BACKGROUND AND OBJECTIVE: There are limited data on the relationship between the severity of community-acquired pneumonia (CAP) and biomarkers of inflammation and coagulation. The aim of this study was to evaluate the association between the severity of CAP and serum levels of antithrombin III (AT-III), protein C (P-C), D-dimers (D-D) and CRP, at hospital admission.METHODS: This was a prospective observational study in 77 adults (62.3% men), who were hospitalized for CAP. The severity of CAP was assessed using the confusion, uraemia, respiratory rate >or=30 breaths/min, low blood pressure, age >or=65 years (CURB-65) score.RESULTS: Forty patients (52%) had severe CAP (CURB-65 score 3-5). Serum levels of AT-III were lower and levels of D-D and CRP were higher in patients with severe CAP than in patients with mild CAP (CURB-65 score 0-2) (P < 0.001 for all comparisons). Levels of P-C were lower in patients with severe CAP compared with those with mild CAP, but the difference was not significant (P = 0.459). At a cut-off point of 85%, AT-III showed a sensitivity of 80% and a specificity of 75%, as a determinant of the need for hospitalization. At a cut-off point of 600 ng/mL, D-D showed a sensitivity of 90% and a specificity of 75% and at a cut-off point of 110 mg/L, CRP showed a sensitivity of 83% and a specificity of 79%, as determinants of the need for hospitalization.CONCLUSIONS: Serum levels of AT-III, D-D and CRP at admission appear to be useful biomarkers for assessing the severity of CAP.