Date Published: January 27, 2017
Publisher: Public Library of Science
Author(s): Géraldine Daneau, Said Aboud, Irena Prat, Willy Urassa, Luc Kestens, Carlo Federico Perno.
CD4 T-cell counts are widely used to assess treatment eligibility and to follow-up HIV-infected patients. The World Health Organization prequalification of in vitro diagnostics program conducted a performance evaluation of the FACSPresto (BD Biosciences), a new point-of-care instrument to measure absolute CD4-T cell (CD4) counts and percentages in venous and capillary blood samples from HIV-infected patients.
Patients were recruited in Belgium (200 patients) and in Tanzania (247 patients). Venous blood samples were analyzed in two nearby reference laboratories. In addition, nurses/technicians collected a capillary blood sample by finger prick directly into a FACSPresto CD4 cartridge. Assay precision was assessed on fresh blood and on external quality control samples. Trueness (bias) was assessed by comparing results from FACSPresto with the reference (single-platform FACSCalibur). Clinical misclassification was measured at 200, 350 and 500 cells/μL thresholds.
Intra-assay precision was < 6%, and inter-assay < 8%. CD4 results from FACSPresto and reference method resulted in regression slopes of 0.99–1.11 using either venous or capillary blood. Correlation was better for venous than for capillary blood (minimum 0.97 vs 0.93 respectively). Capillary blood resulted in a larger bias than venous blood, with 24 and 83 cells/μL for absolute CD4 counts on capillary blood in Antwerp and Dar es Salaam respectively, vs 12 and 41 cells/μL on venous blood. Bias on CD4% was < 1% on both venous and capillary blood, and was proportionally better than for absolute CD4 counts. Clinical misclassification was in line with the average overestimation, showing a very good specificity, but sensitivity around 70–90%. The rejection rate was 11% on first reading, leading to 6% of all samples without final result after a second reading. The FACSPresto performed very well on venous blood samples, and well on capillary blood samples.
In 2014, there were about 36.9 million people living with HIV . Of these, 32.5 million were eligible for antiretroviral treatment (ART) but, in low- and middle-income countries, only 36% received treatment . ART initiation was for long based on absolute CD4 T-cell counts (CD4 count) apart from clinical examination. Currently, the World Health Organization (WHO) recommends starting treatment for all HIV-infected patients irrespective of CD4 count levels, with priority for patients with severe or advanced HIV clinical disease (WHO clinical stage 3 or 4) and adults with less than 350 cells/μL . The previous guidelines mentioned a threshold of 500 cells/μL, which could be seen as an intermediary step towards treatment for all in settings where the threshold of 350 cells/μL to initiate ART is still applied . Even when the importance of CD4 counts to monitor ART is decreasing, it is still recommended to monitor CD4 counts at regular time intervals until stable viral load suppression is achieved [5;6]. In addition, as long as viral load testing is difficult due to technical and financial constraints, it is expected that low-income regions will continue using CD4 counts as an alternative . CD4 counts are also used to decide on prophylaxis against opportunistic infections, for instance to screen for cryptococcal infection in patients with ≤ 100 CD4 cells/μL and stop prophylaxis when patient presents with > 200 cells/μL, or start co-trimoxazole in case CD4 counts are ≤ 350 cells/μL . The threshold of 200 cells/μL is also important to decide on vaccination benefits and risks .
The performance evaluation of the FACSPresto was conducted simultaneously in two complementary sites selected by WHO: one in Antwerp, Belgium, and the other in Dar es Salaam, Tanzania. Both study sites collected venous and capillary blood samples from each patient, which were analysed in the respective reference labs, with high quality assurance to ensure good working conditions.