COMPARISON OF RUBELLA IMMUNOASSAYS SAUTTER ET AL. TABLE I. Performance characteristics (%) of the 5 methods for determining rubella immune status Method IMx FlAX Rubascan Rubalex Rubazyme IMx Negative Positive Positive Negative Positive Positive Negative Positive Sensitivity 96 (97/101) 90 (89/99)* 100 (101/101) 99 (100/101) 100 (101/101) Specificity 97 (86/89) 100 (89/89) 98 (87/89) 97 (86/89) 99 (88/89) PP 96 (97/101) 100 (89/89) 98 (101/103) 97 (100/103) 99 (101/102) aTwo specimens gave equivocal results using the FlAX method and were elminated from the comparison. *P 0.0008. TABLE 2. Samples with discrepant results FlAX Negative Negative Equivocal Negative Negative Positive Negative Negative Rubascan Positive Positive Positive Positive Negative Positive Negative Negative Rubalex Positive Positive Positive Positive Negative Negative Positive Negative Rubazyme Negative Positive Positive Positive Negative Positive Negative Positive NP 96 (86/90) 90 (89/99) 100 (87/87) 99 (86/87) 100 (88/88) Final interpretation Negative Positive Positive Positive Negative Positive Negative Negative Accuracy 96 (183/190) 95 (I 78/188) 99 (188/190) 98 (186/190) 99 (189/190) No. of samples 2 4 2 4 2 Enzyme Immunoassay (EIA) EIA was performed using Rubazyme (Abbott Di- agnostic Laboratories) kits supplied by the manu- facturer. The method was performed and the results were interpreted according to the manu- facturer’s instructions and have been descr’bed pre- viously, s Each serum sample was tested in dupli- cate. Comparison of Results and Statistical Analysis Specimens were considered positive or negative for rubella antibody when 3 or more methods were in agreement. Each assay method was compared to the "consensus" result in order to determine sensitivity, specificity, accuracy, and predictive values of the method. Statistical analysis and evaluation of the 5 methods were made using the chi-square test (StatView + Graphics, Abacus Concepts, Inc., Berkeley, CA). RESULTS Using a consensus of 3 or more methods, we found a total of 101 specimens to be positive for rubella antibody and 89 specimens to be negative. Perfor- mance characteristics of the methods for the detec- tion of rubella antibody can be seen in Table 1. 190 INFECTIOUS DISEASES IN OBSTETRICS AND GYNECOLOGY The FIAX, with a sensitivity of 90%, was the least sensitive of the methods evaluated (P 0.0008). The other 4 methods were compa- rable in sensitivity, which ranged from 96% to 100%. The specificity, positive predictive value (PPV), negative predictive value (NPV), and accu- racy of the 5 methods were as follows: 97%, 96%, 96%, 96% for IMx; 100%, 100%, 90%, 95% for FIAX; 98%, 98%, 100%, 99% for Rubascan; 97%, 97%, 99%, 98% for Rubalex; and 99%, 99%, 100%, 99% for Rubazyme. There was no statisti- cally significant difference in specificity in the 5 methods (P 0.3158). Data on samples giving discrepant results are found in Table 2. There were 17 samples that gave discrepant results in assays. Four samples were neg- ative, and 2 were equivocal (even on repeat testing) by the FIAX method but positive by the other 4 methods. Four additional samples were negative by FIAX and IMx but positive by all other methods. The other 7 discrepancies were distributed among the other assays. DISCUSSION The Advisory Committee for Immunization Prac- tices of the Center for Disease Control, Atlanta,