Fig 1.
(a) 1181 consecutive routine diagnostic samples (serum or plasma) tested on the Siemens HIV 4th generation platform at the primary testing laboratories were re-tested on the Roche platform at the referral laboratory UN (using the same primary tube). Inconclusive specimens were further investigated with HIV viral load and western blot where specimen volume permitted. A subset of 26 specimens which were screen negative, second test positive was retested on the Siemens (initial screening assay) at the JE laboratory. (b) 210 known HIV antibody negative, NAT negative specimens from the same donor were tested on the Siemens and Roche platforms at JE and UN respectively. 105 specimens were tested on each analyser over a 10 day period and handled as normal clinical specimens. After the initial HIV serology test, the specimens were sent through the same analyser and tested for a second time.
Fig 2.
Determining sero-positivity at first and second testing episodes.
The number of negative results decreased and the number of positive results increased on second testing on the Roche analyser at UN laboratory.
Fig 3.
S/CO values of the positive test in S+/R- and S-/R+ discordant samples were generally close to the cut-off (weakly reactive).
(a) S-/R+ samples had S/CO values between 1.0 and 99.1. (The positive range of Roche assay is 1->1000); (b) S+/R- samples had S/CO values between 1.1 and 5.8 (the positive range of Siemens assay is 1->12).
Fig 4.
Western blots of selected specimens: negative, positive and low positive controls are labelled N, P and LP respectively.
Specimens of interest are labelled A to G. Specimens A, B and C were concordantly weakly positive on both serology assays and HIV viral loads were high. Their western blots were scored as positive (A) and indeterminate (B, C). Specimens D, E, F and G were also WB indeterminate, but RNA negative. A follow up specimen on D was antibody negative.
Table 1.
Characteristics of the RNA positive samples.
Table 2.
S/CO values of false positive samples from Siemens analyser at JE laboratory.