Background: The first decision confronting clinicians in the management of patients with community acquired pneumonia (CAP) is whether the patient is to be hospitalized or not. We sought to validate the pneumonia scoring system and assess the power of procalcitonin (PCT) level to predict in-hospital mortality (IHM) and intensive vasopressor and respiratory support (IVRS) requirements in patients with CAP. Materials and Methods: A total of 120 patients with CAP were evaluated for severity of illness based on the defined scoring systems including pneumonia severity index (PSI), confusion, urea, respiratory rate, blood pressure, age>65 (CURB-65), confusion, respiratory rate, blood pressure, age>65 (CRB-65), infectious diseases society of America/American thoracic society 2007 criteria (IDSA/ATS 2007) and systolic blood pressure, multilobar infiltrate, albumin, respiratory rate, tachycardia, confusion, low oxygen, low pH (SMART-COP).Demographic, clinical, laboratory and radiographic data were collected prospectively. The accuracy of each scoring system in predicting IVRS requirement and IHM was assessed from the area under the receiver operating characteristic (ROC) curve (AUC). Level of PCT was determined by semi-quantitative PCT-Q method (BRAHMS). The accuracy of the defined scoring systems, PCT levels and each scoring system plus PCT levels in prediction of IHM and IVRS requirement was analyzed. Results: The accuracy of PCT levels in predicting IHM and IVRS requirement based on AUC was 0.542 and 0.658, respectively and the best threshold was ≥ 2ng/mL for both of them. Adding the level of procalcitonin to different scoring systems (based on the defined scoring systems) improved the accuracy of all systems. Conclusion: We do not suggest using the PCT level alone as a predictor for mortality and IVRS requirement. Instead, we suggest PSI plus PCT and IDSA/ATS 2007 plus PCT as accurate predictors for IHM and SMART-COP plus PCT for IVRS requirement in patients who presented with CAP. Key words: Hospital Mortality; Pneumonia; Procalcitonin; Severity of Illness Index

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