Follow-up of pregnant women exposed to chicken pox: an audit of relationship between level of antibody and development of chicken pox.

The purpose of this study was to validate through natural exposure a cut-off level of varicella zoster IgG as protective against infection with varicella zoster virus (VZV). Laboratory testing to determine VZV immune status of pregnant women exposed to varicella is recommended. Quantitative assays are now available which are sensitive and specific. More than 200 consecutive requests for screening in pregnant patients with recent varicella contacts were followed-up by questionnaire. DiaSorin LIAISON and VZV time resolved fluorescence immuno assay (VZV TRFIA) were used to measure VZV antibody level. One hundred fifty out of 209 (72%) questionnaires were returned; 14 patients developed varicella, 129 did not and seven were not known. Patients who had been given VZIG and developed varicella on follow-up had a mean antibody level before VZIG of 28 mIU/ml and 62 mIU/ml, by LIAISON and TRFIA, respectively. The mean IgG level of those that did not develop varicella was 885 and 866 mIU/ml by LIAISON and TRFIA, respectively. Those with levels <100 mIU/ml were more likely to develop chicken pox than those with levels >100 mIU/ml (relative risk of 10.4 for LIAISON and 8.8 for TRFIA). On the basis of the relatively small numbers in this study, quantitative assays, using a 100mIU/ml cut-off, can differentiate between those who are susceptible and those who are protected against exposure, however follow-up studies should include sampling for VZV DNA and IgM.