Clinical biochemistry: albumin and the anion gap: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Clinical biochemistry: albumin and the anion gap

Sample item

A patient with cirrhosis and a serum albumin of 1.8 g/dL has lactic acidosis, but the calculated anion gap is only mildly raised. Why can the gap understate the acidosis here?

ALow albumin increases sodium retentionBLow albumin increases bicarbonate generationCAlbumin is the main unmeasured anion, so its fall lowers the gapDLow albumin raises the measured chloride sharply

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Albumin is the main unmeasured anion, so its fall lowers the gap

Albumin carries most of the negative charge counted as unmeasured anions, so hypoalbuminaemia lowers the anion gap by about 2.5 mmol/L for every 1 g/dL fall. The gap should be corrected upward before interpretation. Otherwise a true high gap acidosis may be missed.

Option by option

A. Low albumin increases sodium retention
Sodium retention does not lower the gap.
B. Low albumin increases bicarbonate generation
Bicarbonate generation is not the explanation.
C. Albumin is the main unmeasured anion, so its fall lowers the gap (correct)
Correct. Less albumin means fewer unmeasured anions and a falsely low gap.
D. Low albumin raises the measured chloride sharply
Chloride is not raised by hypoalbuminaemia in this way.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Albumin is the main unmeasured anion, so its fall lowers the gap. Albumin carries most of the negative charge counted as unmeasured anions, so hypoalbuminaemia lowers the anion gap by about 2.

Why is "Low albumin increases sodium retention" not the answer?

Sodium retention does not lower the gap.

Why is "Low albumin increases bicarbonate generation" not the answer?

Bicarbonate generation is not the explanation.

Why is "Low albumin raises the measured chloride sharply" not the answer?

Chloride is not raised by hypoalbuminaemia in this way.

What concept does this question test?

It tests Clinical biochemistry: albumin and the anion gap in Biochemistry. 5 mmol/L for every 1 g/dL fall. The gap should be corrected upward before interpretation. Otherwise a true high gap acidosis may be missed.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Clinical biochemistry: albumin and the anion gap?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.