This publication represents the first part of an update to the clinical practice guideline on the diagnosis and management of group A streptococcal (Streptococcus pyogenes or GAS) pharyngitis, developed by the Infectious Diseases Society of America (IDSA). Diagnosis of GAS pharyngitis by clinician judgment alone is unreliable, and unselective testing incurs cost and inconvenience for individuals at low risk of having GAS infection. Clinical scoring systems have been used to quantify the probability of a positive GAS throat culture based on standardized criteria such as the presence of fever; tonsillar enlargement or exudate; tender and enlarged anterior cervical lymph nodes; and the absence of cough. The goal of this article is to determine whether a scoring system should be used to decide which patients should have a diagnostic test performed by rapid antigen detection test, molecular methods, and/or throat culture. We performed a systematic review of randomized and nonrandomized studies that compared the use of a clinical scoring system to clinician judgment alone in predicting the outcome of a throat culture. Evidence from studies in children and adults suggests the diagnostic accuracy of a clinical scoring system is comparable to or slightly higher than clinician judgment alone. However, the studies are limited due to small size, lack of uniformity in outcome measures, and incomplete data. The consensus of the panel is that the balance of benefits and harms favors use of a clinical scoring system as part of the evaluation of patients with sore throat. The principal utility of using a scoring system is to identify patients with low probability of GAS pharyngitis and to reduce unnecessary testing.