Which laboratory tests should be ordered to evaluate a suspected parasitic infection?

Laboratory Testing for Suspected Parasitic Infections

The specific laboratory tests ordered depend on the clinical presentation and suspected parasite, but for most parasitic infections, microscopy remains the gold standard, supplemented by antigen detection, serology, or PCR depending on the organism. 1

Blood Parasites

Malaria and Babesiosis

• Order Giemsa-stained thick and thin blood films immediately as the gold standard for diagnosis of both malaria and babesiosis 1
• Thick films detect parasites rapidly (examining 20-30 layers of blood simultaneously), while thin films allow species identification 1
• Repeat testing every 12-24 hours for a total of 3 examinations before ruling out malaria 1
• Calculate percentage parasitemia using thick or thin films to guide management and monitor treatment response 1
• Rapid antigen detection tests can be used but lack sensitivity with low parasitemia and non-falciparum malaria; follow positive results with confirmatory blood films within 12-24 hours 1
• Blood specimens must be processed within 1 hour, as prolonged EDTA exposure alters parasite morphology 1

Filariasis

• For Wuchereria bancrofti and Brugia malayi: Order microscopy of Giemsa-stained thick and thin blood films, with blood collection between 10 PM and 2 AM when microfilariae are circulating 1
• Examination of concentrated blood specimens (Knott technique, Nuclepore filtered blood, or buffy coat) increases sensitivity 1
• For Onchocerca volvulus: Order microscopy of "skin snip" after incubation in saline at 37°C from areas near nodules 1

Other Blood Parasites

• African trypanosomiasis: Microscopy of Giemsa-stained thick and thin blood films or buffy coat preparations; parasitemia is often low requiring repeated examinations 1
• Chagas disease: Microscopy of Giemsa-stained thick and thin blood films or buffy coat preparations, plus serology through CDC (parasitemia is very low in chronic infection) 1

Intestinal Parasites

Stool Examination

• Order enzyme immunoassays (EIAs) for Giardia and Cryptosporidium rather than routine ova and parasite (O&P) examination 2
• EIA testing yields significantly more positive results (3.4%) compared to O&P only (1.4%) and demonstrates superior sensitivity 2
• Reserve O&P examination for patients with negative immunoassay results and persistent symptoms, or those at increased risk for non-Giardia, non-Cryptosporidium infections 2
• For intestinal amebiasis, accurate diagnosis usually requires PCR or antigen detection tests rather than microscopy alone 3

Helminth Infections

• Traditional microscopy may need supplementation with agar plate culture for strongyloidiasis 3
• Serology (EIA) through CDC or reference laboratories for trichinosis; encysted larvae can be seen in muscle biopsies 1

Tissue and Visceral Parasites

Leishmaniasis

• Cutaneous disease: Order microscopic examination of Giemsa-stained smears from biopsy touch impressions or aspirate from the leading edge of ulcer 1
• Contact CDC for collection kit prior to biopsy; culture and PCR are available and necessary for species identification in South/Central American cases 1
• Visceral disease: Order microscopic examination of Giemsa-stained bone marrow aspirate/biopsy or splenic aspirate 1
• Positive rK39 serology is both sensitive and specific for visceral leishmaniasis in endemic areas 1

Toxoplasmosis

• Order serology (IFA, EIA, ELFA) for detection of IgM and IgG 1
• IgG avidity testing and serial titers distinguish between recent and past infection 1
• Note that positive IgG is seen in 15-40% of US population due to previous exposure 1

Other Tissue Parasites

• Toxocariasis: Serology from CDC or referral laboratories (larvae rarely seen in tissue biopsies) 1
• Echinococcosis: Serology plus microscopic examination of tissue biopsies or cyst aspirates for protoscolices and/or hooklets 1

Critical Timing and Transport Considerations

• Blood films for malaria/babesiosis must be made from fresh blood within 1 hour to preserve parasite morphology 1
• Thick blood films dry slowly and require protection from smearing, spillage, and dust 1
• For rapid antigen tests, blood may be stored at 2°C-30°C for up to 3 days for some commercial assays 1

When to Involve Reference Laboratories

• Contact CDC Division of Parasitic Diseases (https://www.cdc.gov/dpdx/) for esoteric testing not available in routine hospital laboratories 1
• Large public health and reference laboratories have greater experience and resources for validating esoteric parasitic tests compared to community laboratories 1
• Direct phone or email communication can expedite specimen processing and result reporting, especially in urgent clinical situations 1
• CDC provides telediagnostic consultation services accessible through the DPDx website for both laboratorians and clinicians 1

Common Pitfalls

• Physicians frequently order O&P testing when test performance and epidemiology support immunoassays or no testing 2
• During a Cryptosporidium outbreak, no cases were detected by O&P alone, highlighting the inadequacy of this approach for these pathogens 2
• Microscopy for parasites requires skilled and experienced technologists to obtain maximum accuracy and efficiency 1
• PCR for blood protozoa is highly sensitive but cannot rule out Chagas disease, as this condition may be present without parasitemia 3