For Cryptosporidium testing when stool microscopy is negative, which testing is recommended?

• A. Antigen testing or PCR; DFA on stool
• B. Culture on standard media
• C. Serology
• D. Blood smear

Answer: (A)

Stool microscopy can miss Cryptosporidium when the parasite burden is low or shedding is intermittent, so if suspicion remains after a negative smear, move to more sensitive stool-based tests. Antigen detection tests (such as ELISA or rapid immunochromatographic assays) look for Cryptosporidium proteins in the stool and are typically more sensitive than routine microscopy. Molecular testing by PCR on stool detects parasite DNA with high sensitivity and can also help with species identification. Direct immunofluorescence on stool, using fluorescent antibodies, visualizes oocysts directly and is another highly sensitive option in labs that perform it. These approaches are preferred because they increase detection when microscopy misses the infection. In contrast, culture on standard media isn’t routinely used for Cryptosporidium, serology isn’t helpful for acute diagnosis, and a blood smear wouldn’t detect intestinal Cryptosporidium.