The detection limit of the IC kit was determined using serially diluted CHIKV recovered from cell culture supernatants; the limit was approximately 10
4 PFU/ml. Additional figure file explains limit of detection of the immunochromatography rapid diagnosis kit for chikungunya virus antigen in more detail (Additional file
2). From sera collected for this study, total 104 (Group 1) were diagnosed as CF by either qRT-PCR or IgM. These 104 sera (Group 1) were further categorized into 3 groups, based on different combinations of test: 1) Group 2 (
n = 79), CHIKV positive by qRT-PCR, 2) Group 3 (
n = 50), CHIKV positive by IgM, 3) Group 4 (
n = 25), CHIKV positive by both qRT-PCR and IgM. Efficacy of IC kit was assessed compared to qRT-PCR result serving as the gold standard. The sensitivity, specificity and overall agreement (OAA) of the IC kit were evaluated to each group 1–4, respectively, and those for Group 1 were 72.1, 94.7 and 75.6%, respectively (Table
1). All samples from healthy volunteers were negative by IC kit (
n = 4). The IC kit targets viral antigen, CHIKV E protein. To compare efficacy of IC kit to other antigen detection method, CHIKV positive sera by qRT-PCR (Group 2) were extracted from Group 1. For Group 2, the sensitivity, specificity and OAA of the IC kit for CHIKV positive sera by qRT-PCR (irrespective of the presence of anti-CHIKV IgM antibodies) were 93.7, 95.5 and 94.3%, respectively. For comparison, efficacy of the IC kit was calculated against CHIKV positive sera by IgM (Group 3). The sensitivity, specificity and OAA of the IC kit for CHIKV positive sera by IgM (irrespective of the presence of CHIKV RNA) were 46.0, 27.4 and 35.0%, respectively. Low sensitivity of the IC kit to Group 3 may be due to the presence anti-CHIKV IgM antibodies. To test this, we assessed the sensitivity using Group 4 sera (CHIKV positive by both qRT-PCR and IgM). There was no significant reduction in the sensitivity of the IC kit for sera in Group 4 (88.0%). The existence of IgM did not affect the sensitivity of IC kit if samples contained qRT-PCR detectable viral antigen.
Table 1
Sensitivity, specificity and over all agreement with real time RT-PCR of immunochromatography kit for chikungunya virus antigen using clinical serum samples in various clinical categories
Group 1*2 | qRT-PCR or IgM qRT-PCR and/or IgM | 72.1 (75/104) | 94.7 (18/19) | 75.6 |
Group 2 | qRT-PCR | 93.7 (74/79) | 95.5 (42/44) | 94.3 |
Group 3 | IgM | 46.0 (23/50) | 27.4 (20/73) | 35.0 |
Group 4 | qRT-PCR and IgM | 88.0 (22/25) | 44.9 (44/98) | 53.7 |