When to suspect the possibility of a false positive antigen test:
(1) positive serum antigen assay but negative urine antigen
(2) inconsistency between antigen test finding and clinical findings
(3) presence of a known cause for a false-positive result
Causes of a false-positive serum antigen test:
(1) Rheumatoid factor (RF)
(2) rabbit antithymocyte globulin (due to formation of antibodies to rabbit immunoglobulin)
(3) heterophile antibodies to the animal whose antibodies are used in the detection kit
(4) possibly recent immune globulin therapy
Detection of antigen is done using antibodies developed in rabbit, mouse or other animals. Human antibodies against these source animals can result in a false positive reaction.
Causes of a false-positive urine antigen test due to cross-reacting antigens:
(1) blastomycosis
(2) penicilliniosis
(3) paracoccidioimycosis
(4) African histoplasmosis (H. capsulatum var. duboisii)
