False-positive results in serologic tests for Rocky Mountain spotted fever during pregnancy.

The data from this study demonstrate that false-positive results from tests for Rocky Mountain spotted fever increase with the duration of pregnancy. The sera of 4.0% (2/50) of women in their first trimester of pregnancy, 10.9% (5/46) in their second trimester, and 12.1% (12/99) in their third trimester yielded false-positive results from latex agglutination assays for Rickettsia rickettsii infections. The cause of these false-positive results was not determined by this study. These false-positive titers were not associated with clinical findings or other laboratory abnormalities. Even though these sera did not contain antibodies to R rickettsii detectable by indirect fluorescent antibody testing, such false-positive results from serologic tests for Rocky Mountain spotted fever can obscure the patient's correct diagnosis. This may lead to the unnecessary use of potentially toxic antibodies and prevent initiation of appropriate therapy.