Clinical microbiology: catheter-related bloodstream infection: 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: catheter-related bloodstream infection

Sample item

A patient has a central venous catheter and fever. Paired blood cultures are drawn at the same time from the catheter and from a peripheral vein. The catheter sample flags positive at 14 hours and the peripheral sample at 19 hours, both growing the same coagulase-negative staphylococcus. What is the interpretation?

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

The answer and the reasoning

B. Catheter-related bloodstream infection, since the catheter culture became positive at least 2 hours earlier

A differential time to positivity of 2 hours or more, with the catheter sample positive first and growing the same organism, indicates that the catheter is the source of the bacteraemia, because the higher inoculum in the catheter lumen grows faster. Here the difference is 19 minus 14, which is 5 hours. Matching positive cultures from two sites make contamination unlikely.

Option by option

  • A. Skin contamination, since coagulase-negative staphylococci are always contaminants

    Coagulase-negative staphylococci can be true pathogens when growing in paired cultures from a catheter patient.

  • B. Catheter-related bloodstream infection, since the catheter culture became positive at least 2 hours earlier (correct)

    A 5-hour earlier positivity from the catheter exceeds the 2-hour threshold and indicates a catheter source.

  • C. Secondary bacteraemia from a urinary source

    A urinary source is not supported by the culture pattern and organism.

  • D. A negative result since both samples were positive

    Positive cultures at both sites with the same organism are a positive result, and the timing assigns the source.

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 B, Catheter-related bloodstream infection, since the catheter culture became positive at least 2 hours earlier. A differential time to positivity of 2 hours or more, with the catheter sample positive first and growing the same organism, indicates that the catheter is the source of the bacteraemia, because the higher inoculum in the catheter lumen grows faster.

Why is "Skin contamination, since coagulase-negative staphylococci are always…" not the answer?

Coagulase-negative staphylococci can be true pathogens when growing in paired cultures from a catheter patient.

Why is "Secondary bacteraemia from a urinary source" not the answer?

A urinary source is not supported by the culture pattern and organism.

Why is "A negative result since both samples were positive" not the answer?

Positive cultures at both sites with the same organism are a positive result, and the timing assigns the source.

What concept does this question test?

It tests Clinical microbiology: catheter-related bloodstream infection in Microbiology. Here the difference is 19 minus 14, which is 5 hours. Matching positive cultures from two sites make contamination unlikely.

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: catheter-related bloodstream infection?

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.