What is the clinical significance of Blastocystis hominis in stool?

• A. Pathogenicity is controversial; detection by microscopy or PCR is used, but clinical correlation is needed.
• B. Blastocystis hominis is universally pathogenic.
• C. It cannot be detected by PCR.
• D. It is a commensal organism with no clinical relevance.

Answer: (A)

The key point is that Blastocystis hominis often behaves as a bystander in the gut. Its presence in stool does not automatically mean disease, because many people carry it without symptoms. The clinical significance is therefore controversial: some individuals with diarrhea or other GI symptoms may have Blastocystis contributing to their illness, but in others it may be incidental. Detection methods include microscopy, which can identify the organism’s various forms in stool, and PCR, which is more sensitive and can confirm its presence (and help with subtyping). Because symptoms, other conditions, and co-infections can explain GI illness, laboratory findings must be interpreted in the clinical context. Thus, the best statement reflects that pathogenicity is controversial, detection can be by microscopy or PCR, and clinical correlation is needed to determine relevance.