Field evaluation of Standard Q Filariasis Antigen Test for Lymphatic Filariasis (LF) during a pre-transmission assessment survey in Sierra Leone, 2022
Authors: Dembele Benoit , Bah Mohamed Salieu , Conteh Abdulai , Kamara Habib , Kargbo-Labour Ibrahim , Souza Ashley , Houck Patricia , Mensah Ernest O. , Turay Victoria , Elder E. Scott , Gass Katherine , Reid Steven D , Shott Joseph P. , Zhang Yaobi , Won Kimberly Y. , Weaver Angela 
Published: December 2024

Background

As part of a multi-country evaluation, the SD Biosensor STANDARD™ Q Filariasis Ag Test (QFAT) was compared with the Abbott Bioline TM Filariasis Test Strip (FTS) for classifying lymphatic filariasis (LF) prevalence at a population level and for ease of use in field conditions in Sierra Leone.

Methods and principal findings

The evaluation was done in two districts, Bombali and Karene, where repeat pre-transmission assessment surveys (pre-TAS) were planned. Two sites with high LF antigen prevalence in 2020 (4.1% in the village of Kagbo and 7.7% in the village of Makorba Yelimi) were chosen. Convenience sampling was used to recruit 350 community members ≥5 years in each site. Blood was collected by fingerstick (20μl for QFAT and 75 μl for FTS). The reading time for both tests was 10 minutes. For all positive or invalid results, a repeat test was performed for both tests. In total, 728 participants (5 - 91 years) were tested by QFAT and FTS. The positive rate was 4.8% (17/357) and 3.5% (13/367) for FTS and 3.4% (12/357) and 4.1% (15/367) for QFAT in Kagbo and Makorba Yelimi, respectively. All participants testing positive for FTS or QFAT underwent further testing by night blood smear to detect microfilariae using microscopy. None of the positive participants had circulating microfilariae. Nearly half (14/30) of those who tested positive with FTS during this survey also tested positive with FTS in re-pre-TAS in 2020. Four FTS and three QFAT samples were indeterminate (meaning a positive result followed by a negative result). In field conditions, QFAT was easy to handle and recorded zero invalid tests compared to FTS (six invalids). Using the FTS results as a reference standard, the sensitivity and specificity of the QFAT was 78.6% and 99.4% respectively. The concordance between FTS and QFAT was 0.81 (Cohen’s Kappa). The discrepancy found between the two tests in terms of positive tests was not statistically significant (p=0.78).

Conclusions / significance

The results suggest that the QFAT is a credible LF diagnostic test when compared to the routinely used FTS; use of either test would result in the same program decision.

Authors summary

Lymphatic filariasis (LF) is a vector-borne disease targeted for elimination as a public health problem by 2030. The Global Program to Eliminate LF recommends tools to measure the impact of interventions and to achieve elimination. A reliable and easy to use diagnostic tool is key for the success of the global program. Currently only one rapid antigen test is used for programs inWuchereria bancroftiendemic counties. This study was part of a multi-country field evaluation of the SD Biosensor STANDARD™ Q lateral flow assay rapid diagnostic test. The primary objective was to determine comparability of the SD Biosensor STANDARD™ Q Filariasis Ag Test (QFAT) to the Abbott Bioline™ Filariasis Test Strip (FTS) in its ability to classify LF prevalence at a population level. In addition, information was collected on the utility and ease of use of the QFAT in field settings. The evaluation was done in two districts (Bombali and Karene) in Serra Leone, which were undergoing repeat pre-transmission assessment surveys (pre-TAS). The results of this study confirm the performance of QFAT as a suitable alternative to the currently recommended FTS. In field conditions, using QFAT seems effective given that it records zero invalid tests compared to the FTS (six invalid tests).