A cluster of primary varicella cases among healthcare workers with false-positive varicella zoster virus titers

BACKGROUND: Five cases of primary varicella zoster virus (VZV) were diagnosed among hospital healthcare workers (HCWs). All had complied with a pre-employment VZV screening program and had been considered immune. OBJECTIVES: To summarize the investigation of VZV among un-immunized HCWs and to provide recommendations for avoiding false-positive serologic tests. DESIGN: Risk of transmission of VZV to susceptible HCWs is minimized through serologic screening. Varicella vaccine is recommended for susceptible HCWs. A commercially available latex bead agglutination assay (LA) is widely used because it is rapid and easy to perform. LA was compared with the whole-cell varicella ELISA standardized in the Centers for Disease Control and Prevention (CDC) National Herpes Laboratory. SETTING/POPUIATION: Large inner-city, tertiary care hospital with a diverse employee population. RESULTS: In a year, 5 HCWs presented with laboratory confirmed primary varicella infection. Four had VZV exposures 2 weeks prior to presentation. All had documented positive VZV titers by LA performed at hire. None were offered VZV vaccination. The original LAs were judged false-positives. INTERVENTION/FOLLOW-UP INVESTIGATION: Fifty-three consecutive VZV LA samples from the hospital laboratory were retested at the CDC. Forty-four samples concurred. Of the remaining 9, 4 were positive by hospital LA but negative by CDC IgG ELISA. Four were equivocal by hospital LA but negative by CDC IgG ELISA and IA. One was positive by hospital LA but negative by LA and equivocal by ELISA at the CDC. CONCLUSION: LA may be prone to false-positive results and inappropriate for screening hospital HCWs.