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Background: Anti-HIV-1 IgM antibody is an important immunoassay target for early HIV antibody detection., Objectives: The objective of this study is to determine if the early HIV antibody sensitivity of the 60s INSTI test is due to detection of anti-HIV-1 IgM in addition to IgG., Study Design: To demonstrate HIV gp41 IgM antibody capture by the INSTI HIV-1 gp41 recombinant antigen, an HIV-IgM ELISA was conducted with commercial HIV-1 seroconversion samples. To demonstrate that the INSTI dye-labelled Protein A-based colour developer (CD) has affinity to human IgM, commercial preparations of purified human immunoglobulins (IgM, IgD, IgA, IgE, and IgG) were blotted onto nitrocellulose (NC) and probed with the CD to observe spot development. To determine that INSTI is able to detect anti-HIV-1 IgM antibody, early seroconversion samples, were tested for reduced INSTI test spot intensity following IgM removal., Results: The gp41-based HIV-IgM ELISA results for 6 early seroconversion samples that were INSTI positive determined that the assay signal was due to anti-HIV-1 IgM antibody capture by the immobilised gp41 antigen. The dye-labelled Protein-A used in the INSTI CD produced distinct spots for purified IgM, IgA, and IgG blotted on the NC membrane. Following IgM removal from 21HIV-1 positive seroconversion samples with known or undetermined anti-HIV-1 IgM levels that were western blot negative or indeterminate, all samples had significantly reduced INSTI test spot intensity., Conclusions: The INSTI HIV-1/HIV-2 Antibody Test is shown to detect anti-HIV-1 IgM antibodies in early HIV infection which enhances its utility in early HIV diagnosis., (Copyright © 2015 The Authors. Published by Elsevier B.V. All rights reserved.)