A unique phenomenon in hbsag screening–the hook effect

Screening for HBsAg is done routinely in patients attending various departments of Hospital. Different methods of conducting HBsAg tests include solid phase assays, flow through, agglutination, and lateral flow. These rapid tests need a backup of standard EIA testing and /or DNA based methods like PCR as gold standard for evaluating their sensitivity and specificity. In our setup we perform rapid tests (J mitra’s Hepacard) based on Immunochromatographic method, Elisa and PCR for confirmation and further evaluation. When Screening for HBsAg was done in a 32 year orthopaedic patient showing pink coloured line in test reigion of Hepacard, without developing colour band in the control line. In the absence of control line we could not interpret the test result immediately. So Elisa was performed on this sample which gave reactive result for HBsAg antigen. This prompted us to do serial dilution of serum sample (1:10, 1:20) and on repeating the test, positive results with control line was observed in Hepacard. Hence we assume that inconclusive result observed earlier using Hepacard is due to Hook effect. Hook effect might have occurred due to large quantities of HBsAg antigen in serum, which impairs antigen-antibody binding, resulting in low antigen determination. Further when Quantitative PCR was done in this sample high concentration of HBV DNA was present. HenceHook effect has to be kept in mind when screening of HBsAg is done routinely.