Does Culture Confirmation of High-sensitivity Rapid Streptococcal Tests Make Sense? A Medical Decision Analysis
Authors: Webb Kenneth H. 
Published: August 2004

Objective. Since the 1990 publication of a decision analysis, in which the treatment of pharyngitis in children was evaluated, a number of assumptions important in that analysis have changed. Updating many of the assumptions and costs used in that analysis to reflect the conditions currently found in a large, suburban pediatric practice, a cost-effectiveness analysis was performed in which four strategies for the treatment of pharyngitis were considered: treat all, high-sensitivity antigen test, culture, and high-sensitivity antigen test with culture confirmation.

Design. Decision analysis.

Results. Using microbiology data from the 13 published studies in which a high-sensitivity antigen test (Strep A OIA; BioStar Inc., Boulder, CO) and blood agar plate culture were evaluated against a variety of gold standards, the sensitivity and specificity of the high-sensitivity antigen test were 89.1% and 95%, respectively. The sensitivity and specificity of blood agar plate throat culture were 83.4% and 99%, respectively. Penicillin V was used as the treatment of choice for uncomplicated pharyngitis; erythromycin was used in cases of penicillin allergy. Rates of suppurative and nonsuppurative complications reflect those currently seen in the United States. Other assumptions and cost data were taken from a large, suburban pediatric practice and its affiliated tertiary care medical center, except where noted.

Despite the potential induction of resistance and the high number of allergic reactions associated with the treat-all strategy, this strategy had the lowest average cost per patient encounter and was the most cost-effective in terms of dollars per suppurative and nonsuppurative complication prevented. Of the strategies in which a diagnostic test was used, the high-sensitivity antigen test strategy had the lowest average cost and was the most cost-effective. The high-sensitivity antigen test with culture confirmation strategy had the highest average cost and was the least cost-effective.

In the sensitivity analyses, a number of assumptions used in the original model were varied within a reasonable range. Under most conditions, the treat-all strategy remained the most cost-effective strategy used. One notable exception: when the wholesale cost of the antibiotic exceeded $10.76, as would be seen if any cephalosporin were used as the primary therapy of uncomplicated pharyngitis, the high-sensitivity antigen test strategy became the most cost-effective strategy.

Under most conditions, the high-sensitivity antigen test strategy was the most cost-effective of the strategies in which a diagnostic test was used. Notable exceptions included: 1) conditions in which there was a low probability of streptoccal infection, 2) the use of an antigen test whose sensitivity is inferior to that of culture, and 3) during an epidemic of acute rheumatic fever.

Culture confirmation of a negative high-sensitivity antigen test is the most cost-effective testing strategy only under conditions in which the probability of acute rheumatic fever approaches those levels last seen in the United States more than 40 years ago.

Conclusions. Although most cost-effective, the treat-all strategy is not recommended because of concerns about antibiotic resistance, which could not be included in the model, and the high number of allergic reactions found in children who did not have streptococcal infection. Use of the high-sensitivity antigen test without culture confirmation of all negative results was the most cost-effective strategy in which a diagnostic test was used with respect to prevention of suppurative and nonsuppurative complications of streptococcal pharyngitis. Culture confirmation of negative high-sensitivity antigen tests was not cost-effective under any of those conditions currently seen in the United States.