The serologic window between HCV infection and the detection of specific antibodies varies from patient to patient. With current assays, seroconversion occurs on average between 7 and 8 weeks after onset of an infection 
. About 30% to 50% of patients have undetectable anti-HCV antibodies at the onset of clinical symptoms 
. It is widely accepted that IgM provides a first line of defense during microbial infections and IgM detection has proven valuable for early diagnosis of many viral infections 
. Therefore, it is predicted that improved IgM detection by an anti-HCV EIA assay may narrow this serologic window.
The IgM detection efficacy of current anti-HCV assays using monoclonal or polyclonal HRP-labeled anti-human Ig antibodies as conjugates has not been evaluated. In this study, we used a NEIBM, LD5 which showed high affinity for IgM in addition to IgG () 
, to make conjugates, HRP-LD5. The HRP-LD5 also expressed a much stronger binding capacity to hIgM than did HRP-goat anti-human PcAb (). Consistently, the established HRP-LD5-based anti-HCV assay showed a higher detection rate than that of the commercial diagnostic kit (Chang Zheng) in hemodialysis patients (, ), and it was attributed to the enhanced IgM detection (, ).
Hemodialysis patients are a high-risk population for HCV infection. The anti-HCV prevalence seen in these patients varied from 3.3% in New Zealand 
, 39% in South America 
, 44%–60% in Far-Eastern countries 
to as high as 80.0% in Egypt 
. HCV is highly transmissible among these patients, and HCV infections at many different stages, including recent infections, are observed in this population. Therefore, a comparison study was conducted among them. Our results showed that the HRP-LD5 based anti-HCV assay had a statistical significant higher detection efficacy (55.9% positive detection rate) than did the domestic diagnostic kit (Chang Zheng) (positive detection rate of 53.3%) ().These positive detection rates are consistent with the 59.7% and 58.8% anti-HCV prevalence reported in other Chinese hemodialysis patient studies 
The five patients who tested positive using the HRP-LD5 based assay alone were demonstrated to have anti-HCV IgM antibodies and HCV RNA. Follow-up samples taken from these patients three weeks later were positive using both the HRP-LD5-based assay and the Chang Zheng kit (). These results indicate that these five patients were likely to be newly HCV infected cases, which account for 2.56% of the 195 hemodialysis patients in this study, consistent with the result in a follow-up study of Chinese Shanghai hemodialysis patients that showed the anti-HCV seroconversion rates at 6, 12, 18, 24 and 30 months were 6.4%, 11.9%, 20.7%, 35.4% and 54.5%, respectively 
Indirect EIAs have been developed to detect IgM against many microbes. Low sensitivity or specificity is common problem for IgM detection, and the lack of applicable anti-hIgM monoclonal and polyclonal antibodies is a major obstacle for effective IgM detection 
. An indirect anti-HCV IgM EIA assay has also been developed and extensively studied. Many reports have shown it to be useful for early diagnosis of HCV infection 
, whereas others do not support this conclusion 
. For unknown reasons, commercially available anti-IgM antibodies have low affinity constants (Ka), varying from 2×105
, which is much lower than that of anti-hIgG monoclonal and polyclonal antibodies, which is usually above 1×109
. Unlike the anti-IgM antibodies used in anti-HCV IgM ELISA assay, LD5 has well defined binding mode, i.e., simultaneously bound to the κ light chain and VH
3 of hIgM. Consequently, it showed a high binding affinity to hIgM with the affinity constant (Ka) of 3.23×109
and an equilibrium dissociation constant (KD
) of 0.31 nM, which was as high as that of SpA to hIgG with the Ka of 3.45×109
and the KD
of 0.29 nM (). Consistent with this binding property, the HRP-LD5 based assay displayed a higher IgM detection efficacy than the established IgM assay (, ).
To our knowledge, this is the first report of the application of an NEIBM, LD5, to establish an anti-HCV detection assay with enhanced detection efficacy due to improved IgM detection. Our results also imply that NEIBMs with a high affinity to IgM could have potential application for detection of antibodies against other viral infections, such as hepatitis E virus or dengue virus, to improve of detection efficacy and early diagnosis.