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 the Western region at the King Fahd General Hospital, Jeddah. Two hundred and fourteen WB-indeterminate samples were followed up by repeat WB testing of subsequent samples from the same patients over a 3-12 month period. One hundred and forty-two samples (66.4%) gave negative results. Sixty-five samples (30%) remained indeterminate. Only seven samples (3.3%) not initially meeting WHO criteria for positivity turned clear-cut positive, with high EIA readings on follow-up. It was discovered initally 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 eliminated by rinsing the lines between samples, separating samples with high EIA from those with low EIA, and rerunning indeterminate samples. A reduction of indeterminates from 21% to 8.5% subsequently followed. After this improvement, most of the samples that remained indeterminate had 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 last year and a half of the study, 1.4% of the total samples (21/1506) had repeatedly high EIA readings but were negative by WB. In addition, 16 samples (1.1%) were positive for HIV-2. A separate computer-based system for the storage of data was very helpful in ascertaining proper follow-up of indeterminate WB results.