Sensitivity of a Giardia antigen test in persistent giardiasis following an extensive outbreak.

The utility of a rapid antigen test for diagnosing cases of persistent giardiasis, as defined by detection of cysts by conventional microscopy following standard formalin-ether concentration or the positive rapid antigen test, was investigated following a large, waterborne outbreak of giardiasis. The sensitivity and specificity of the rapid test as compared with microscopy were 60.7% and 96.7%, respectively, in this patient group. The low sensitivity contrasts with previous reports, and may be partly explained by low cyst numbers.