Clostridium difficile: Sending multiple stool specimens for diagnosis is often unnecessary

You suspect that your patient has Clostridium difficile infection (CDI). How many stool samples should you submit for testing?

Tests for CDI can be placed in one of two major categories – stool toxin and organism detection assays. Among the stool toxin assays are the cytotoxicity assay, which is considered to be the gold standard, PCR, and enzyme immunoassay (EIA). Common antigen testing and anaerobic culture are the two types of organism detection assays.

In most clinical laboratories, the diagnosis of CDI is based on a stool EIA detecting toxin A and/or B. Although the newer PCR-based test and the gold standard cytotoxicity assay are more sensitive and specific, EIA is preferred by laboratories because it is relatively simple to perform, less expensive, and associated with faster turnaround time.

While there is some debate about the optimal number of tests that should be ordered, there is no question that multiple tests are commonly ordered by healthcare professionals. In recent years, several studies have examined the diagnostic utility of repeat stool EIA tests when the initial stool sample test result is negative. In one study, a second test was ordered in 1,934 instances following an initial negative test result (Nemat). Only 95 (4.91%) of these tests were positive. In the same study, a third test was ordered in 793 instances following two negative test results. Only 24 (3.03%) were positive. The authors wrote that their “findings strongly support the utility of limiting the workup of suspected CDI to a single stool test with only repeat testing in cases of high clinical suspicion, and avoiding the routine ordering of multiple stool samples.”

In a recent retrospective study of over 10,000 patients, Dr. Joe Dylweski found that repeat testing for C. difficile, was also very common. His research involved testing using the cytotoxicity assay, which has a longer turnaround time when compared to EIA. Repeating the cytotoxicity assay following an initial negative test result was found to of very low yield.

Dr. Dylweski also noted that healthcare professionals commonly ordered repeat tests even before the results of the initial test were known. “The order is ‘C. diff times three’. So the patient has three bowel movements on the same day, and they send all three samples down on the same day.” He urged clinicians to wait until the results of the initial test become available before repeating the test (Finn).

References

Nemat H, Khan R, Ashraf M, Matta M, Ahmed S, Edwards B, Hussain R, Lesser M, Pekmezaris R, Dlugacz Y, Wolf-Klein G. Diagnostic value of repeated serum immnoassays in Clostridium difficile infection. Am J Gastroenterol 2009; 104(8): 2035-41.

Finn R. Early repeat C. difficile testing rarely useful. Hospitalist News 2009; (12): 10.


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