The complement fixation test is an immunological meical test that can be used to detect the presence of either specific antibody or specific antigen in a patient's serum. It was widely used to diagnose infections, particularly with microbes that are not easily detected by culture methods, and in rheumatic diseases. However, in clinical diagnostics labs it has been largely superseded by other serological methods such as ELISA and by DNA-based methods of pathogen detection, particularly PCR.