ObjectiveTo develop and evaluate a simple V3 peptide-based enzyme immunoassay (EIA) for large-scale serotyping of HIV-1 specimens from Thailand.DesignSerologic reactivities with synthetic peptides derived from the V3 loop of gp120 were used for typing HIV-1 specimens.MethodsSynthetic peptides PND-A and PND-B, derived from the consensus amino-acid sequences of the V3 loop of gp120 from two major genomic variants of HIV-1 in Thailand (A and B), were evaluated in an EIA on 61 Thai HIV-1 sera for which genotypes had been determined by polymerase chain reaction. The peptide EIA was then applied to sera from 188 HIV-1-infected patients, selected in non-random, convenience samples of known risk groups from four geographic regions of Thailand.ResultsThe sensitivities and specificities of PND-A and PND-B were 86% (30 out of 35) and 96% (25 out of 26) and 92% (24 out of 26) and 94% (33 out of 35), respectively, with 100% predictive values of a monoreactive positive test for both peptides. The assay classified 101 specimens as serotype A, 39 as serotype B, eight as serotype AB (dually reactive), and 40 as untypable (non-reactive). Excluding dual reactors and non-reactors, 92% (77 out of 84) of specimens from patients probably infected by sexual contact were serotype A; conversely, 76% (28 out of 37) of injecting drug users were serotype B.ConclusionThe serologic results corroborated previous findings, in a smaller subset of samples, of an apparent segregation of viral subtypes by mode of transmission, suggesting two separate HIV-1 epidemics in Thailand. This peptide EIA could be a valuable epidemiologic tool in determining the dynamics of the rapid spread of HIV-1 in Thailand.