Clinical microbiology: blood culture interpretation: practice question

A Microbiology 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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Microbiology · Clinical microbiology: blood culture interpretation

Sample item

A 60-year-old woman has fever and hypotension after a hospital stay. Two blood culture sets are drawn from separate venepuncture sites before antibiotics. One of four bottles grows coagulase-negative staphylococci at 38 hours, and the other three bottles stay sterile. There is no prosthetic material or catheter in situ. What is the most likely interpretation?

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

The answer and the reasoning

C. Skin contaminant introduced during collection

Coagulase-negative staphylococci are the commonest skin contaminants of blood cultures. Growth in only one of several bottles, and late growth, favours contamination, whereas true infection typically grows in multiple sets, often early. Collecting two or more sets from separate sites before antibiotics, with proper skin antisepsis, is what allows this interpretation.

Option by option

  • A. Definite endocarditis needing urgent valve surgery

    Endocarditis would typically give bacteraemia in most or all sets and clinical valve involvement.

  • B. Pseudobacteraemia due to unsuitable culture broth

    Pseudobacteraemia is usually due to contaminated reagents or devices and would give growth across many specimens.

  • C. Skin contaminant introduced during collection (correct)

    Correct. Single-bottle, late growth of a skin commensal is most likely contamination.

  • D. Catheter-related bloodstream infection needing line removal

    No intravascular catheter is present, and a line infection would give positive sets.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Skin contaminant introduced during collection. Coagulase-negative staphylococci are the commonest skin contaminants of blood cultures.

Why is "Definite endocarditis needing urgent valve surgery" not the answer?

Endocarditis would typically give bacteraemia in most or all sets and clinical valve involvement.

Why is "Pseudobacteraemia due to unsuitable culture broth" not the answer?

Pseudobacteraemia is usually due to contaminated reagents or devices and would give growth across many specimens.

Why is "Catheter-related bloodstream infection needing line removal" not the answer?

No intravascular catheter is present, and a line infection would give positive sets.

What concept does this question test?

It tests Clinical microbiology: blood culture interpretation in Microbiology. Growth in only one of several bottles, and late growth, favours contamination, whereas true infection typically grows in multiple sets, often early. Collecting two or more sets from separate sites before antibiotics, with proper skin antisepsis, is what allows this interpretation.

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 microbiology: blood culture interpretation?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.