Confirmatory serological testing of blood donors positive on TPHA screening in Singapore

Seventy-two blood donors who were tested positive by the Singapore Blood Transfusion Service (SBTS) for Treponema pallidum haemagglutination (TPHA) test, were evaluated at the Department of Sexually Transmitted Diseases Clinic (DSC) between November 1994 to December 1996. All underwent syphilis serological testing, including rapid plasma reagin test (RPR), TPHA test and fluorescent treponemal antibody-absorption (FTA-Abs) test. All except one (98.6%) were confirmed TPHA positive by the DSC. Of the 71 TPHA-confirmed-positive donors, 53 (74.6%) were subsequently tested positive for FTA-Abs and 18 (25.4%) were tested negative for FTA-Abs. Twenty-two (31%) of the 71 TPHA-positive blood donors had reactive RPR and 49 (69%) had non-reactive RPR. Of the 22 TPHA-positive donors who had reactive RPR, 19 (86%) had positive FTA-Abs (13 late latent syphilis, 4 serological scar, one late congenital syphilis, one secondary syphilis), and 3 (14%) had negative FTA-Abs (all late latent syphilis). Of the 49 TPHA-positive donors who had non-reactive RPR, 34 (69%) had positive FTA-Abs (24 late latent syphilis, 9 serological scar, one late congenital syphilis) and 15 (31%) had negative FTA-Abs (12 late latent syphilis, 2 serological scar, one false-positive TPHA). Only one TPHA-positive donor referred by the SBTS subsequently turned out to have negative syphilis serology at the DSC. Overall, 68 (95.8%) TPHA-positive donors who had a past history of sexual exposure were managed as treated or untreated syphilis, regardless of their RPR or FTA-Abs results. However, FTA-Abs was found to be useful in the management of 3 (4.2%) TPHA-positive blood donors in the absence of a history of sexual exposures.