Improving Practical Diagnostic Measure for Enteric Parasites: Development of Fluorescent Antibody Microscopy (FAM) for Giardiasis and Cryptosporidiosis

Background: Giardiasis and cryptosporidiosis are often misdiagnosed by stool ova and parasite test (classical O&P). Multiplex PCR and rapid antigen immunochromatography (rapid-IC) could offer high accuracy; however, its cost and restricted coverage, especially for parasites, limit their use. This study evaluated the efficacy of newly developed “fluorescent antibody microscopy (FAM)” for these protozoa. Methodology/Principal Findinges: DyLight 488-conjugated antibodies against Giardia cysts and Cryptosporidium oocysts were used for FAM examination. Diagnostic accuracy was assessed by reference to multiplex PCR results and compared with that of rapid-IC. FAM identified Giardia and/or Cryptosporidium in 35 of 694 stool samples tested. Also, FAM-negative stools examined in a randomly chosen month (49 samples) were selected as negative control. For Giardia, all FAM results were identical to those of rapid-IC, including three false negatives [100% positive predictive value (PPV), and 95.2% negative predictive value (NPV)]. For Cryptosporidium, FAM showed 100% PPV, and 98.6% NPV, which were comparable with those of rapid-IC. Conventional PCR for Giardia sequences produced negative results for three samples considered to yield FAM false negative, raising another possibility of multiplex PCR false positives.