Testing for Strep Throat Before Using Antibiotics

Family Pratice News,  May 15, 2000  by Miriam E. Tucker

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NEW YORK -- Management of pharyngitis is "extremely important and surprisingly controversial," Dr. Alan L. Bisno said at a meeting on infectious diseases sponsored by the Center for Bio-Medical Communication.

"It's a very common illness and we have effective treatment, said Dr. Bisno, professor and vice chair of the department of medicine at the University of Miami.

"Disturbingly, there is evidence that physicians are not using it in an optimal fashion," he added.

About 20% of acute pharyngitis is due to group A streptococcus (GAS), about 40%-45% is due to viruses, and about 5% is caused by other bacteria. A third are of unknown etiology.

Viruses such as adenovirus, Epstein-Barr virus, and herpes simplex virus can cause symptoms that mimic GAS-related acute pharyngitis, as can less common bacterial infections such as Neisseria gonorrhoeae or group C and G streptococcus, Dr. Bisno noted.

About 80%-85% of children and 95% of adults with acute pharyngitis don't have "strep throat," yet studies suggest that more than half of these patients end up with an antibiotic prescription, said Dr. Bisno, who chaired the Acute Pharyngitis Guideline Panel of the Infectious Diseases Society of America (Clin. Infect. Dis, 25[3]:574-83, 1997).

"The implications of blunderbuss treatment of this extremely common, largely nonstreptococcal illness on human microbial ecology require serious consideration," he commented.

The IDSA guidelines call for use of a positive throat culture or rapid antigen detection test (RADT) to confirm the diagnosis of GAS.

The guidelines also say that a negative RADT test should always be confirmed with a throat culture, but many people have questioned the need for this additional step, citing the low risk of acute rheumatic fever in the United States, Dr. Bisno noted.

The rationale for a backup throat culture is that although the specificity of RADTs is high, sensitivity ranges from 75%-95%, compared with 90%-95% for a throat culture, so some cases will be missed.

Skipping a backup throat culture might be justified in adults, since only about 5% will have GAS. But physicians who decide routinely not to use throat cultures should check with their local labs to make sure that the RADTs being used have a high sensitivity. These physicians also should commit to not treating patients with a negative rapid test; otherwise, the whole point of the test is missed, Dr. Bisno said.

Group A streptococcal pharyngitis typically appears in winter and early spring in children aged 5-15 years, although infection also can occur in younger children and adults, particularly those who are in contact with younger children.

These patients typically have acute onset of fever and pain on swallowing, with tonsillar and pharyngeal erythema. About half of them have exudate. Cervical adenitis and scarlatiniform rash are less common, he noted.

Clinical features that point away from a diagnosis of "strep throat" include the absence of fever or pharyngeal edema and the presence of conjunctivitis, cough, hoarseness, anterior stomatitis, diarrhea, or "common cold," all of which suggest a viral etiology.

But the clinical manifestations of streptococcal and nonstreptococcal pharyngitis overlap too much to allow accurate clinical diagnosis, Dr. Bisno added.

Treatment of strep throat also is controversial, with some people questioning the designation of penicillin as the treatment of choice. But the IDSA, the American Heart Association, and the American Academy of Pediatrics all recommend penicillin as first-line treatment, and erythromycin for penicillin-allergic patients. If compliance is a problem, intramuscular benzathine penicillin G is recommended.

Penicillin is safe, inexpensive, and effective in treating strep throat and preventing rheumatic fever. It is a narrow-spectrum agent that does not promote antimicrobial resistance, Dr. Bisno pointed out.

A 5-day regimen of the newer macrolide azithromycin has been approved for treatment of GAS pharyngitis. This regimen may be useful in penicillin-allergic patients, but "I worry that it's too easy to write a prescription for this, and it will become the treatment of choice not just for GAS but for all sore throats. That would be very dangerous. You get resistance a lot sooner to macrolides than to penicillin," he said.

Moreover, a cost comparison done in early 2000 showed that 5 days of azithromycin cost $54.08, compared with $507 for 10 days of penicillin V and $14.85 for 10 days of erythromycin.

"Penicillin is dearly the drug of choice," Dr. Bisno said.

As for patients with clinical failure and recurrent strep throat episodes, some are probably carriers, while others are experiencing reinfections. True "failures" are rare.

The IDSA guidelines recommend clindamycin for patients with multiple repeated culture-positive episodes of acute pharyngitis.

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