To the Editor: The Western blot (WB) is the standard supplemental test used in many settings for confirming the presence of HIV antibodies detected on screening tests such as enzyme immunoassay (EIA) and particle agglutination (PA) (1-3). However, WB is often not used in developing countries because of its high cost and technical complexity. Recently, the World Health Organization (WHO) proposed alternative HIV testing strategies that do not require WB testing (4). One of these proposed strategies (strategy II) uses a second screening assay instead of WB as supplemental HIV test for all samples reactive on an initial screening test to establish HIV seropositivity in asymptomatic persons in areas with HIV prevalence >10% and in patients with suspected HIV-related disease. We evaluated the effectiveness of HIV testing strategy II at Bamrasnaradura Infectious Disease Hospital (BIH), a public tertiary care center in a Bangkok suburb serving HIV/AIDS patients as well as general medical and infectious disease patients.
In December 1993, we established a database on HIV/AIDS patients attending BIH to monitor trends in HIV infection among admitted patients and clinical management of AIDS patients (5). As part of this project, all adult patients admitted to nonsurgical wards received pretest counseling and were offered voluntary HIV testing with written informed consent. Posttest counseling was offered to all consenting patients. Specimens from patients were screened for HIV antibodies by PA testing (SERODIA-HIV, Fujirebio Inc., Tokyo, Japan) at the BIH laboratory. All sera reactive by PA were retested by EIA (Genetic Systems HIV-1/HIV-2 EIA, Genetic System Corporation, Redmond, WA, U.S.A.) and WB (Novapath HIV-1 Immunoblot, Bio-Rad Laboratories, Hercules, CA, U.S.A.) at the HIV/AIDS Collaboration laboratory. A specimen was interpreted as WB-positive when any two of the following bands were present: p24, gp41, and gp120/gp160 (6). Thus, data from this study provided an opportunity to evaluate WHO HIV testing strategy II under field conditions.
From December 1993 though May 1995, 4,818 serum specimens from patients admitted to BIH were screened for HIV antibody by PA; the 1,173 (24.3%) reactive specimens were tested by EIA and WB (Fig. 1). Of these, 1,170 specimens (99.7%) were reactive and three were nonreactive by EIA. Among 1,170 EIA-reactive specimens, 1,169 (99.9%) were positive by WB. The three EIA-nonreactive specimens and one EIA-reactive specimen were indeterminate on WB testing. Thus, the positive predictive value of a testing strategy using EIA as the supplemental test was 99.9% [1,169 of 1,170, exact 95% confidence interval (CI) = 99.5-100], and the positive predictive value of PA testing alone was 99.7% (1,169 of 1,173, exact 95% CI = 99.1-99.9).
The near 100% positive predictive value of PA and EIA testing lends support to the practicality of the WHO alternative testing strategy for confirming HIV seropositivity among hospital patients with suspected HIV-related disease in Thailand and in other countries with a comparable HIV/AIDS situation. However, studies that define the sensitivity and the negative predictive value remain necessary to assess the utility of this algorithm either to screen blood donors or to assess patients with possible HIV infection.
*Amphorn Ittiravivongs; †Sirirat Likanonsakul; *‡Timothy D. Mastro; †Somsit Tansuphasawadikul; *‡Nancy Young; *Thananda Naiwatanakul; *§Dwip Kitayaporn; *Khanchit Limpakarnjanarat
*HIV/AIDS Collaboration; Nonthaburi, Thailand
†Bamrasnaradura Infectious Disease Hospital; Department of Communicable Disease Control; Ministry of Public Health; Nonthaburi, Thailand
‡Centers for Disease Control and Prevention; Atlanta, Georgia, U.S.A.
§Faculty of Tropical Medicine, Mahidol University; Bangkok, Thailand
The use of trade names and commercial sources is for identification only and does not imply endorsement by the Public Health Service or the U.S. Department of Health and Human Services.
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