Recent experience with the complement fixation test in the laboratory diagnosis of rickettsial diseases in the United States.

Sera from patients suspected of having rickettsial infections were tested in the complement fixation test with antigens prepared from the rickettsiae of Rocky Mountain spotted fever (SF), rickettsial pox (RP), murine typhus, epidemic typhus, and from Rickettsia canada (RC). Eight units of antigen were used in all cases and two units in man. Only those patients with antibody titers of 1:16 or higher were included in the study. Largely on the basis of comparative titers, the patients were divided into two groups: 102 with SF and 35 with infections by one of the members of the typhus group. The antibody titers were higher with SF antigen than RP antigen in 72% of the SF patients, and in only two SF patients was the RP titer higher, and then by only one tube (twofold dilution). There seemed little advantage in including the RP antigen in the battery of rickettsial antigens. Cross-reaction with at least one of the typhus antigens was observed in the sera from 64% of the SF patients. It was extensive enough to be confusing (within one tube) in 17% with eight units of antigen, but the differentiation was more distinct with two units of antigen. The cross-reaction with typhus antigens was as frequent in children with SF as it was in adults; thus, it is unlikely that these cross-reactions resulted from previous typhus vaccination. The serological differentiation between murine typhus and epidemic typhus was frequently difficult, but the epidemiological background was distinct. Five patients had higher titers to RC antigen, and four of these may possibly have had RC infections.