Clinical biochemistry: EDTA contamination of a sample: 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: EDTA contamination of a sample

Sample item

A well outpatient's serum shows potassium 9.5 mmol/L, calcium 0.9 mmol/L and very low alkaline phosphatase, with a normal ECG. The sample was drawn after an EDTA tube. What is the most likely explanation?

AIn vivo hyperkalaemia from renal failureBContamination of the serum sample with potassium EDTACDelayed separation of serum from the clotDHaemolysis during venepuncture

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

The answer and the reasoning

B. Contamination of the serum sample with potassium EDTA

K2EDTA contamination adds potassium and chelates calcium, magnesium and zinc, which lowers measured calcium and the activity of metalloenzymes such as alkaline phosphatase. The normal ECG and well patient show that the values are spurious. Haemolysis raises potassium but does not lower calcium or alkaline phosphatase.

Option by option

A. In vivo hyperkalaemia from renal failure
True hyperkalaemia of this degree would cause ECG changes and would not lower calcium and ALP in this way.
B. Contamination of the serum sample with potassium EDTA (correct)
Correct. EDTA adds potassium and chelates calcium and the metal cofactors of alkaline phosphatase.
C. Delayed separation of serum from the clot
Delayed separation raises potassium and lowers glucose, but it does not lower calcium and alkaline phosphatase in this way.
D. Haemolysis during venepuncture
Haemolysis raises potassium but leaves calcium and ALP largely unaffected.

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 B, Contamination of the serum sample with potassium EDTA. K2EDTA contamination adds potassium and chelates calcium, magnesium and zinc, which lowers measured calcium and the activity of metalloenzymes such as alkaline phosphatase.

Why is "In vivo hyperkalaemia from renal failure" not the answer?

True hyperkalaemia of this degree would cause ECG changes and would not lower calcium and ALP in this way.

Why is "Delayed separation of serum from the clot" not the answer?

Delayed separation raises potassium and lowers glucose, but it does not lower calcium and alkaline phosphatase in this way.

Why is "Haemolysis during venepuncture" not the answer?

Haemolysis raises potassium but leaves calcium and ALP largely unaffected.

What concept does this question test?

It tests Clinical biochemistry: EDTA contamination of a sample in Biochemistry. The normal ECG and well patient show that the values are spurious. Haemolysis raises potassium but does not lower calcium or alkaline phosphatase.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: EDTA contamination of a sample?

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.