Which testing strategy is recommended when Cryptosporidium is clinically suspected but stools are negative on microscopy?

• A. Repeat stool microscopy
• B. Blood serology
• C. Urine antigen testing
• D. Stool antigen testing (ELISA) or PCR; DFA may be used

Answer: (D)

When Cryptosporidium is suspected but stool microscopy is negative, you look for a test with higher diagnostic sensitivity. Stool antigen tests (like ELISA) and molecular tests (such as PCR) detect parasite components or its DNA directly in stool, which catches infections that microscopy can miss due to low parasite load, intermittent shedding, or technical limits of staining. Direct fluorescence assays (DFA) can also be more sensitive than standard microscopy and may be used if available. Blood serology isn’t useful for acute diagnosis because antibodies take time to develop and don’t prove current infection, and urine antigen testing isn’t a standard approach for Cryptosporidium. So the recommended strategy is stool antigen testing or PCR, with DFA as an alternative if available.