and whether PaCO₂ is reported in mmHg or kPa. Then describe the pH as acidemia, alkalemia or within the reference interval. Acidemia describes the measured state; acidosis describes a process that tends to lower pH. Two opposing processes may coexist. MSD explains the distinction.

Next ask which component explains the direction: carbon dioxide or bicarbonate. Do not stop after assigning a name. Compare the remaining component with an expected response for that particular primary disorder, and then examine relevant chemistry results.

For an exercise, put four headings on the page: pH; primary process; compensation; gap. Reserve a fifth line for “what these numbers cannot establish”. That final line prevents a biochemical pattern becoming an unsupported diagnosis.

Worked case: metabolic acidosis with an expected response

Consider a fictional arterial sample and simultaneous chemistry values:

• pH 7.29.
• PaCO₂ 26 mmHg.
• Bicarbonate approximately 12 mmol/L.
• Sodium 140 mmol/L and chloride 104 mmol/L.
• Albumin 4.0 g/dL.

Use a teaching pH interval of 7.35–7.45 and, for the later gap comparison, a laboratory anion-gap reference centred on 12 mmol/L with potassium excluded.

The pH shows acidemia. The low bicarbonate explains an acidifying process. The low PaCO₂ points in the opposite direction, so it could be an accompanying respiratory response rather than the primary cause of the acidemia.

For metabolic acidosis, use Winter's expected PaCO₂ relationship:

Expected PaCO₂ = (1.5 × bicarbonate) + 8 ± 2 mmHg.

At bicarbonate 12, this gives 18 + 8 = 26 ± 2 mmHg, or 24–28 mmHg. The measured 26 falls inside that range. The conclusion is “metabolic acidosis with a respiratory response within the expected range”, not “normal acid–base balance”. MSD provides the compensation framework.

Calculate the gap separately

With potassium excluded:

Anion gap = sodium − (chloride + bicarbonate).

For this case: 140 − (104 + 12) = 24 mmol/L. Relative to the stated laboratory assumption, this is increased. It suggests an excess of unmeasured anions, but does not identify which ones. Albumin affects the expected gap, so use the laboratory convention and consider albumin rather than treating one normal value as universal. OpenAnesthesia discusses gap interpretation and metabolic acidosis.

The case deliberately supplies albumin at 4.0 g/dL to avoid making an albumin correction the hidden task. It does not supply lactate, ketones, renal function or an exposure history. Naming a specific cause would therefore go beyond the data.

Check that the ABG values belong together

The bicarbonate–CO₂ relationship can be expressed approximately as:

pH = 6.1 + log₁₀[HCO₃⁻/(0.03 × PaCO₂)], with PaCO₂ in mmHg.

Using 12 and 26 gives 6.1 + log₁₀(12/0.78), approximately 7.29. This is a consistency check, not a replacement for interpretation. Blood-gas analysers commonly calculate bicarbonate from measured pH and PaCO₂; chemistry measurements have their own method and sampling context. Clinical Methods explains ABG measurement and the underlying relationship.

Now hold bicarbonate at 12 but change PaCO₂ to 40 mmHg. The expected range remains 24–28, so 40 is unexpectedly high for compensation. The new values suggest an additional respiratory acidosis. Their corresponding pH would be about 7.10, not the original 7.29. When constructing practice cases, change dependent values consistently.

Avoid three shortcuts

First, do not apply Winter's equation to every acid–base disturbance. It estimates the respiratory response to metabolic acidosis. Other primary processes need their corresponding relationships and consideration of timing.

Second, do not enter PaCO₂ in kPa into a formula defined in mmHg. A value around 3.5 kPa is near 26 mmHg; those are two expressions of approximately the same pressure, not two very different patients.

Third, do not use “compensated” as a synonym for harmless, resolved or isolated. Expected compensation is a numerical relationship; the clinical problem and possible additional processes still need assessment.

Original self-test

1. The first case has acidemia and metabolic acidosis. True: pH is low and bicarbonate explains the direction.
2. PaCO₂ 26 proves a separate primary respiratory alkalosis. False: it fits the expected response in this case.
3. The calculated anion gap is 24 mmol/L. True: 140 − 116 = 24.
4. That gap alone proves lactic acidosis. False: the unmeasured anions have not been identified.
5. Changing PaCO₂ while fixing bicarbonate leaves pH unchanged. False: they are mathematically linked.

Place this exercise within the physiology revision guide, then practise distinguishing a measurement from its interpretation in the EDAIC question bank.

Sources

• MSD Manual: Acid-Base Disorders.
• OpenAnesthesia: Metabolic Acidosis.
• NCBI Clinical Methods: Arterial Blood Gases.

Prepared with AI assistance for examination revision; this article is not a patient-care protocol or a substitute for current primary guidance. Sources checked 7 October 2026.