A commercially available enzyme immunoassay for the diagnosis of giardiasis was evaluated in a clinical trial. The ProSpecT/Giardia diagnostic test (Alexon, Inc., Mountain View, Calif.) was compared with the standard ova and parasite (O&P) microscopic examination. Additionally, several widely used stool fixatives and a commonly used transport medium were assessed for compatibility with the immunoassay. A total of 325 stool specimens were collected and used to evaluate assay performance. Of those, 93 specimens were collected from symptomatic Giardia O&P-positive patients and 232 specimens were randomly collected from patients as part of a routine health screening procedure. All 93 Giardia O&P-positive stool specimens were strongly positive by visual and spectrophotometric examination using the immunoassay. Of the 232 randomly collected specimens, 16 were positive by O&P examination and immunoassay, 6 were negative by O&P examination but positive by immunoassay, and 1 was positive by O&P examination and negative by immunoassay. There was substantial supportive evidence that indicated that the six immunoassay-positive, O&P-negative specimens were true-positives. When these six specimens were accepted as true-positives, the immunoassay detected almost 30% more cases of Giardia infection than did O&P examination. Its sensitivity and specificity were 96 and 100%, respectively, while the sensitivity and specificity of O&P examination were 74 and 100%, respectively. The immunoassay also performed well on specimens treated with 10% neutral Formalin, sodium acetate-Formalin fixative, and Cary-Blair transport medium. However, the test was not compatible with polyvinyl alcohol-treated specimens. Overall, the ProSpecT/Giardia test was a sensitive, specific immunoassay which was easy to run and interpret. It offers a simple solution to traditional difficulties encountered in diagnosing Giardia infection.

A Giardia lamblia-specific antigen (GSA 65) was isolated from stools of G. lamblia-positive patients by crossed-and line-immunoelectrophoresis and counterimmunoelectrophoresis (CIE) in agarose by using rabbit antiserum prepared against G. lamblia cysts. CIE with rabbit anti-GSA 65 monospecific antiserum revealed that GSA 65 was present in aqueous stool eluates of giardiasis patients and in cysts and trophozoites of the parasite. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of immunoaffinity-purified antigen followed by Western blotting showed that the molecular weight of this molecule was about 65,000. GSA 65 was detectable by CIE in stool eluates of 36 of 40 giardiasis patients but not in eluates of 10 G. lamblia-negative asymptomatic controls. GSA 65 was detected in stool eluates of 2 of 18 individuals with chronic diarrhea who were negative for parasites by microscopic examination. Cross-specificity studies with other genera of parasitic protozoa performed by using CIE and immunofluorescence indicated that GSA 65 was present only in strains of G. lamblia. Based on these findings, GSA 65 may prove to have an important application in the design of sensitive diagnostic tests for giardiasis.

We recently reported the isolation and identification of a Giardia lamblia-specific antigen (GSA 65) that is shed in the stool of giardiasis patients. In the present study, this antigen was affinity purified from sonic extracts of axenically cultured G. lamblia trophozoites and characterized to better understand its biological function and its potential usefulness in the design of coprodiagnostic assays for giardiasis. GSA 65 was resistant to proteolytic digestion with trypsin, chymotrypsin, and protease but was sensitive to treatment with NaIO4 as assessed by Western blotting. This antigen was also stable during prolonged storage at 4 and-20°C in 10% Formalin or distilled H2O as assessed by counterimmunoelectrophoresis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing gel banding patterns, in conjunction with protein and carbohydrate assays and lectin binding studies, confirmed that this antigen is a highly glycosylated glycoprotein. The resistance of GSA 65 to proteolytic degradation, together with previous immunofluorescence data that indicate the antigen is an integral part of the G. lamblia cyst wall, suggests that this molecule may play a role in maintaining the integrity of the cyst in vivo. The ability of GSA 65 to maintain its antigenic structure under a wide variety of conditions makes it an ideal antigen around which to design sensitive immunodiagnostic assays for giardiasis.

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