BACKGROUND: Georgia Department of Public Health (GDPH) has been conducting surveillance for Zika virus since January 2016. The Georgia Public Health Laboratory (GPHL) has capacity to perform RT-PCR and MAC-ELISA (IgM) testing for Zika. Positive, equivocal, or inconclusive IgM results are not confirmatory for Zika infection due to cross-reactivity with other flaviviruses. Plaque Reduction Neutralization Test (PRNT), performed at the Centers for Disease Control and Prevention (CDC), is then required to confirm Zika virus infection. It can take weeks for GDPH to receive PRNT results, delaying notification and response to confirmed cases. Analysis of PRNT result data to determine the relationship between IgM results and subsequent confirmation of Zika virus infection may be helpful to inform management of suspect cases while PRNT is pending.
METHODS: Lab results on Georgia residents tested for Zika virus are recorded in the Zika Active Monitoring System (ZAMS) within the State Electronic Notifiable Disease Surveillance System (SendSS). PRNT results and their corresponding IgM results were compiled using both electronic and paper lab reports and analyzed using statistical software.
RESULTS: As of December 20, 2016, PRNT results have been received on 110 GA patients originally found to be positive, equivocal, or inconclusive by IgM testing. Of 62 positive IgM samples, 45 were positive for Zika by PRNT (72.6%), 9 were negative for any flavivirus (14.5%), and 8 were positive for dengue only (12.9%). Of 21 equivocal IgM samples, 8 were positive for Zika (38.1%), 8 were negative for any flavivirus (38.1%), and 5 were positive for dengue only (23.8%). Of 27 inconclusive IgM samples, 1 was positive for Zika (3.7%) and 26 were negative for any flavivirus (96.3%).
CONCLUSIONS: Analysis of available data in Georgia show that IgM-positive Zika results are often indicative of true Zika infections as opposed to inconclusive IgM results, which are largely negative for any flavivirus. IgM-equivocal Zika results, however, lead to a roughly even distribution of true Zika and/or dengue infections and no flavivirus infection. The differences in confirmed infections obtained from inconclusive and equivocal IgM results align with our understanding of inconclusive vs. equivocal IgM results as defined by the CDC. This information has helped GDPH epidemiologists prioritize pending patients for additional follow up and provide guidance for physician and patient education. Updating this analysis regularly is critical to relaying the most accurate information to healthcare providers and patients.