Tox-A Test for Clostridium difficile.

In recent articles (1, 4), the results of studies comparing several of the Clostridium difficile toxin enzyme-linked immunosorbent assays (ELISAs) with tissue culture assay were presented and points were made concerning the low specificity and high rate of indeterminate results with the Tox-A Test. We point out that the Tox-A Tests used in those studies were labeled "For Investigational Use Only" and that they are not the same test that has been approved for in vitro diagnostic use. The approved Tox-A Test exhibits higher specificity and fewer indeterminate results than those in these two articles. This statement is based on the results from a number of studies performed at various locations around the country. In our clinical trials, which were performed at six different locations and which involved the analysis of 1,130 specimens, the Tox-A Test exhibited a sensitivity and specificity of 91.9 and 97.4%, respectively, and the predictive positive and negative values were 84.5 and 98.7%, respectively, when the test was compared with tissue culture and/or toxigenic culture. The overall correlation of the Tox-A Test with tissue culture assay and/or toxigenic culture was 96.7%, and the indeterminate rate was 1.7%. In studies presented at the 93rd ASM General Meeting last year (2, 3), the Tox-A Test was compared with other C. difficile toxin ELISAs, including the Baxter EIA, the Analytab (Cytoclone) EIA, and the Premier test. The results showed that the Tox-A Test exhibited performance characteristics similar to those of the other ELISAs. Specificities of 98.3 and 100% were reported for the Tox-A Test in those studies. In one of the studies, indeterminate rates of 5.1, 0.5, and 2.6% were reported for Cytoclone, Premier, and the Tox-A Test, respectively. The results from a reference laboratory that routinely uses the Tox-A Test indicated a sensitivity of 80.3% and a specificity of 98.0% compared directly with tissue culture assay when used with more than 400 specimens. In addition, the predictive positive and negative values were 87.5 and 96.7%, respectively; the overall correlation was 95.4%; and the indeterminate rate was 3.0%.