Background:  Most Africans are not monitored for viral response to ART because of the cost and difficulty of providing an HIV RNA polymerase chain reaction (PCR) service in resource-limited settings. Filter paper transfer (FPT) of dried whole blood or plasma specimens to well-supported central laboratories has been proposed. We evaluated FPT in Ugandans who were established on ART, in a busy urban clinic in Kampala.

Methods:  This was a cross-sectional study of 402 patients on ART for a median duration of 11 months. “Gold standard” viral load testing was performed locally on liquid plasma using standard reverse transcriptase (RT) polymerase chain reaction (PCR) assay compared with results obtained after FPT of both whole blood and plasma to Europe and real time PCR (RT-PCR). Plasma for gold standard and plasma was separated from an EDTA sample by centrifuge within 6 hours. Whole blood and plasma were spotted onto filter paper, air dried, and stored in sealed envelopes at ambient temperature. Samples were sent to Europe fortnightly for RNA extraction and quantification. Plasma specimens were collected only after an interim analysis of early whole blood results.

Results:  Including 306 whole blood and 218 plasma (122 whole blood/ plasma pairs), 402 patient samples underwent both gold standard and FPT. By gold standard, 39 of the 402 (9.7%) patients had detectable viral loads (>500 copies/mL). Whole blood yielded 4 false negative results (all <2000 copies/mL) and 64 false positives (median 1002 copies/mL; range 510 to 3510). Whole blood had a sensitivity, specificity, positive predictive value, and negative predictive value of 0.86 (0.67 to 0.96), 0.77 (0.71 to 0.81), 0.27 (0.18 to 0.46), and 0.98 (0.95 to 1.00), respectively. McNemar’s test showed a significant difference between gold standard and whole blood. Plasma yielded 1 false positive (593 copies/mL) and no false negative results. Sensitivity, specificity, positive predictive value, and negative predictive values for plasma were 1.00 (0.84 to 1.00), 0.99 (0.97 to 1.00), 0.95 (0.77 to 1.00), and 1.00 (0.77 to 1.00), respectively. McNemar’s test showed no significant difference between plasma and gold standard. Bland Altman plots confirmed the high number of low-level false positives for whole blood, but demonstrated that there is good agreement between whole blood and gold standard and plasma and gold standard for high (>5000 copies/mL) viral loads (within 2 SD).

Conclusions:  FPT of plasma specimens for RNA RT-PCR may provide a practical means of monitoring the viral loads of Africans taking ART. FPT using whole blood would not require venesection or centrifuge equipment. Unfortunately whole blood is associated with a high false positive rate. This may be due to the detection of cell-associated HIV DNA by the RT-PCR assays.