Which organism’s infection is primarily diagnosed by serology rather than stool or blood smear?

• A. Toxoplasma gondii
• B. Entamoeba histolytica
• C. Giardia lamblia
• D. Cryptosporidium parvum

Answer: (A)

Infections caused by certain parasites are most practically diagnosed by detecting the host’s immune response rather than by trying to visualize the organism in stool or blood. Toxoplasma gondii is diagnosed mainly through serology, testing for Toxoplasma-specific IgG and IgM antibodies. The organism forms tissue cysts and is not reliably shed in stool, nor is it typically seen on routine blood smears, so serologic tests provide the most consistent, accessible evidence of infection. Serology helps distinguish recent infection from remote exposure and, in pregnancy, supports assessment of congenital risk (with additional tests like IgG avidity or PCR as needed).

In contrast, the other listed organisms are typically identified by examining stool samples for cysts or trophozoites or by stool antigen tests, because they are shed in stool and can be visualized or detected directly.