Clinical biochemistry: pleural fluid analysis: 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: pleural fluid analysis

Sample item

A 50-year-old man has a pleural effusion. Pleural fluid protein is 3.5 g/dL with serum protein 6.0 g/dL, and pleural fluid LDH is 280 U/L with serum LDH 400 U/L. How is the fluid classified under Light's criteria?

ATransudate, because the LDH ratio is below 1BExudate, because the protein ratio exceeds 0.5 and the LDH ratio exceeds 0.6CTransudate, because the pleural protein is below 4 g/dLDExudate, only if the pleural glucose is below 60 mg/dL

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

The answer and the reasoning

B. Exudate, because the protein ratio exceeds 0.5 and the LDH ratio exceeds 0.6

Light's criteria classify fluid as exudate if any of the following is met: pleural to serum protein above 0.5, pleural to serum LDH above 0.6, or pleural LDH more than two thirds of the upper limit of normal serum LDH. Here the protein ratio is 3.5/6.0 = 0.58 and the LDH ratio is 280/400 = 0.70, so two criteria are met. Exudates arise from pleural or adjacent inflammation, infection or malignancy.

Option by option

A. Transudate, because the LDH ratio is below 1
The LDH ratio threshold is 0.6, not 1, and here it is exceeded.
B. Exudate, because the protein ratio exceeds 0.5 and the LDH ratio exceeds 0.6 (correct)
Correct. Both the protein ratio of 0.58 and the LDH ratio of 0.70 exceed the cut-offs.
C. Transudate, because the pleural protein is below 4 g/dL
No absolute pleural protein cut-off of 4 g/dL defines a transudate in Light's criteria.
D. Exudate, only if the pleural glucose is below 60 mg/dL
Glucose is not part of Light's criteria.

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, Exudate, because the protein ratio exceeds 0.5 and the LDH ratio exceeds 0.6. Light's criteria classify fluid as exudate if any of the following is met: pleural to serum protein above 0.

Why is "Transudate, because the LDH ratio is below 1" not the answer?

The LDH ratio threshold is 0.6, not 1, and here it is exceeded.

Why is "Transudate, because the pleural protein is below 4 g/dL" not the answer?

No absolute pleural protein cut-off of 4 g/dL defines a transudate in Light's criteria.

Why is "Exudate, only if the pleural glucose is below 60 mg/dL" not the answer?

Glucose is not part of Light's criteria.

What concept does this question test?

It tests Clinical biochemistry: pleural fluid analysis in Biochemistry. 5, pleural to serum LDH above 0.6, or pleural LDH more than two thirds of the upper limit of normal serum LDH. Here the protein ratio is 3.5/6.0 = 0.58 and the LDH ratio is 280/400 = 0.70, so two criteria are met. Exudates arise from pleural or adjacent inflammation, infection or malignancy.

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: pleural fluid analysis?

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.