Background:
The Focus HerpesSelect 2 ELISA IgG test detects human IgG antibodies to HSV-2 in human sera. An Index Value cut-off 1.1 determines positive results and may increase false positives in developing countries

Objective:
To evaluate the Focus® cut-off used to test a population in India, and estimate false positives.

Method:
Sera from 2,186 participants of The NIMH Collaborative HIV/STD Prevention Trial in Chennai, India were obtained from men and women. Samples were collected every12-months and tested using the Focus Kit. Index Values > 1.1 were positive for HSV-2; Index Values of 0.9 -1.1 were indeterminate, and index values < 0.9 were negative. Data comparing the Index Values at visits 1 and 2 were evaluated at the Johns Hopkins Reference Lab.

Result:
Of 2,186 results, 1,480 (67.6%) tested negative at both visits, and 511 (23.4%) tested positive at both visits. Of the 511 samples, 103 (20.1%) index values were < 3.5, and 408 (79.8%) were >3.5. Of the 2,186 participants, 86 (3.93%) tested positive at visit 1, and negative/indeterminate at visit 2. Of these 86 samples, 75 (87.2%) had Index Values < 3.5, and 11(12.8%) were > 3.5 at visit 1. Of 2,186 there were 32 (1.46%) negative/indeterminate results at visit 1 that tested positive at visit 2. Of the 32 results, 12 (37.5%) had Index Values > 3.5 at visit 2, possible seroconverters, whereas the remaining 20 (62.5%) had Index Values <3.5 at visit 2, possible false positives.

Conclusion:
Using 1.1 as a cut-off may increase the number of false positive results, and seroreverters in the Chennai population. Further evaluation is warranted, to determine if the cut-off is too low, estimate the possible effects on seroreversion/false positives, and confirm testing by Western Blot.

Implications:
When kit performance is as expected, an adjusted cut-off comparative to the population tested may increase testing accuracy, and PPV.