Cryptosporidium antigen detection in human feces by reverse passive hemagglutination assay

A reverse passive hemagglutination (RPH) assay was developed for Cryptosporidium oocyst antigen with an antioocyst monoclonal antibody (MAb; MAb-C1) coupled to stabilized sheep erythrocytes. RPH was compared with microscopy of auramine-phenol-stained smears of 56 oocyst-positive fecal samples, each of which was tested blindly by RPH with two oocyst-negative samples received on the same day (a total of 112 controls). Thirty-nine additional fecal samples from human immunodeficiency virus type 1 antibody-positive patients with diarrhea (10 of which were positive in auramine-phenol-stained smears) were stored at -20 degrees C before testing. Thirty specimens with a variety of other fecal pathogens (all negative for oocysts) were also tested. Of the 237 samples tested, 69 were positive by one or both methods: 65 by RPH and 66 by microscopy. The kappa coefficient of agreement between the methods was very high at 0.926. The sensitivity of RPH was 93.9%, the specificity was 98.2%, the positive predictive value was 95.4%, and the negative predictive value was 97.7%. Visible oocyst numbers and RPH titers were measured after storage of fecal samples and oocyst concentrates for 8 days at 4 degrees C. Oocyst morphology was generally poor in specimens from the human immunodeficiency virus type 1 antibody-positive group, and it degenerated during the 8-day storage experiments. MAb-C1-reactive antigen eluted from oocysts to give progressively higher reciprocal titers during storage, and it was partially removed from the oocysts by concentration. RPH is a promising technique for the detection of Cryptosporidium antigen in human feces and may be useful when specimens are stored before testing. Studies of the sensitivity of Cryptosporidium immunoassays should take into account the possible release of antigen from oocysts.