Rubella Surveillance and Diagnostic Testing among a Low-Prevalence Population, New York City, 2012-2013

Abstract

The New York City Department of Health and Mental Hygiene (DOHMH) receives clinical and laboratory reports for rubella. Because rubella immunoglobulin M (IgM) assays may produce false-positive results and rubella infections may be asymptomatic, interpretation of positive IgM results can be challenging. Rubella reports received by DOHMH in 2012 to 2013 were reviewed. The rubella IgM testing purpose was determined through case investigation. Results of IgM testing by indirect enzyme-linked immunosorbent assay (ELISA) and capture enzyme immunoassay (EIA) were compared to determine positive predictive value (PPV) and specificity. DOHMH received 199 rubella reports; 2 were true cases. Of all reports, 77.9% were tested for rubella IgM erroneously, 19.6% were tested for diagnostic purposes, 2.0% had unknown test purpose, and 0.5% were not tested. PPV of indirect ELISA was 6% overall, 14% for diagnostic tests, and 0% for tests ordered erroneously. PPV of capture EIA was 29% overall, 50% for diagnostic tests, and 0% for tests ordered erroneously. Overall, specificity was 52% for indirect ELISA and 85% for capture EIA. Limiting rubella IgM testing to patients for whom rubella diagnosis is suspected and using a more specific IgM assay have the potential to reduce false-positive rubella IgM results.

Keywords: immunoassays; immunoglobulin M; rubella; serology; surveillance.

FIG 1

Classification status and rubella IgM testing by New York City Department of Health and Mental Hygiene's Public Health Laboratory and Centers for Disease Control and Prevention Laboratory, 2012–2013. Abbreviations: CDC, Centers for Disease Control and Prevention Laboratory; DOHMH, Department of Health and Mental Hygiene; IgM, immunoglobulin M; NYC, New York City. *, includes the following: no laboratory testing, n = 1; tested by CDC without DOHMH testing, n = 2; tested by labs other than DOHMH or CDC, n = 97; **, categories not mutually exclusive; ***, includes one individual who was unable to be reached to determine international travel history; ****, one of two was reverse transcriptase PCR positive.