June 1997
CHICAGO — A new type of rapid strep test may be as sensitive as cultures taken in an office setting, a recent study showed, which means this test may not need culture confirmation.
There are many antigen detection (rapid) tests to confirm a clinical diagnosis of group A streptococcal (GAS) pharyngitis, but these tests are not sensitive enough to exclude GAS. In the office setting they may be used to help decide whether to begin antibiotics, but the American Academy of Pediatrics recommends that negative results be confirmed by culture. A positive is positive, but a negative result may be a false negative.
The blinded study was conducted in six pediatric offices in Connecticut and Chicago using 2,113 patients with acute pharyngitis. The three Connecticut offices collected throat swabs from 983 patients whose ages ranged from 1 to 51 years old; and the three Chicago offices collected swabs from 1,130 patients ranging in age from 1 to 38 years old.
Forty-seven percent of the Connecticut patients and 53% of the Chicago patients were male. Researchers said most of the older patients were parents of children brought in for sore throat evaluation.
In Connecticut, 350 of the 983 patients examined had GAS in their pharynx; while 655 of the Chicago patients had GAS isolates. Among the patients tested in Connecticut and Chicago, the streptococcal A optical immunoassay (OIA) test — the rapid test used — was found to be significantly more sensitive but significantly less specific than the blood agar plate cultures in picking up GAS.
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The study found that among patients in Connecticut, the sensitivities of the OIA test and cultures were 94% and 89%, respectively. Among patients in Chicago, the sensitivities of the two tests were 79% for OIA vs. 72% for the cultures. In each of the six pediatric offices, the OIA test was more sensitive than the culture. Combining the data from Connecticut and Chicago, the overall sensitivities of the OIA test and cultures were 84% and 78%, respectively.
"The incremental increase in positive test results and the identification of patients with GAS pharyngitis would have been substantially greater if negative cultures had been confirmed with OIA testing than if the negative OIA tests had been confirmed with cultures," said Michael Gerber, MD, of the University of Connecticut's School of Medicine and one of the paper's authors. "Because the sensitivity of the office culture is considered sufficient for use alone in the diagnosis of GAS pharyngitis, it follows that another test with sensitivity equivalent to the office culture and with adequate specificity should be equally acceptable when used alone for this diagnosis."
Previous evaluations of the accuracy of OIA were carried out, primarily in diagnostic microbiology laboratories rather than physicians' offices, which set this study apart. In the labs, there was considerable variability in the criterion standard used and in the manner in which the cultures were performed. Consequently, in some studies the sensitivity of the OIA was at least as high as that of culture and in others the sensitivity was substantially less than that of the culture. Also, the OIA has been compared directly with other antigen detection tests and has consistently been found to be significantly more sensitive.
The results of this large-scaled, office-based investigation indicate that the sensitivity of the OIA test was at least equivalent to that of the routine cultures. It also points out the influence that variations in laboratory methods and criterion standards have on the evaluation of GAS tests.
Because OIA is more expensive, labor-intensive and time consuming than many rapid strep tests, it may not be practical for every office setting. However, these findings suggest that with properly trained personnel, negative OIA test results may not always need to be routinely confirmed with cultures, which in turn could save time and expense. The study results were reported in The Journal of the American Medical Association.
For more information:
- Gerber M, Tanz A, Robert R et al. Optical immunoassay test for group A b-hemolytic streptococcal pharyngitis. JAMA.1997;46:899-903.
Editor's Note: This further confuses the issue of which is the preferred test and whether culture is necessary as a back-up. We need the advice of an advisory committee prior to changing current policy. Culture is still the "gold standard." — P. Brunell
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