Method and Organism Discussions

Discussion #2B: Reporting Blastocystis spp. Results

Interpretation and Reporting of Results

Although many people worldwide are infected, most probably remain asymptomatic. The main problem is correct identification and differentiation between this organism and pathogenic intestinal protozoa.

Infection with Blastocystis may be the cause of diarrhea, cramps, nausea, fever, vomiting, and abdominal pain and may require therapy. The incidence of this organism appears to be higher than suspected in stools submitted for parasite examination; it is considered the most common protozoan worldwide (review of published literature). In symptomatic patients in whom no other etiologic agent has been identified, Blastocystis should certainly be considered the possible pathogen. When a symptomatic infection responds to therapy, the improvement may also represent elimination of some other undetected pathogenic organism (E. histolytica, G. lamblia, D. fragilis)

With approximately 17 subtypes identified (half pathogenic, half nonpathogenic), this may provide an explanation for why some patients are symptomatic and some are asymptomatic. This is probably the most common intestinal parasite found in humans (worldwide) and is often seen in a higher percentage of patients (as compared with other intestinal protozoa). Percent positive patients are usually higher than either Giardia lamblia (G. duodenalis, G. intestinalis) and/or Dientamoeba fragilis. Giardia and Dientamoeba are often rotate between number 2 and number 3 in terms of positive patients, with Blastocystis continuing to be number 1.

Blastocystis should be quantitated when reported (rare, few, moderate, many, packed); both large and small numbers of organisms may cause symptoms. However, clinical specimens should be examined thoroughly for the presence of other potential pathogens before pathogenic status is assigned. Therapy is also effective in eradicating other intestinal protozoa (Giardia, etc.). However, recent studies indicate persistence of symptoms in some patients could be due to drug resistance with metronidazole demonstrated in subtype 3. Unfortunately, the various subtypes cannot be differentiated on microscopic morphology.

Quantitation of parasites, cells, yeast cells, and artifacts

Additional Information Regarding Parasitemias

Rare = often not reported; usually higher numbers present in symptomatic patients

Few = ≤2 per 10 oil immersion fields (× 1,000)

Moderate = 3 to 9 per 10 oil immersion fields (×1,000)

Many = ≥10 per 10 oil immersion fields (×1,000)

Packed = ≥ 20 per 10 oil immersion fields (× 1,000)

Report Comments per Blastocystis spp.

Blastocystis spp. is recommended

Report comments are highly recommended, particularly if the term Blastocystis hominis rather than Blastocystis spp. is used.

Blastocystis spp. is recommended

Blastocystis spp. contains approximately 17 different subtypes, none of which can be differentiated on organism morphology; some subtypes have been found to cause symptoms and some have not. If no other pathogens are found, Blastocystis may be the cause of patient symptoms

AND

Other organisms capable of causing diarrhea should also be ruled out.

Clinical significance may be relevant; approximately half of the human subtypes may cause symptoms

Status as a pathogen remains controversial; approximately half of the human subtypes can cause symptoms, while half do not

Until there are testing options to differentiate between the subtypes, it is important that the physicians know that some subtypes of Blastocystis may cause symptoms. Quantitate these organisms (rare, few, moderate, many, packed).

Report Examples per Blastocystis spp. (including report comments)

Example:

Moderate Blastocystis spp.; Clinical significance may be relevant; approximately half of the 17 human subtypes are pathogenic and may cause symptoms

Example:

Many Blastocystis spp.; Pathogenic and/or nonpathogenic subtypes (17 human subtypes) cannot be differentiated on microscopy; approximately half of the 17 human subtypes are pathogenic and may cause symptoms.

References

Garcia, LS, 2021. Practical Guide to Diagnostic Medical Parasitology, 3^rd ed., ASM Press, Washington, DC