Background/Purpose: Few studies have addressed the predictive value of white blood cells (WBCs) and C-reactive protein (CRP) at different cutoff values in appendicitis. Our purpose was to determine the cutoff values for WBC and CRP at different periods during clinical evolution of appendicitis and to establish their use for the diagnosis of appendicitis and differentiation of simple from perforated appendicitis. Methods: We studied 198 patients operated on for appendicitis, which were further divided into 4 subgroups according to the time from the onset of symptoms to diagnosis. Receiver operating characteristic curves were constructed for CRP and WBC; the best cutoff points were used to calculate the sensitivity and specificity to discriminate patients with and without appendicitis and patients with simple and perforated appendicitis. Results: White blood cell and CRP individually and together had a high sensitivity to differentiate patients with and without appendicitis. The specificity of WBC and CRP taken individually and together to differentiate patients with simple and perforated appendicitis was high, but the sensitivity was low. Conclusions: White blood cell and CRP could be used to support the clinical diagnosis of appendicitis, and, depending on the time from the onset of symptoms to diagnosis, to differentiate patients with and without appendicitis and discriminate simple from perforated appendicitis. (c) 2007 Elsevier Inc. All rights reserved.