Clinical information on the history and physical examination of persons presenting to the emergency departments of the Prince of Wales and United Christian Hospitals was abstracted from chart review by trained nursing and medical officers at each hospital. The reviewers were blinded to the final diagnosis and used a standardized protocol for abstracting information. Candidate variables, collected through standardized recording forms used by triage clinic personnel, included age (<18 years, 18 to 64 years, and ≥ 65 years), sex, health care worker (yes vs. no), contact history with patients with SARS (yes vs. no), and history of travel to other SARS-affected areas (as defined by the WHO at the time) within 2 weeks of symptom onset or the time of presentation to the triage clinic (yes vs. no). Additional variables were the presence of fever (defined as either a positive self-reported history or self-measurement at home or a tympanic temperature of at least 38 °C at triage), cough, sputum, dyspnea, sore throat, rhinorrhea, chills or rigor, myalgia, anorexia, malaise, diarrhea, vomiting, abdominal pain or headache (coded as dichotomized responses), pulse rate, systolic and diastolic blood pressure, respiratory rate, and oxygen saturation on room air. Investigations consisted of chest radiography (normal, haziness, or unilateral or bilateral pneumonic consolidation as interpreted by the emergency department physician) and simple hematologic and biochemical blood tests, including hemoglobin level, leukocyte count, absolute lymphocyte count, monocyte and neutrophil counts, and platelet count. Because we did not have sufficiently complete data (<60%) on some laboratory determinations for the entire sample (alkaline phosphatase level, alanine aminotransferase level, aspartate aminotransferase level, international normalized ratio, lactate dehydrogenase level, prothrombin time, serum albumin level, and total bilirubin level), we excluded these determinations from the analysis for all patients.