Development and validation of a simple, two-step algorithm to identify streptococcal infection in adults with sore throat

Abstract

Objectives: Development and validation of a clinical algorithm to identify infection with group A β-haemolytic streptococci in patients with sore throat, using a clinical score and measurement of C-reactive protein for selected patients.

Design: Two phased study. Phase 1: Development of a clinical score from structured clinical records by logistic regression analysis and Bayesian analysis. Phase 2: Prospective validation of the score developed in phase 1.

Setting: One GP practice in Germany (two GPs) for development of the score, five German GP practices (six GPs) for validation of the score.

Subjects: Consecutive patients aged 16 or older presenting with newly developed sore throat, n=116 (phase 1), n=265 (phase 2).

Main outcome measures: Infection by group A β-haemolytic streptococci diagnosed by means of throat swab cultures on selective Columbia agar.

Results: Development of a two-step algorithm consisting of a clinical score of four parameters (throat mucosa, uvula, soft palate, tonsils) and near-patient quantitative measurement of C-reactive protein for selected patients with an ambiguous score. In the validation phase the algorithm has a diagnostic accuracy of 89.4%, a sensitivity of 0.74 (95% confidence interval (CI)=0.53-0.89), a specificity of 0.95 (95% CI=0.88-0.99), positive predictive value of 0.86 (95% CI=0.65-0.97) and a negative predictive value of 0.91 (95% CI=0.81-0.96). To achieve the accuracy, a measurement of C-reactive protein is necessary in 30% of patients with sore throat.

Conclusions: The algorithm, consisting of a clinical score and selective measurement of C-reactive protein, achieves a diagnostic accuracy of almost 90% in diagnosing infection by group A streptococci in adults with sore throat.

Key Words:

• sore throat
• diagnostic algorithm
• group A β-haemolytic streptococci
• CRP measurement
• pharyngitis