Laboratory diagnosis: 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 · Laboratory diagnosis: blood culture interpretation

Sample item

A 62-year-old man with fever and a urinary source has two blood culture sets (four bottles) drawn from separate sites before antibiotics. At 36 hours, one aerobic bottle grows coagulase-negative staphylococci and the other three bottles stay negative. He has no prosthetic material. How should this result be interpreted?

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

The answer and the reasoning

A. A probable skin contaminant, and no specific therapy is directed at this isolate

Coagulase-negative staphylococci are common skin flora. Growth in only one of several bottles, with no intravascular device or prosthesis, indicates a contaminant. By contrast, Staphylococcus aureus in even one bottle is never regarded as a contaminant. Poor skin antisepsis is the usual cause.

Option by option

  • A. A probable skin contaminant, and no specific therapy is directed at this isolate (correct)

    Correct. A single positive bottle of skin flora without a device is a contaminant.

  • B. Evidence of infective endocarditis that requires transoesophageal echocardiography

    Endocarditis would give multiple positive sets and compatible clinical signs, not a lone bottle.

  • C. True bacteraemia that needs two weeks of intravenous vancomycin

    Treating a single positive bottle as true bacteraemia would lead to unnecessary vancomycin.

  • D. An invalid test because the blood volume was inadequate

    Nothing in the history suggests an inadequate volume, and the three negative bottles support contamination.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, A probable skin contaminant, and no specific therapy is directed at this isolate. Coagulase-negative staphylococci are common skin flora.

Why is "Evidence of infective endocarditis that requires transoesophageal ech…" not the answer?

Endocarditis would give multiple positive sets and compatible clinical signs, not a lone bottle.

Why is "True bacteraemia that needs two weeks of intravenous vancomycin" not the answer?

Treating a single positive bottle as true bacteraemia would lead to unnecessary vancomycin.

Why is "An invalid test because the blood volume was inadequate" not the answer?

Nothing in the history suggests an inadequate volume, and the three negative bottles support contamination.

What concept does this question test?

It tests Laboratory diagnosis: blood culture interpretation in Microbiology. Growth in only one of several bottles, with no intravascular device or prosthesis, indicates a contaminant. By contrast, Staphylococcus aureus in even one bottle is never regarded as a contaminant. Poor skin antisepsis is the usual cause.

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 Laboratory diagnosis: 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.