Density-dependent reactivity of the STANDARD Q Filariasis Antigen Test in individuals with Loa loa microfilaraemia in an area with no evidence of lymphatic filariasis in Gabon : a cross-sectional study

Abstract Background Lymphatic filariasis (LF) programmes increasingly rely on the STANDARD Q Filariasis Antigen Test (QFAT) to detect Wuchereria bancrofti circulating filarial antigen. Older antigen tests cross-react in people with heavy Loa loa microfilaraemia, which complicates LF mapping in Central Africa. Field evaluations of QFAT have so far been conducted outside loiasis-endemic areas, and recent confirmatory mapping in Gabon found antigen-positive children without W. bancrofti infection. Methods We performed QFAT on stored plasma samples (− 30°C, one freeze–thaw cycle) collected within the PHYLECOG project from 50 adults with microscopically confirmed L. loa microfilaraemia in the Ntem and Haut-Ntem departments, Woleu-Ntem province, northern Gabon. Microfilarial density was quantified by direct examination and leukoconcentration of venous blood collected between 10:00 and 15:00. We compared density between QFAT-positive and QFAT-negative participants and modelled the probability of a positive test as a function of log-transformed density. Results Twelve of 50 participants (24.0%; 95% confidence interval [CI] 14.3–37.4%) were QFAT-positive. No participant below 3,000 microfilariae (mf)/mL was positive (0/30); positivity was 14.3% (1/7) at 3,000–8,000 mf/mL, 75.0% (6/8) at > 8,000–30,000 mf/mL and 100% (5/5) above 30,000 mf/mL. Eleven of 13 hypermicrofilaraemic participants (≥ 8,000 mf/mL; 84.6%) were QFAT-positive, compared with 1 of 37 (2.7%) below this threshold. Median microfilaraemia was 21,300 mf/mL in QFAT-positive and 600 mf/mL in QFAT-negative participants ( P < 0.001). Each tenfold increase in microfilaraemia was associated with higher odds of a positive QFAT (odds ratio adjusted for age and sex 105; 95% CI 5.2–2,105), and the modelled probability of positivity reached 50% at about 9,800 mf/mL. Microfilaraemia discriminated QFAT status well (area under the curve 0.97; 95% CI 0.92–1.00), and no participant below 4,900 mf/mL was positive. All three participants with Strongyloides stercoralis were QFAT-positive, but all three were also hypermicrofilaraemic. The four participants co-infected with Mansonella perstans were QFAT-negative. Conclusions QFAT reacts in a density-dependent manner in people with L. loa microfilaraemia and is mostly positive in hypermicrofilaraemic individuals, as its predecessors are. Because W. bancrofti was not investigated, these results describe reactivity, not diagnostic accuracy. In loiasis co-endemic areas, QFAT-positive results should be confirmed by nocturnal W. bancrofti microscopy or molecular testing and interpreted alongside daytime L. loa quantification before they inform programme decisions.