What laboratory tests are used to diagnose parasites?

Laboratory Testing for Parasitic Infections

The diagnostic approach for parasites depends critically on the suspected organism and clinical presentation, with microscopic examination of stool (ova and parasite exam) or blood films (Giemsa-stained thick and thin smears) serving as the gold standard for most parasitic infections, supplemented by immunoassays for specific pathogens like Giardia and Cryptosporidium, and serology for tissue parasites. 1

Primary Diagnostic Categories

Blood Parasites (Malaria, Babesia, Trypanosomiasis)

Microscopic examination of Giemsa-stained thick and thin blood films is the gold standard for diagnosing blood-borne parasites, allowing species identification and quantification of parasitemia. 1, 2

• Thick blood films concentrate parasites by lysing red blood cells, enabling rapid detection across 20-30 layers of blood simultaneously—this is the most sensitive microscopic method. 1
• Thin blood films preserve red blood cell morphology, allowing precise species identification and calculation of percentage parasitemia, which is critical for treatment decisions. 1, 2
• Collect at least 3 specimens drawn 12-24 hours apart if initial films are negative and malaria is strongly suspected, ideally during febrile episodes. 2

Rapid diagnostic tests (RDTs) provide results within 15 minutes and are useful for initial screening when microscopy expertise is unavailable, but must be confirmed with blood films within 12-24 hours. 1, 2 The BinaxNow Malaria test is the only FDA-approved RDT in the United States. 2

Critical pitfall: RDTs can remain falsely positive for several days after parasite eradication since antigens persist, so never use RDTs to monitor treatment response—only microscopy should be used. 1, 2

Nucleic acid amplification tests (NAATs) offer superior sensitivity (detecting ~0.2-6 parasites per microliter) and are available through reference laboratories and the CDC, though none are FDA-cleared. 1, 2 NAATs are useful when blood films are negative or difficult to obtain, but blood films must still be examined when NAAT is positive to determine percentage parasitemia. 1

Intestinal Parasites (Giardia, Cryptosporidium, Entamoeba, Helminths)

Enzyme immunoassays (EIAs) for Giardia and Cryptosporidium are more sensitive than traditional ova and parasite (O&P) examination and should be the first-line test for these common pathogens. 3, 4

• Giardia and Cryptosporidium EIAs detect more cases (3.4% positivity) compared to O&P alone (1.4% positivity), with superior sensitivity and specificity approaching 100%. 3, 4
• Reserve stool O&P examination for patients with negative immunoassay results and persistent symptoms, or individuals at increased risk for non-Giardia, non-Cryptosporidium infections (travel to endemic areas, HIV-positive status, institutionalization, prior parasitic disease, smoking). 3, 5
• Collect at least 3 sequential stool specimens over 14 days when parasitic infection is suspected, as a single specimen detects only 58.3% of infections, two specimens detect 78.9%, and three specimens are needed to achieve 100% detection. 6

Important caveat: Stool specimens should be transported to the laboratory within 24 hours in appropriate transport medium. 1 For Strongyloides in immunocompromised patients, specialized techniques like Baermann method or agar plate culture may be required. 1

Tissue and Systemic Parasites

Serology is the primary diagnostic method for tissue parasites that are difficult to detect by microscopy. 1

• Toxoplasmosis: IgM and IgG serology via IFA, EIA, or ELFA from CDC or reference laboratories; IgG avidity testing and serial titers distinguish recent from past infection. 1
• Toxocariasis (visceral larva migrans): Serology from CDC or reference laboratories, as larvae are rarely seen in tissue biopsies. 1
• Trichinosis: Serology (EIA) from CDC or reference laboratories; encysted larvae may be visible in muscle biopsies. 1
• Chagas disease (chronic phase): Serology through CDC, as parasitemia is very low in chronic infection; IgG antibody persists for decades. 1

Critical limitation: Serology shows significant cross-reactivity among helminths, including filaria, resulting in decreased specificity. 7 Clinical decisions should not be based on reactive serologic results alone—interpretation must consider travel history, exposure risk, and clinical presentation. 7

Practical Testing Algorithm

1. For acute diarrhea: Order Giardia and Cryptosporidium EIAs first; reserve O&P for negative EIA with persistent symptoms or specific risk factors. 3, 5

2. For fever with travel to malaria-endemic areas: Immediately perform thick and thin blood films (stat request); use RDT for initial screening if microscopy expertise unavailable, but confirm with blood films within 12-24 hours. 1, 2

3. For suspected tissue parasites: Order specific serology based on clinical syndrome and exposure history; contact CDC or reference laboratories for specialized testing. 1

4. For negative initial tests with high clinical suspicion: Repeat testing with multiple specimens (3 stool specimens over 14 days for intestinal parasites; 3 blood specimens at 12-24 hour intervals for blood parasites). 2, 6

Key Laboratory Considerations

Large public health and reference laboratories are more likely to have the experience and volume to properly validate esoteric tests compared to community laboratories. 1 The CDC DPDx website (https://www.cdc.gov/dpdx/) provides a list of available diagnostic tests and offers telediagnostic consultation services. 1

For urgent cases: Direct phone or email communication with public health laboratories can hasten specimen processing and result reporting. 1 Requests for malaria and babesiosis diagnosis should be considered "stat" and testing performed as rapidly as possible, as P. falciparum can be rapidly fatal. 2