Figure 2. Investigation of sample cross-contamination at a study site.

The figure shows two examples of results from two Western blot runs that were performed at the central laboratory as part of an investigation of discordant test results. Results from various laboratory tests are shown above the Western blot strips. HIV rapid tests were performed at a laboratory at the study site in Soweto, South Africa using whole blood; N indicates that both rapid tests were non-reactive, R indicates that both rapid tests were reactive. Samples were subsequently processed to produce plasma aliquots for storage which were later shipped to a central laboratory in the United States for analysis. Results from the ARCHITECT Combo HIV Ag/Ab test are shown (COMBO); N indicates that the Combo test was non-reactive, R indicates the Combo test was reactive. Samples were also tested using the Vitros EIA Human Immunodeficiency Virus Type 1 and/or 2 (HIV-1/2) Antibody Detection in Human Serum and Plasma (EIA); N indicates that the EIA test was non-reactive, R indicates the EIA test was reactive. Western blots were interpreted as negative (N) or positive (P) based on the pattern of bands observed. The banding pattern typically varies among different HIV-positive samples. The panel on the left shows that samples 11–15 were likely to have been cross-contaminated by transfer of plasma from sample 10 into those samples during aliquot preparation (sequential unintended transfer of plasma from tube to tube). Similar findings are shown in the panel on the right; samples 17–19 were likely to have been cross-contaminated by transfer of plasma from sample 16 into those samples. Further investigation at the study site confirmed that a technologist working at the study site prepared sample aliquots without changing pipette tips. All of the samples that may have been processed on the days that this technologist was working in the laboratory were excluded from the endpoint analysis.