Which diagnostic approach is most appropriate for Giardia lamblia infection?

• A. Stool antigen testing or microscopy showing Giardia cysts or trophozoites
• B. Blood culture
• C. CSF analysis
• D. Urinalysis for red blood cells

Answer: (A)

Diagnosing Giardia lamblia infections hinges on testing the stool. The parasite lives in the small intestine and is shed in feces, so the most informative samples come from stool analysis. Stool antigen testing detects Giardia-specific proteins in the stool and is highly sensitive and specific, often preferred because it remains reliable even when parasite numbers are low. Microscopy can also identify the parasite by finding cysts or motile trophozoites in the stool, especially when multiple samples are examined and proper staining or concentration techniques are used.

In practice, a combination of stool antigen testing and microscopy increases the chance of detection. Other tests—such as blood culture, CSF analysis, or urinalysis for red blood cells—do not diagnose giardiasis because Giardia is an intestinal parasite and these tests assess unrelated organ systems or conditions.