Identification of algae which interfere with the detection of Giardia cysts and Cryptosporidium oocysts and a method for alleviating this interference.

Fifty-four algal species were tested for cross-reaction in the American Society for Testing and Materials Giardia/Cryptosporidium indirect immunofluorescence assay, and 24 showed some degree of fluorescence. Two species, Navicula minima and Synechococcus elongatus, exhibited a bright apple green fluorescence. The addition of goat serum to the assay mixture blocked the fluorescence of most nontarget organisms tested and also decreased the background fluorescence. Goat serum did not interfere with the fluorescence of Giardia cysts or Cryptosporidium oocysts or the identification of cyst and oocyst internal structures.