The laboratory diagnosis of parasitic infections relies on a combination of microscopic, immunologic, and molecular methods. Selection of the appropriate diagnostic approach depends on the suspected parasite, the clinical presentation, and the geographic history of the patient.
Stool examination is the primary diagnostic method for intestinal parasites. The direct wet mount of fresh stool in saline and iodine allows detection of motile trophozoites and cysts. Concentration techniques increase sensitivity by separating parasites from fecal debris. The formalin-ethyl acetate sedimentation method is suitable for both protozoan cysts and helminth eggs. Zinc sulfate flotation is effective for protozoan cysts but distorts trematode eggs.
Permanent stained smears using trichrome or iron hematoxylin provide detailed morphological features for species identification of protozoa. Modified acid-fast stains are used for Cryptosporidium, Cyclospora, and Cystoisospora. The Kato-Katz technique provides quantitative egg counts for soil-transmitted helminths and schistosomes. The cellophane tape method detects Enterobius vermicularis eggs in perianal skin.
Blood examination detects blood-borne parasites. Thick and thin blood films stained with Giemsa or Field stain are used for malaria, babesiosis, and trypanosomiasis. The thick film concentrates parasites for detection, while the thin film allows species identification. Quantification of parasitemia guides treatment monitoring. Microfilariae detection uses Giemsa-stained thick blood films or Knott concentration, with timing based on known periodicity.
Tissue diagnosis requires biopsy specimens. Schistosoma eggs are identified in tissue sections. Trichinella larvae are detected in muscle biopsies. Leishmania amastigotes are identified in bone marrow, lymph node, or splenic aspirates stained with Giemsa. Amebic liver abscess fluid may show trophozoites at the abscess margin.
Antigen detection methods provide rapid diagnosis for many parasitic infections. ELISA and immunochromatographic lateral flow tests detect Giardia, Cryptosporidium, and Entamoeba antigens in stool. Malaria RDTs detect HRP2 and pLDH. Circulating cathodic antigen tests detect active Schistosoma infection.
Serological tests detect antibodies against parasitic antigens and are useful for tissue parasites. ELISA, immunofluorescence, and Western blot are used for Toxoplasma, Strongyloides, filariasis, schistosomiasis, and cysticercosis. Serology cannot distinguish past from current infection in endemic areas.
Molecular methods offer the highest sensitivity and specificity. Nested PCR, real-time PCR, and multiplex gastrointestinal panels detect and differentiate multiple parasites simultaneously. Genus-specific and species-specific primers target ribosomal RNA genes, repetitive sequences, or mitochondrial genes. Quantitative PCR measures parasite burden and monitors treatment response.
