How is a positive patient reaction indicated in a complement fixation test?

A positive patient reaction in a complement fixation test is indicated by inhibition of hemolysis. In this test, serum containing antibodies is mixed with a known antigen and complement. If antibodies specific to the antigen are present, they will bind to the antigen and activate the complement. The complement is then "fixed" or consumed in the immune complex formation, which prevents it from participating in hemolysis of red blood cells.

Thus, if hemolysis does not occur, it indicates that the complement has been fixed and is no longer available to lyse the red blood cells, signaling a positive reaction for the presence of the specific antibodies in the patient's serum. This outcome is critical, as it showcases the immunological interaction and confirms the presence of antibodies against the tested antigen.

In contrast, the other options do not indicate a positive patient reaction in this context. The presence of hemolysis would suggest that no specific antibodies were present to fix the complement, resulting in a negative reaction. An absence of reaction does not provide evidence of a positive result as well. A delayed reaction would be more indicative of a potential variation in immune response but does not specifically reflect the principles of the complement fixation test.