What laboratory tests are done to check for parasites?

Laboratory Tests for Parasite Detection

The most effective approach to parasite detection involves microscopic examination of multiple stool specimens (ideally three) collected 12-24 hours apart, combined with specific antigen detection tests and molecular methods for suspected parasites. 1

Stool Examination for Intestinal Parasites

Microscopic Examination (O&P Testing)

• Ova and Parasite (O&P) examination is the traditional method for detecting intestinal parasites
• Requires collection of three stool specimens over a 24-hour period for optimal sensitivity 2
• First specimen detects approximately 58% of parasites, second specimen adds 21%, and third specimen adds another 21% 2
• Specimens should be properly preserved and transported to maintain parasite morphology

Antigen Detection Tests

• More sensitive than microscopy for specific parasites:
  • Giardia lamblia
  • Cryptosporidium
  • Entamoeba histolytica 1, 3
• The Triage Micro Parasite Panel can detect all three parasites simultaneously with high accuracy 3

Molecular Testing

• Multiplex PCR panels detect multiple pathogens simultaneously with higher sensitivity than conventional methods
• Can detect 15-22 pathogens including parasites, bacteria, and viruses
• Provide results within hours (some within 1 hour) 1
• Particularly useful when rapid diagnosis is needed or conventional methods have failed

Blood Parasite Testing

Microscopic Examination

• Thick and thin blood films stained with Giemsa are essential for detecting blood parasites
• Examine at least 100 microscopic fields before reporting negative results
• For patients without previous exposure, examine at least 300 fields 4
• Useful for detecting:
  • Malaria (Plasmodium species)
  • Babesiosis (Babesia species)
  • Trypanosomiasis (Trypanosoma species)
  • Filariasis 4

Rapid Diagnostic Tests (RDTs)

• Immunochromatographic tests that detect parasite antigens
• Useful for initial screening when microscopy expertise is not immediately available
• Positive results should be confirmed with microscopy 1

Molecular Methods

• PCR available for detection of blood parasites with low parasitemia
• Particularly useful for Babesia and Plasmodium species identification 4

Tissue Parasite Testing

Tissue Sampling

• Collection of tissue aspirates or biopsy specimens for:
  • Microscopic examination of smears and impression preparations
  • Histopathology
  • Parasite culture
  • Molecular testing 1

Serologic Testing

• Useful for parasites that are difficult to detect directly:
  • Toxoplasmosis (IgM and IgG)
  • Trichinosis
  • Cysticercosis
  • Echinococcosis
  • Toxocariasis 4
• Not recommended as stand-alone diagnostic tests 1

Testing Algorithm

1. For intestinal parasites:

   • Begin with antigen detection tests for common parasites (Giardia, Cryptosporidium, E. histolytica)
   • Collect three stool specimens for O&P examination
   • Consider molecular multiplex panels when rapid diagnosis is needed

2. For blood parasites:

   • Examine both thick and thin blood films
   • Consider RDTs for initial screening
   • If initial testing is negative but suspicion remains high, collect multiple specimens (≥3) drawn 12-24 hours apart
   • Consider molecular testing for definitive species identification

3. For tissue parasites:

   • Use multiple diagnostic approaches (microscopy, histopathology, serology, molecular testing)
   • Molecular amplification assays provide highest sensitivity

Common Pitfalls and Considerations

• False negatives in O&P examination occur when:

  • Insufficient specimens are collected (fewer than three)
  • Improper collection or preservation techniques are used
  • Inexperienced laboratory personnel perform the examination 2

• Stool O&P testing should be limited to:

  • Patients with negative immunoassay results but persistent symptoms
  • Individuals at increased risk for non-Giardia, non-Cryptosporidium infections 5

• Antigen or DNA may remain detectable for days to weeks after successful treatment, making these tests unsuitable for monitoring treatment response 1

• For hospitalized patients, O&P examinations have very low yield after 3 days of hospitalization and should generally be avoided unless there is a specific clinical indication 6