Immunology: complement fixation test 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 · Immunology: complement fixation test interpretation

Sample item

In a complement fixation test, patient serum, antigen and a measured amount of guinea pig complement are incubated, and sheep red cells coated with anti-sheep antibody are then added. No haemolysis occurs, while the serum control tube is lysed. How is this interpreted?

AInvalid: the serum is anticomplementaryBNegative: antibody was absentCPositive: the sheep red cells were agglutinatedDPositive: specific antibody was present and used up the complement

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

The answer and the reasoning

D. Positive: specific antibody was present and used up the complement

If antibody to the antigen is present, antigen-antibody complexes fix the complement, leaving none to lyse the indicator sheep red cells, so absence of haemolysis is a positive result. If antibody is absent, complement remains free and lyses the indicator cells. A lysed serum control rules out an anticomplementary serum.

Option by option

A. Invalid: the serum is anticomplementary
A serum control that lyses shows that the serum is not anticomplementary, so the result is valid.
B. Negative: antibody was absent
If antibody were absent, free complement would have lysed the indicator cells.
C. Positive: the sheep red cells were agglutinated
Agglutination is not the endpoint; the readout is presence or absence of haemolysis.
D. Positive: specific antibody was present and used up the complement (correct)
Correct. No lysis with a clear serum control means antibody fixed the complement.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Positive: specific antibody was present and used up the complement. If antibody to the antigen is present, antigen-antibody complexes fix the complement, leaving none to lyse the indicator sheep red cells, so absence of haemolysis is a positive result.

Why is "Invalid: the serum is anticomplementary" not the answer?

A serum control that lyses shows that the serum is not anticomplementary, so the result is valid.

Why is "Negative: antibody was absent" not the answer?

If antibody were absent, free complement would have lysed the indicator cells.

Why is "Positive: the sheep red cells were agglutinated" not the answer?

Agglutination is not the endpoint; the readout is presence or absence of haemolysis.

What concept does this question test?

It tests Immunology: complement fixation test interpretation in Microbiology. If antibody is absent, complement remains free and lyses the indicator cells. A lysed serum control rules out an anticomplementary serum.

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 Immunology: complement fixation test 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.