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BACKGROUND: Rubella was declared eliminated in the United States in 2004; however, the possibility of importation requires a surveillance system for the timely detection and control of rubella and congenital rubella, a serious condition that may result from rubella infection during pregnancy. Prenatal rubella IgG testing for immunity is the standard of care. Rubella is characterized by a generalized rash, lymphadenopathy, and arthralgia, although 50% of cases may be asymptomatic. In New York City (NYC), laboratories with test results consistent with rubella infection (e.g., positive IgM) and providers who suspect rubella infection clinically are required to report to the Department of Health and Mental Hygiene (DOHMH). Reports are investigated and recorded in an electronic surveillance database. Positive IgM results from commercial laboratories or from the NYC DOHMH Public Health Laboratory in symptomatic persons or pregnant women without evidence of immunity are retested at the Centers for Disease Control and Prevention. This surveillance system was evaluated based on validity, data quality, timeliness, and usefulness.
METHODS: The evaluation included all rubella reports to the DOHMH in 2012 and 2013. Reports were classified as confirmed or discarded according to CSTE criteria. To assess validity, the positive predictive value (PPV) was calculated for all reports to the DOHMH and separately by testing purpose. Data quality was assessed by completeness of key variables. Timeliness was measured by the days between specimen collection for serologic testing and case classification. Usefulness was determined by the ability to lead an effective response for confirmed cases.
RESULTS: A total of 215 rubella reports were received by the DOHMH, of which two were confirmed (PPV=1%). Both were symptomatic and internationally imported. Of all reports with IgM testing performed, 53(25%, PPV=5%) were tested for diagnostic purposes, 149(69%, PPV=0%) were inappropriately tested for immunity, and 13(6%, PPV=0%) did not specify the test purpose. Data completeness ranged from 93%-100%. Mean time from specimen collection to classification was 21 days. The surveillance system detected confirmed cases, enabling appropriate control measures, and no known secondary cases occurred.
CONCLUSIONS: The rubella surveillance system in NYC was useful for detecting confirmed cases and implementation of control measures. Data quality was high; however, the PPV of reports to the DOHMH was low and the majority of IgM test results were false-positives. Improvements in PPV and reduction in unnecessary testing could be achieved if IgM tests are utilized only when a clinically compatible diagnosis of rubella infection is considered.