A lab technician is running controls for a complement fixation test. To check for the absence of anti-complementary activity of the antigen, the technician should perform the test:

In a complement fixation test, the goal is to determine whether specific antibodies are present in the serum sample that could fix complement. To ensure that the antigen itself does not have any anti-complementary activity, it is crucial to run the test without the serum. When the test is performed without serum, it allows for the evaluation of the antigen’s behavior in isolation, confirming that any complement fixation observed in subsequent tests is due to the presence of antibodies rather than any inherent properties of the antigen itself.

Performing the test with serum introduces other variables, such as potential interference from the serum’s proteins or antibodies that might result in complement fixation independently of the target antibodies. Using diluted serum could also lead to misleading results, as the concentration of antibodies might not be adequate to facilitate a definitive interpretation. Positive controls serve to validate the test methodology but do not address the need to assess the antigen's properties without serum interference, thereby confirming that the antigen is suitable for testing with patient samples.