Postzone effect in Histoplasma urine lateral flow antigen tests can cause weak or false negative results.

In this article, we evaluate a possible postzone effect for the Histoplasma antigen lateral flow assay (LFA). We re-tested urine samples that were Histoplasma-positive with the LFA, both for an undiluted and diluted sample. Of the 12 samples, a postzone effect was observed in six, and in one sample this led to a false negative result in the undiluted sample. For two samples, dilution of the sample led to a weaker LFA result. We recommend cautiousness when interpreting negative samples based on LFA alone, especially when clinical suspicion of histoplasmosis is high.