University of Michigan
Department of Pediatrics
Evidence-Based Pediatrics Web Site

Question

• A 6 year old boy presents to the office in January with a sore throat and fever. You do a rapid strep test which is negative. Do you need to send the throat swab to the lab for culture confirmation? In other words, in children with signs/symptoms of streptococcal pharyngitis, is culture confirmation of a negative rapid strep test cost-effective in preventing strep complications?

Clinical Bottom Lines

1. "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." 1
2. Exceptions to #1 include conditions in which there is a low probability of strep infection (e.g., summer months), the use of an antigen test with inferior sensitivity to that of culture (<0.82), and during an epidemic of acute rheumatic fever.
3. Culture confirmation of negative rapid strep tests does prevent more strep complications than use of a single diagnostic test.

Summary of Key Evidence

1. Key article is a medical decision analysis utilizing newest sensitivity/specificity data of rapid strep test optical immunoassays (OIA) and incidence of rheumatic fever to determine cost-effectiveness of different diagnostic strategies: treat all vs. culture vs. OIA test vs. OIA with culture confirmation.
2. Current OIAs have higher sensitivity than original rapid strep tests (77-97% vs 50-70%). Based on weighted average of 13 published studies, the performance of the "gold standard" (blood agar culture, sensitivity 83.4%) now appears no better/equivalent to high sensitivity immunoassays (sensitivity 89.1%)
3. Sensitivity analysis finds the decision "sensitive" to rapid test performance, prevalence of GABHS (summer vs. winter), and incidence of rheumatic fever.
4. At a probability of acute rheumatic fever of 0.58% (580/100,000), OIA plus culture is more cost-effective than OIA alone; at a probability of ARF of 0.16%, OIA plus culture is more cost-effective than culture.
5. At a sensitivity of OIA of <82%, culture alone is more cost-effective.
6. At a cost of antibiotics of $15.05, culture is more cost-effective than OIA.

Additional Comments

• Antibiotic therapy has been shown to shorten the clinical course, reduce the rate of transmission and prevent complications of GABHS such as peritonsillar abscess and acute rheumatic fever.1
• There is a wide variability in the published sensitivities for rapid strep OIA; thus generalizing to your specific patient population could change the analysis.
• At the University of Michigan, the cost for throat culture is $17 and for a rapid strep test, $18. The authors in this study did not report the costs used for their analysis.
• Cost-effectiveness is the utility in this analysis (i.e. public health perspective); the value to the individual patient is not incorporated (e.g. if patients have insurance, cost may not be an issue to them).
• 2000 Red Book comments: "Rapid diagnostic tests using new techniques, such as optical immunoassay and chemiluminescent DNA probes, have been developed. Published data suggest that these tests may be as sensitive as standard throat cultures on sheep blood agar and more sensitive than other rapid tests for GAS. Some experts believe that the optical immunoassay test is sufficiently sensitive to be used without throat culture backup. Physicians who use this test without culture backup may wish to compare their results with those of culture to validate adequate sensitivity in their practice."2

Citation

1. Webb KH. Does Culture Confirmation of High-sensitivity Rapid Streptococcal Tests Make Sense? A Medical Decision Analysis. Pediatrics 1998; 101:1-10.
2. 2000 Red Book: Report of the Committee on Infectious Diseases, 25th edition.
3. Gelehrter S. Clinical Scoring Systems Can Help in the Diagnosis of Strep Pharyngitis. CAT, Peds EBM Site.

CAT Author: Amy E. Cooke, MD

CAT Appraisers: Robert E. Schumacher, MD

Date appraised: October 30, 2000