Evaluation for Giardia in a Patient with Loose Stools
Test for Giardia using either a Giardia antigen test or PCR—this is a strong recommendation with high-quality evidence for all patients presenting with chronic diarrhea. 1
Why Testing for Giardia is Essential
The American Gastroenterological Association provides a strong recommendation with high-quality evidence that all patients presenting with chronic diarrhea should be tested for Giardia. 1 This recommendation is based on several compelling factors:
- Giardia is the most common parasitic cause of chronic diarrhea throughout the United States, making it a high-yield test regardless of geographic location. 1
- Modern diagnostic tests have excellent performance characteristics, with sensitivity and specificity both exceeding 95% for Giardia antigen tests and PCR-based assays. 1
- The infection is readily treatable, making early diagnosis clinically meaningful for patient outcomes. 1
Optimal Testing Strategy
Use either a Giardia antigen test or polymerase chain reaction (PCR) test—these are superior to traditional microscopy. 1
- Giardia antigen enzyme immunoassays and PCR tests for the Giardia small subunit ribosomal RNA demonstrate sensitivity and specificity >95%. 1
- Traditional microscopic examination of stool for ova and parasites is labor-intensive, requires skilled technicians, is expensive (estimated $1,836 per significant organism identified), and has lower sensitivity than modern antigen or molecular tests. 1
- Do NOT order a general "ova and parasites" stool examination unless the patient has recent travel to or immigration from high-risk areas—the AGA specifically recommends against this for patients without such exposure. 1
Clinical Context and Risk Factors
While testing is recommended for all patients with chronic diarrhea, certain risk factors increase the pretest probability of Giardia infection: 1
- Exposure to endemic areas
- Drinking untreated or contaminated water
- Hikers drinking untreated water from streams or lakes
- Exposure in daycare settings
- Men who have sex with men
- Swimming in community pools 2
Additional Diagnostic Considerations
When evaluating a patient with loose stools concerned about Giardia, a comprehensive workup should also include:
- IgA tissue transglutaminase (tTG) with total IgA to screen for celiac disease, which can present similarly (strong recommendation, moderate-quality evidence). 1
- Fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease (conditional recommendation, low-quality evidence). 1
- Complete blood count and basic metabolic panel to assess for anemia, electrolyte disturbances, and signs of malabsorption. 1
Common Pitfalls to Avoid
- Do not rely on a single negative stool examination in periodic cyst excretors—Giardia may not be detected unless repeated examinations are performed, though modern antigen and PCR tests have largely mitigated this issue. 3
- Do not order traditional ova and parasite microscopy as the first-line test—it is inferior to antigen detection and PCR. 1
- Do not skip testing even in asymptomatic patients if they are food handlers or have potential to transmit infection, though treatment of asymptomatic carriers is generally not recommended in other contexts. 4