Improved diagnostic accuracy of group A streptococcal pharyngitis with use of real-time biosurveillance

Abstract

Background: Clinical prediction rules do not incorporate real-time incidence data to adjust estimates of disease risk in symptomatic patients.

Objective: To measure the value of integrating local incidence data into a clinical decision rule for diagnosing group A streptococcal (GAS) pharyngitis in patients aged 15 years or older.

Design: Retrospective analysis of clinical and biosurveillance predictors of GAS pharyngitis.

Setting: Large U.S.-based retail health chain.

Patients: 82 062 patient visits for pharyngitis.

Measurements: Accuracy of the Centor score was compared with that of a biosurveillance-responsive score, which was essentially an adjusted Centor score based on real-time GAS pharyngitis information from the 14 days before a patient's visit: the recent local proportion positive (RLPP).

Results: Increased RLPP correlated with the likelihood of GAS pharyngitis (r(2) = 0.79; P < 0.001). Local incidence data enhanced diagnostic models. For example, when the RLPP was greater than 0.30, managing patients with Centor scores of 1 as if the scores were 2 would identify 62, 537 previously missed patients annually while misclassifying 18, 446 patients without GAS pharyngitis. Decreasing the score of patients with Centor values of 3 by 1 point for an RLPP less than 0.20 would spare unnecessary antibiotics for 166, 616 patients while missing 18, 812 true-positive cases.

Limitations: Analyses were conducted retrospectively. Real-time regional data on GAS pharyngitis are generally not yet available to clinicians.

Conclusion: Incorporating live biosurveillance data into clinical guidelines for GAS pharyngitis and other communicable diseases should be considered for reducing missed cases when the contemporaneous incidence is elevated and for sparing unnecessary antibiotics when the contemporaneous incidence is low. Delivering epidemiologic data to the point of care will enable the use of real-time pretest probabilities in medical decision making.

Appendix Figure 3. Proportion of patients in the Validation set (n=27,081) testing positive for Group A Streptococcal pharyngitis by recent local proportion positive (RLPP) and grouped and labeled by Centor score

Each line represents patients with the same Centor score across varying RLPP. The numbers at the far right side of the graph identify the Centor score for each line. The proportion of patients who tested positive increases both as the clinical score increases, and as the RLPP increases. Pearson’s coefficient was used to measure strength of correlation. The r², representing the proportion of the variation in GAS pharyngitis positivity that can be attributed to the RLPP, is 0.33, (p=0.012) for Centor 4, 0.70 (p<0.001) for Centor 3, 0.82 (p<0.001) for Centor 2, 0.68 (p<0.001) for Centor 1, and 0.35 (p=0.005) for Centor 0. The slopes of the lines for Centor 4, 3, 2, 1, and 0 are 0.99, 0.80, 0.75, 0.43 and 0.28. Each data point represents a median of 140 patients (range 45–555 patients, interquartile range 82–290).

Figure 1. Proportion positive by study week for nine different locations

The x axis is the study week from January 1, 2007– December 31, 2007, and the y axis is weekly proportion positive for group A streptococcal pharyngitis. Each graph shows the proportion of patients who tested positive each week in one of nine markets. The axes have been standardized to allow comparison across markets. The horizontal line is the average across all markets (0.25), and is provided for reference and to facilitate comparison.

Figure 2. Proportion of patients testing positive for Group A Streptococcal pharyngitis by recent local proportion positive (RLPP) and grouped and labeled by Centor score

Each line represents patients with the same Centor score across varying RLPP. The numbers at the far right side of the graph identify the Centor score for each line. The proportion of patients who tested positive increases both as the clinical score increases, and as the RLPP increases. Pearson’s coefficient was used to measure strength of correlation. The r², representing the proportion of the variation in GAS pharyngitis positivity that can be attributed to the RLPP, is 0.81, (p<0.001) for Centor 4, 0.86 (p<0.001) for Centor 3, 0.86 (p<0.001) for Centor 2, 0.70 (p<0.001) for Centor 1, and 0.47 (p<0.001) for Centor 0. The slopes of the lines for Centor 4, 3, 2, 1, and 0 are 1.21, 0.88, 0.64, 0.37 and 0.29. Each data point represents a median of 223 patients (range 41–1152 patients, interquartile range 115–518).