Background There is almost no data about the influence of antimicrobial pre-treatment (APT) on levels of inflammatory markers in community acquired pneumonia (CAP). The aim of this study was to investigate the influence of APT on inflammatory markers in CAP. Methods 991 hospitalized patients (64.3 ± 17.6 years, 61% male) with CAP were enrolled. In all patients procalcitonin (PCT), C-reactive protein (CRP), and leukocyte count (WBC) were determined. Patients were followed-up for 28 days for survival. Results 232 patients (23.4%) had APT, 759 had no APT. Patients without APT had significantly higher levels of PCT and WBC but not of CRP compared to those with APT. In patients without APT, survivors compared to non-survivors had lower values of PCT (0.20 ng/mL; 0.02–169.10 vs. 0.83 ng/mL; 0.04–516.30, p 9 /L; 1.3–49.9 vs. 14.9 × 10 9 /L; 3.7–34.5, p = 0.047) and CRP (107.0 mg/mL; 0.3–567.0 vs. 143.5 mg/mL; 5.0–589.0, p = 0.006). However, in patients with APT, the values of PCT, WBC and CRP were not significantly different in survivors and non-survivors. Cox regression analysis confirmed that PCT, CRP and WBC were predictive for 28 day mortality in patients without APT but not in those with APT. Conclusions PCT and WBC but not CRP levels are higher in patients without APT compared to those with APT. PCT, CRP and WBC are predictive for 28 days mortality exclusively in patients without APT. Interpretation of inflammatory parameters has to take into account possible APT.
