Background: The performance of Giardia diagnostic tests that detect either cysts or fecal antigens has not been thoroughly examined.
Hypothesis/objectives: We examined the concordance and agreement among 4 Giardia diagnostic tests (2 cyst and 2 coproantigen detection methods) in a colony of dogs chronically and subclinically infected with Giardia.
Animals: Twenty dogs with chronic, subclinical Giardia infection.
Methods: Giardia diagnostic tests were performed repeatedly on each dog over 120 days. Fecal cyst detection methods (ZnSO4 flotation and fluorescent antibody [FAB] coproscopy) were performed 3 times per week. Coproantigen methods (Giardia SNAP test and quantitative ELISA) were performed weekly. Results were analyzed and compared among methods.
Results: When compared with FAB coproscopy, all of the in-house diagnostic tests had excellent positive predictive values (PPVs, 95-99%) at the study prevalence (89%). At lower prevalence rates, ZnSO4, SNAP, and ELISA tests all had good negative predictive values (NPVs), but poor PPVs. There was poor to good agreement among tests by kappa analysis.
Conclusion and clinical relevance: Our findings show that most commonly used in-house Giardia diagnostic tests have poor agreement with the gold standard method (FAB coproscopy). The in-house tests have good NPVs, but poor PPVs, at prevalence rates common in most clinical settings.