Diagnostic accuracy of ARK Checker® C/G–DyLight® 488: simultaneous detection of Giardia and Cryptosporidium by fluorescent antibody microscopy

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 diagnostic accuracy; however, their cost and restricted coverage, especially for parasites, limit routine use. This study evaluated the efficacy of adapted fluorescent antibody microscopy using ARK Checker® C/G-DyLight® 488 (FAM-TEST). The reagent is a liquid-form conjugated antibodies which directly detect Giardia cysts and Cryptosporidium oocysts under fluorescence microscopy. METHODS: A total of 694 stool samples submitted for microbiological examination were screened by FAM-TEST, in which specimens were mixed with DyLight-488-labeled antibodies and examined microscopically. Samples with discordant results among these methods were additionally tested by conventional PCR with Sanger sequencing to verify infection status. RESULTS: Diagnostic accuracy was assessed by reference to multiplex PCR results and compared with that of rapid-IC. FAM-TEST identified Giardia and/or Cryptosporidium in 35 samples. In addition, 49 FAM-TEST-negative samples from a randomly selected month were included as negative controls. For Giardia, all FAM-TEST results were identical to those of rapid-IC, including three false negatives [88.0% sensitivity, 100% specificity, 100% positive predictive value (PPV), and 95.2% negative predictive value (NPV)]. For Cryptosporidium, FAM-TEST showed 93.8% sensitivity, 100% specificity, 100% PPV, and 98.6% NPV, which were comparable with those of rapid-IC. Notably, conventional PCR for Giardia sequences produced negative results for three samples considered to yield FAM-TEST false negatives, raising another possibility of multiplex PCR false positives or extremely low pathogen burden. CONCLUSION: Concurrent use of FAM-TEST with classical O&P offers a cost-effective, practical diagnostic approach for enteric parasites, which especially strengthens diagnostic accuracy for giardiasis/cryptosporidiosis.