Evaluation of Og4C3 antigen ELISA as a tool for detection of bancroftian filariasis under lymphatic filariasis elimination programme.

Diagnosis of Lymphatic Filariasis by microscopic examination of thick blood films (TBF) collected between 8.30 pm to 12 midnight, though highly specific is operationally problematic. We evaluated the TropBio Og4C3 serum ELISA as a tool for detection of W. bancrofti microfilaria carriers using Dried Blood Spots (DBS). The study was carried out in two parts (i) to test the sensitivity and specificity of the ELISA test for detection of circulating filarial antigen (CFA) in microfilaria (Mf) carriers vis-à-vis the conventional thick blood film (TBF) microscopy and its persistence in different categories of individuals during the course of disease viz., Endemic normals (n=51), microfilaria (Mf) carriers (n=27), acute cases (n=27), chronic cases (n=50) and a control group of non-endemic normals (n=48) using sera samples and ii) to study the utility of finger prick Dried Blood Spots (DBS) collected on filter paper for detection of Mf carriers and its comparison with another antigen detection assay, the Immunochromatographic test (ICT). Considering the non-endemic normals and microfilaria carriers, the ELISA test was found to have 100% sensitivity and 94.12% specificity for detection of Mf carriers in sera samples. The CFA was absent in majority of the subjects tested under other categories with a positivity of 7.8% among endemic normals, 11.12% among acute cases, 7.84% among chronic cases and 6.25% among nonendemic normals. Comparison of finger prick DBS and sera samples by ELISA vis-à-vis the ICT, carried out on Mf carriers (n=91) and endemic normals (n=97), showed a positivity of 88 (96.7%) in DBS as against 86 (94.5%) in sera samples and 88 (96.7%) by ICT, amongst Mf carriers, with a statistically significant correlation in antigen units between sera and DBS samples (r = 0.959, p = 0.000) amongst the microfilaria carriers. Out of 97 endemic normals, 19 (19.6%) sera and 17 (17.5%) DBS samples tested positive by ELISA while as 12(12.4%) tested positive by ICT, again with a statistically significant correlation between the antigen units in sera and DBS samples (r = 0.942, p = 0.000). DBS prepared from 25 microl of blood were found to be as sensitive as 50 microl for antigen detection. Antigen positivity detected from DBS collected during day and night from known microfilaria carriers (n=27) showed a statistically insignificant difference (p = 0.125) and a significant correlation in antigen units (r = 0.820 and p = 0.013). In view of the comparable results of ELISA, ICT and TBF microscopy, it is concluded that the TropBio Og4C3 ELISA using finger prick DBS can be used as an alternate to TBF microscopy for detection of bancroftian Filariasis under the LFE programme.