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. Dr. Marcy was speaking 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 to that approach would be in the case of a very sick child presenting with doughnut-like papules that have white centers. “These are diagnostic,” Dr. Marcy explained.
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 pediatric 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 medical practice at Kaiser Foundation Hospital in Panorama City, Calif., Dr. Marcy said he does not bother giving the rapid test at all. 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 from the throat culture, Dr. Marcy prescribes Tylenol to prevent fever and pain. “I tell parents about preventing rather than chasing the symptoms,” he said, calling Tylenol “as good as penicillin” during the wait.
He also posts a chart published in this newspaper in June 2002 that illustrates how long cold and flu symptoms, including sore throat, persist. The chart explains that these are viral illnesses for which antibiotics will not work.
“Parents look at it and say, 'I don't need to see you,'” Dr. Marcy recounted, calling the chart on cold and flu symptoms “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 physicians to take throat cultures with two swabs, Dr. Marcy noted. He further explained that the samples must be taken from the patient's 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 5 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 versus amoxicillin. This has to be done.”
