Evaluation of latex-Rickettsia rickettsii test for Rocky Mountain spotted fever in 11 laboratories.

A latex-Rickettsia rickettsii test for detection of antibodies to Rocky Mountain spotted fever (RMSF) was evaluated during the 1980 RMSF season in 11 laboratories in nine states where the disease is endemic. In a double-blind study, all sera submitted to each laboratory for RMSF testing were also examined by the latex-R. rickettsii test. A portion of each specimen was then sent to the New York State laboratory for testing by latex-R. rickettsii and by the reference microimmunofluorescence test. Results were exchanged at the end of the examination period. At the usual ratio of reactive to nonreactive sera encountered in a diagnostic laboratory on a day-to-day basis, the efficiency of the latex-R. rickettsii test relative to microimmunofluorescence was 96.79% for New York and 93.30% for the collaborating laboratories. Both the latex and microimmunofluorescence tests detected antibodies to RMSF within 7 to 9 days of onset. With the latex-R. rickettsii test--but not necessarily with microimmunofluorescence--a high titer (greater than or equal to 128) on a single serum was diagnostic of active RMSF. Changes in serum titer for patients with multiple sera were similar for both tests. The test detects rickettsial antibodies in patients with active infection, but in most cases it does not detect antibody in patients with past infection. Test reactivity could not be uniquely linked to a particular immunoglobulin class.