ObjectiveTo determine the value of procalcitonin (ProCT) as a marker of infection in critically ill patients.DesignProspective, observational study.SettingMedicosurgical department of intensive care (31 beds).PatientsOne hundred eleven infected and 79 noninfected patients.InterventionsNone.Measurements and Main ResultsProCT and C-reactive protein (CRP) concentrations were monitored daily. The best cutoff values for ProCT and CRP were 0.6 ng/mL and 7.9 mg/dL, respectively. Compared with CRP, ProCT had a lower sensitivity (67.6 vs. 71.8), specificity (61.3 vs. 66.6), and area under the receiver operating characteristic curve (0.66 vs. 0.78, p < .05). The combination of ProCT and CRP increased the specificity for infection to 82.3%. In the infected patients, plasma ProCT, but not CRP, values were higher in nonsurvivors than in survivors. Infected patients with bacteremia had higher ProCT concentrations than those without bacteremia, but similar CRP concentrations. ProCT levels were particularly high in septic shock patients.ConclusionsProCT is not a better marker of infection than CRP in critically ill patients, but it can represent a useful adjunctive parameter to identify infection and is a useful marker of the severity of infection. (Crit Care Med 1999; 27:498-504)