FOLLOW-UP OF HIV WESTERN-BLOT INDETERMINATE RESULTS

Indeterminate results obtained with the Western blot (WB) confirmatory test on HIV enzyme immunoassay (EIA)-positive samples, constituted 15 .6% (444/2849) over a 2.5-year period at the referral laboratory for t he Western region at the King Fahd General Hospital, Jeddah. Two hundr ed and fourteen WB-indeterminate samples were followed up by repeat WB testing of subsequent samples from the same patients over a 3-12 mont h period. One hundred and forty-two samples (66.4%) gave negative resu lts. Sixty-five samples (30%) remained indeterminate. Only seven sampl es (3.3%) not initially meeting WHO criteria for positivity turned cle ar-cut positive, with high EIA readings on follow-up. It was discovere d initially that a significant proportion of indeterminates was due to low-grade cross-contamination between samples as a result of aerosol backflow during aspiration in the washing procedure. This was eliminat ed by rinsing the lines between samples, separating samples with high EIA from those with low EIA, and rerunning indeterminate samples. A re duction of indeterminates from 21% to 8.5% subsequently followed. Afte r this improvement, most of the samples that remained indeterminate ha d low EIA readings, and few bands of mainly anti-gag (p55, p24 or p18) or anti-pol (p51) antibodies, while the few turning positive all had anti-gp160, in addition to anti-p24 or p55. Interestingly, over the la st year and a half of the study, 1.4% of the total samples (21/1506) h ad repeatedly high ETA readings but were negative by WB. In addition, 16 samples (1.1%) were positive for HIV-2. A separate computer-based s ystem for the storage of data was very helpful in ascertaining proper follow-up of indeterminate WB results. .