Zinc sulfate (ZnSO4) centrifugation followed by microscopic examination is considered by many to be the optimal method for detection of Giardia species cysts. However, false-negative results may occur when the small cysts are overlooked or false-positive results may occur when other matter, such as yeast bodies and plant material, are confused with Giardia cysts. In this study, eight cats were infected with a Giardia species that had originally been isolated from a human. Feces were collected on weeks 0, 3, 6, 9, 12, and 15 and stored at 4°C until assayed 4 to 8 months later. ZnSO4 assays were conducted according to published techniques. Giardia-antigen assay (Heska Corporation, Fort Collins, CO) and immunofluorescent antibody assay (IFA) (Meridian Diagnostic, Cincinnati, OH) were done according to the manufacturers' instructions. According to IFA results, all cats were negative for Giardia on week 0 and positive for Giardia in all subsequent samples. As compared with the IFA results, the sensitivity and specificity of the Giardia-antigen assay were 95% and 100%, respectively; for the ZnSO4 test they were 87.5% and 75%, respectively. When used with stored feces, both the IFA and Giardia-antigen assay were similar to each other and superior to ZnSO4. The false-negative and false-positive results with ZnSO4 may be attributed to the distortion of cysts during storage and the large number of yeast bodies that grow in the samples during storage.
COMMENTARY: Diagnosis of giardiasis is often difficult, as both false positives and false negatives occur with fecal flotation tests. Availability of an in-clinic test that doesn't rely on recognition of cysts will make diagnosis easier and more accurate.
Comparison of ZnSO4 centrifugation, a fecal antigen assay, and an immunofluorescent antibody assay for the diagnosis of giardiasis in cats. Lappin MR, Jensen WA, Taton-Allen G. ACVIM Proceedings, 2002, p 779.