P-1772. Diagnostic Utility of Fluorescent Antibody Microscopy (FAM) for Giardiasis and Cryptosporidiosis

Abstract Background Giardiasis and Cryptosporidiosis are common diarrheal diseases that are often underdiagnosed due to non-specific symptoms and diagnostic limitations. While multiplex-PCR and rapid antigen tests (rapid-IC) offer high accuracy, their cost and restricted pathogen coverage, especially for parasites, limit their use. This study evaluates newly developed diagnostic method for these protozoa, named fluorescent antibody microscopy (FAM).TABLE 1Sensitivity and specificity of each method for detection of Giardia with reference to multiplex PCRTABLE 2Sensitivity and specificity of each method for detection of Cryptosporidium with reference to multiplex PCR Methods For performing FAM, stool samples are mixed with DyLight 488-conjugated antibodies against G. duodenalis cysts and Cryptosporidiumoocysts, thereafter, the samples are examined by fluorescence microscopy to detect these protozoa. All samples showing positive for FAM, and unbiasedly selected those with FAM negative result were subjected to multiplex-PCR, and rapid-IC. Diagnostic accuracies of FAM and rapid-IC were assessed by the result of multiplex-PCR as a reference standard. Cases with discordant results among these tests were examined by conventional PCR with sequencing to confirm infection. Results FAM was examined for 694 stool samples during study period, which identified 35 “FAM positive samples”. Also, “FAM negative samples” were selected from all cases with negative results at any one month (49 samples). In total, a subset of 84 samples underwent further analysis. For Giardiasis, FAM showed 88.0% sensitivity and 100% specificity, which was exactly the same results with rapid-IC, including 3 samples showing false negative by both tests. (Table 1) For cryptosporidiosis, FAM had 93.8% sensitivity and 100% specificity, which represents comparable diagnostic accuracy with rapid-IC. (Table 2) Interestingly, conventional PCR for G. duodenalis oppositely showed negative results for 3 samples with multiplex-PCR positive, and FAM/rapid-IC negative results, which raised 2 possibilities; false negative results of FAM/rapid-IC due to extremely low pathogen burden, or false positive result of multiplex-PCR. Conclusion FAM, a newly developed low-cost morphological stool examination for Giardia and Cryptosporidium, showed comparably high diagnostic accuracy with rapid-IC. It could be a cost-effective option for routine stool testing. Disclosures All Authors: No reported disclosures