Performance characteristics of STANDARD Q Filariasis Antigen test (QFAT) to detect filarial antigens of Wuchereria bancrofti in the field.

Background: Mapping, monitoring, and evaluation of the Global Programme to Eliminate Lymphatic Filariasis (GPELF) rely on high-throughput diagnostics. While the WHO-recommended Filariasis Test Strip (FTS) is widely used to evaluate the programme, its use is limited by some technical and operational issues. We evaluated the performance characteristics of Q Filariasis Antigen Test (QFAT) compared to FTS for detecting Wuchereria bancrofti filarial antigen in the field.

Methods: The QFAT and FTS kits were tested simultaneously for circulating filarial antigen (CFA) during an epidemiological monitoring survey (EMS) in two blocks of a filariasis endemic district in Karnataka, India, as a part of evaluation of the filariasis elimination programme with three drugs (Ivermectin, Diethylcarbamazine, and Albendazole-IDA). Blocks are considered as the evaluation unit as per the revised national guidelines. Two sentinel and one random site from each block with a sample size of 300 individuals aged ≥20 years were selected for the EMS. The field evaluation of the new kit was carried out in the four sentinel sites. Positive tests with either FTS or QFAT or both were tested for microfilaria (Mf) using night blood samples. The performance of the tests was compared in terms of sensitivity, specificity, and predictive values. The percentage agreement between the tests was verified using Cohen's kappa statistics (k), with a P value of less than 0.05 indicating statistical significance.

Findings: A total of 1227 individuals were tested for CFA using both the QFAT and FTS tests. The QFAT test detected 299 positive individuals at the end of 10 minutes, while the FTS detected 310 positives. The QFAT showed high sensitivity (95.5%), specificity (99.7%), positive predictive value (99.0%), and negative predictive value (98.5%), and the results were in near perfect agreement with those of the FTS (k = 0.97, P <0.001) when the results were read at 10 minutes. There were 17 discordant results that were positive according to either one of the tests. Both antigen tests were positive for all 68 microfilaria-positive samples. None of the QFAT tests were invalid, while three FTS tests were invalid due to non-flow on the test pad. There was no cross-reactivity of the QFAT with Brugia malayi-positive samples (n = 5). The feedback from the technicians indicates that QFAT tests were easier to perform compared to FTS in the field.

Conclusions: The Q filariasis antigen test is a promising tool for detecting the Wuchereria bancrofti antigen. The kits may be further validated for the review of Diagnostic Technical Advisory Group for Neglected Tropical Diseases (DTAG), to be recommended for the Global Programme to Eliminate Lymphatic Filariasis (GPELF).