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Summary The NOW® Filariasis card test is a useful tool for rapid field diagnosis of Wuchereria bancrofti infection, based on detection of specific circulating filarial antigen (CFA) in the patients’ blood. Concern has been raised that a high proportion
of infection negative individuals develop false positive reactions in these tests when the test cards are left for a prolonged period before being examined. We carried out a survey in an endemic Tanzanian village to investigate this phenomenon. Individuals who were positive in the NOW®
Filariasis test at 10 min after specimen application were also positive in the TropBio ELISA for CFA, and thus appeared to be truly positive. Many of the test cards that were negative at 10 min developed a positive line later, but these lines appeared to be falsely positive when
the TropBio test was used as the gold standard. Close examination revealed that true and false positivity lines could be distinguished on their shape and colour. The study thus reaffirmed that test cards should be read after 10 min to avoid false positives, but it also indicated that
experienced test card users should be able to make a correct diagnosis even at a later time.