What lab test should be ordered for a patient suspected of having a parasite in their stool?

Laboratory Testing for Parasites in Stool

Order a stool ova and parasite (O&P) examination with concentrated specimen and permanent stained smears, or alternatively, use nucleic acid amplification testing (NAAT) or enzyme immunoassay (EIA) for specific parasites when available. 1

Optimal Specimen Collection

• The optimal specimen is a fresh diarrheal stool sample (one that takes the shape of the container), as this maximizes diagnostic yield for parasitic organisms 1

• Collect at least 3 stool specimens on consecutive days, as parasite shedding is intermittent and sensitivity increases substantially with multiple samples 2, 3

• Fresh stool is strongly preferred for identification of protozoal agents, as delays in processing can cause degradation of trophozoites 1

Primary Diagnostic Approach

Concentrated stool microscopy with O&P examination including permanent stained smears (such as trichrome stain) is the gold standard for detecting intestinal helminths including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes) 1, 2

For Common Protozoa (Giardia, Cryptosporidium, Entamoeba):

• Enzyme immunoassay (EIA) or NAAT are preferred over microscopy due to superior sensitivity and specificity 4, 5

• For Giardia lamblia: EIA or NAAT on stool 1, 4

• For Entamoeba histolytica: Species-specific immunoassay or NAAT on stool (to distinguish from non-pathogenic E. dispar) 1

• For Cryptosporidium: Direct fluorescent immunoassay, EIA, or NAAT 1

• Cryptosporidium and Giardia testing are often performed together as a combined primary parasitology examination 1, 4

For Less Common Parasites:

• For Cyclospora cayetanensis and Cystoisospora belli: Modified acid-fast stain on concentrated specimen, ultraviolet fluorescence microscopy, or NAAT 1

• For Microsporidia: Modified trichrome stain on concentrated specimen or small bowel biopsy with histologic examination 1

Multiplex Molecular Testing

• Nucleic acid amplification tests (NAAT) or multipanel gastrointestinal PCR assays can detect multiple parasites simultaneously and are particularly useful for organisms difficult to detect by microscopy 2, 4

• Clinical correlation is essential when interpreting NAAT results, as these assays detect DNA and not necessarily viable organisms 1, 4

• NAAT is especially valuable for Strongyloides and certain protozoa that have low microscopic sensitivity 2

Special Considerations for Specific Parasites

Strongyloides:

• Serology should be ordered in addition to stool testing, as concentrated stool microscopy has very low sensitivity for Strongyloides 2

• Specialized Strongyloides stool culture (Baermann technique or agar plate culture) or fecal PCR are more sensitive than routine microscopy 1, 2

Tapeworms:

• Look specifically for eggs or proglottids (segments) in concentrated stool microscopy 2

• Species identification is critical to distinguish Taenia saginata (beef tapeworm) from T. solium (pork tapeworm), as T. solium carries risk of neurocysticercosis 2

• If T. solium is identified or suspected, order cysticercosis serology to assess for systemic involvement 2

When to Order Testing

High-Yield Clinical Scenarios:

• Persistent or chronic diarrhea lasting ≥14 days, especially in travelers returning from endemic areas 1, 4

• Immunocompromised patients with diarrhea require broad parasitic workup including Cryptosporidium, Cyclospora, Cystoisospora, Microsporidia, and helminths 1, 2

• Patients with diarrhea plus bloating, abdominal pain, and weight loss 4

• High-risk exposures: drinking untreated water from streams/lakes, daycare center exposure, men who have sex with men, travel to endemic regions 4

• Patients with eosinophilia or systemic symptoms suggestive of tissue-migrating helminths 2

Lower-Yield Scenarios (Consider Selective Testing):

• Routine inpatient O&P testing has very low yield (approximately 2%) and should be restricted to patients with specific risk factors 6, 7

• Consider testing only if patient has: smoking history, prior parasitic disease, HIV-positive status, travel to endemic area, institutionalization, or diarrhea >7 days 6

Critical Pitfalls to Avoid

• Do not assume a single negative stool test rules out parasitic infection—always collect at least 3 samples over consecutive days, as sensitivity of single specimens is severely limited 2, 3

• Do not use routine stool O&P for pinworm (Enterobius vermicularis) diagnosis—this requires the cellophane tape test applied to perianal area in the morning 8

• Do not treat T. solium with praziquantel without first excluding neurocysticercosis, as killing intestinal worms may worsen CNS disease 2

• Do not order routine O&P testing in uncomplicated acute diarrhea without risk factors, as diagnostic yield is extremely low and cost-ineffective 6, 7

Additional Ancillary Testing

• Complete blood count with differential to assess for eosinophilia, which is common in tissue-migrating helminths 2

• Serology for schistosomiasis, strongyloidiasis, or other tissue-invasive parasites if travel history to endemic areas exists 2

Public Health Considerations

• All specimens positive for bacterial pathogens by culture-independent testing should be cultured to ensure outbreak detection and allow antimicrobial susceptibility testing 1

• Tapeworm cases, particularly T. solium, should be reported to local public health authorities, and household contacts should be screened 2

• Specimens from patients involved in an outbreak should be tested per public health department guidance 1