CAMBRIDGE, MASS. — Backup throat cultures following negative rapid group A strep detection tests may no longer be necessary in some practices, Michael Pichichero, M.D., said at a conference on pediatric infectious diseases.
To determine whether the standard procedure can be discontinued, clinicians can undertake a simple investigation to assess the sensitivity of commercial rapid immunochemical antigen test kits as they are used in their practices, Dr. Pichichero suggested at the conference, sponsored by Boston University, PEDIATRIC NEWS, and FAMILY PRACTICE NEWS.
“In the early 1990s, published studies evaluating the accuracy of rapid strep tests, compared with throat culture, suggested the rapid tests had really good specificity, but the sensitivity in the clinical setting of office-based practices wasn't there,” said Dr. Pichichero of the University of Rochester (N.Y.) Medical Center.
Because of the sensitivity limitations, the American Academy of Pediatrics recommended in its 2000 Red Book—and continues to recommend—that all negative rapid diagnostic tests for group A strep pharyngitis be backed up by throat culture.
Because of the very high specificity of the rapid tests, a positive test result does not require throat culture confirmation, noted Dr. Pichichero, who is also in private practice in Rochester.
Since the early 1990s, the rapid strep tests have matured and new products have emerged, “and at some point it seemed to us that the sensitivity was getting better too,” Dr. Pichichero said.
To test this hypothesis, Dr. Pichichero and his colleagues conducted a retrospective analysis reviewing 11,427 of the rapid group A strep tests performed in their practice between January 1996 and June 1999.
Of the 11,427 tests, 8,385 were negative and 3,042 were positive. Patients testing positive began antibiotic treatment immediately. Nearly all of the patients testing negative—8,234 of 8,385, or 98%—received backup cultures.
“Only 2% of the patients with negative rapid tests cultured positive on follow-up. You can't expect much better sensitivity than that, even with throat culture,” Dr. Pichichero said.
The findings cannot necessarily be generalized across practices, however. “There are some important considerations. First, you have to know who you're culturing. We only culture patients who we think have strep. If you culture patients with upper respiratory infections, for example, you're going to have more issues,” Dr. Pichichero said. “The other important thing is, you have got to use a good test, and a good test is probably not one that costs less than a dollar. You're going to have to spend about $1.50-$2.00—well worth the expense if the return is good sensitivity and a reduction in the costs associated with cultures.”
One way to gauge the performance of rapid strep tests is “to look at your own practice,” he suggested. “Take a look at the last hundred negative rapid tests performed in your office. If the follow-up cultures for 90 or more of them are negative, that should tell you that, for the patients you're testing and the way you're using the tests, you can probably stop the follow-up cultures.”
'First you have to know who you are culturing. We only culture patients who we think have strep.' DR. PICHICHERO