['We determine whether clinical characteristics and physician assessment are useful in the exclusion of spontaneous bacterial peritonitis in emergency department (ED) patients with ascites requiring paracentesis.', 'We conducted a prospective, observational study of ED patients with ascites undergoing paracentesis. Predefined clinical characteristics including historical features and ED vital signs were recorded. Each patient was assessed by 2 separate, blinded physicians for severity of abdominal tenderness and overall clinical suspicion for spontaneous bacterial peritonitis. The primary outcome measures were sensitivity, specificity, and likelihood ratios (LR) of the individual clinical characteristics and the physician assessments. Spontaneous bacterial peritonitis was defined by absolute neutrophil count greater than 250 cells/mm(3) or positive fluid culture result.', 'There were 285 separate physician assessments in 144 patients enrolled with complete data. Spontaneous bacterial peritonitis was diagnosed in 17 (11.8%) patients. Physician clinical impression had a sensitivity of 76% (95% confidence interval [CI] 62% to 91%) and specificity of 34% (95% CI 28% to 40%) for the detection of spontaneous bacterial peritonitis. The lowest negative LR was associated with the presence of any abdominal pain or tenderness (negative LR=0.4); however, the presence of pain/tenderness was also observed in 85% of patients without spontaneous bacterial peritonitis. Six patients (4.2%) with spontaneous bacterial peritonitis had at least 1 physician assessment of little to no risk for spontaneous bacterial peritonitis, and 3 of the 6 subsequently died during their hospitalization.'] Does physician clinical impression rule out spontaneous bacterial peritonitis in patients undergoing emergency department paracentesis?