Anion Gap and Albumin: Why a Normal Gap Can Be Misleading

A reported anion gap needs context. The calculation method, reference interval and albumin concentration affect how it is interpreted. This English companion develops a theme from Serdar Efe's Turkish discussion of normal-gap metabolic acidosis: important information can be missed when clinicians read only the reported pH.

Check which anion gap you are using

A commonly used equation is AG = Na − (Cl + HCO₃). Some laboratories include potassium, producing a different numerical result and reference interval. Use the convention stated on the report and do not compare unlike equations as if they were identical. Match the electrolytes and bicarbonate to the relevant sampling time.

What albumin changes

Albumin contributes to the unmeasured anions represented by the gap. When albumin is low, an apparently normal measured gap can hide an increase in other unmeasured anions. The ATS teaching resource describes an approximate adjustment of 2.5 mEq/L per 1 g/dL decrease in albumin. Using 4 g/dL as the reference, the illustrative equation is corrected AG = measured AG + 2.5 × (4 − albumin in g/dL). This is an approximation, not a direct measurement of an individual acid.

A worked calculation, not a patient diagnosis

Consider hypothetical values of sodium 140 mmol/L, chloride 110 mmol/L, bicarbonate 20 mmol/L and albumin 2 g/dL. The potassium-free measured gap is 140 − (110 + 20) = 10 mEq/L. The albumin adjustment adds 2.5 × (4 − 2) = 5, giving an approximate corrected gap of 15 mEq/L. Whether that is abnormal depends on the laboratory's method and reference interval. The arithmetic alone does not identify a cause.

Return to the normal-gap question

In the original Turkish post, the author reflects on postoperative care, gastrointestinal losses, chloride exposure and renal tubular acidosis. These are prompts to investigate the clinical context. The post's individual bedside experiences should not be treated as a universal fluid or bicarbonate prescription.

A useful review separates what was measured, what was calculated and what remains a hypothesis. Check the fluid history, relevant losses, kidney function and medication history alongside the laboratory pattern. An isolated gap should not close the differential diagnosis.

Keep the interpretation reproducible

• State whether potassium is included.
• Record the laboratory reference interval.
• Give albumin with its units; g/L and g/dL are different.
• Label any correction as approximate.
• Describe the clinical evidence supporting the proposed cause.

The platform's blood gas tool overview explains its conventional and physicochemical modules. Automated output still needs checking against the sample, units and clinical context.

Sources

Serdar Efe, Sıcak yaz günlerinde buz gibi Yoğun Bakım kokteylleri, 8 July 2024; ATS: Interpretation of Arterial Blood Gases. Educational explanation; no patient-specific treatment recommendation.

Continue reading

• Blood Gas Interpretation in the ICU: A Structured Reading Checklist
• Respiratory Pyramid Interface: Understanding the RPI Research and Its Limits

Author: Serdar Efe, MD · Bursa Uludağ University, Türkiye. Publications.

Published: 9 October 2026. English text prepared with AI assistance from the cited platform resources and references.

English: ICU Quality Insights · All English articles