ASPEN, COLO.—Rapid antigen detection tests have a high false-negative rate, and cannot be relied upon to diagnosis strep throat without a confirmatory throat culture, according to S. Michael Marcy, M.D.
“Many people are using antigen detection tests alone. This is not what is recommended yet,” he said, urging caution in adopting the new tests at a conference on pediatric infectious diseases sponsored by Children's Hospital, Denver.
Throat culture is still the preferred method, advised Dr. Marcy of the University of Southern California and the University of California, Los Angeles.
In nearly all cases, he said antibiotics should not be prescribed until group A streptococcal infection is confirmed. One exception would be a very sick child presenting with doughnut-like papules that have white centers. “These are diagnostic,” he said.
The Centers for Disease Control and Prevention and the American Academy of Pediatrics say antibiotics may be prescribed without a culture if an antigen detection test is positive, according to Dr. Marcy.
If it is negative, both recommend the results be confirmed by a throat culture.
“The problem with antigen detection tests, in my opinion, is unless you get the answer immediately, you don't have a huge advantage,” he said.
In practices where tests are processed in a batch, Dr. Marcy said the results typically arrive after the parent has taken the child home. Then the family has to be called back for the confirmatory culture or sent to the pharmacy.
In his own practice at Kaiser Foundation Hospital in Panorama City, Calif., Dr. Marcy said he does not bother with the rapid test. Instead, he does a culture if strep is suspected and the clinical signs do not strongly suggest a viral etiology.
While waiting for the results, he prescribes acetaminophen to prevent fever and pain. “I tell parents about preventing rather than chasing the symptoms,” he said, calling acetaminophen “as good as penicillin” during the wait.
He also posts a chart published that illustrates how long cold and flu symptoms, including sore throat, persist. The chart tells parents that these are viral illnesses for which antibiotics will not work.
“Parents look at it and say, 'I don't need to see you,' “he recounted, calling the graphic “very useful.”
Only about 20% of throat cultures are positive for strep, according to Dr. Marcy. He cited a Finnish study that found a viral infection in 42% of children with febrile exudative pharyngitis; no pathogen was detected in 37%. While 37% had bacterial infections, just 12% of pathogens were group A streptococci (Pediatrics 1987;80:6–12). Coinfections brought the total above 100%.
Current recommendations call for throat cultures to be done with two swabs, Dr. Marcy noted.
He warned that samples must be taken from the right and left tonsils. “If you only touch one side, you will get a false negative 30% of the time. Three separate papers show that. You must touch them both.”
If group A strep is confirmed, amoxicillin is the treatment of choice, Dr. Marcy said. He recommended prescribing 750 mg once a day for 10 days. “Compliance is better” than it is with the twice-a-day option, he said, dismissing controversy over the efficacy of cephalosporin vs. penicillin as dated.
“What needs to be done at this time is [a trial comparing] cephalosporin vs. amoxicillin. This has to be done,” he said.