Background. Acute appendicitis (AA) is the most common acute surgical condition of the abdomen, and the most commonly misdiagnosed. Objective. To analyse the white blood cell count (WBCC) and C-reactive protein (CRP) contribution to the diagnosis of AA in children.Methods. This was a retrospective study of 943 consecutive patients operated on with the preoperative diagnosis of AA, in whom preoperative WBCC and CRP had both been measured. Postoperatively, the patients were divided into three groups: normal appendix (no AA), simple AA and complicated AA. Results. Of the 943 patients, 616 (65.3%) had simple AA. The mean (standard deviation (SD)) age for this group was 9.8 (3.2) years (p<0.01 v. complicated AA), the mean WBCC was 16.5 (5.0) × 10 9 /L (p<0.01 v. complicated AA and no AA), and the mean CRP level was 304.8 (409.5) nmol/L (p<0.01 v. complicated AA). The mean age of the patients with complicated AA (283/943, 30.0%) was 7.9 (3.7) years, the mean WBCC was 17.7 (6.2) × 10 9 /L (p<0.01 v. no AA) and the mean CRP level was 1 076.2 (923.8) nmol/L (p<0.01 v. no AA). The mean age of the patients with no AA (44/943, 4.7%) was 8.8 (3.2) years, the mean WBCC was 13.1 (5.3) × 10 9 /L and the mean CRP was 361.9 (447.6) nmol/L. The WBCC was normal in 113/899 patients with appendicitis (12.6%) and CRP in 139 (15.5%). Both the WBCC and CRP were normal in 17 patients with appendicitis (1.9%). The best receiver operating characteristic (ROC) curve was obtained for WBCC when comparing all AA with no AA: cut-off point 15.0 × 10 9 /L, sensitivity 65%, specificity 68%, area under the curve 0.70. The best ROC curve for CRP was obtained when comparing simple AA with complicated AA: cut-off point 361.9 nmol/L, sensitivity 74%, specificity 74%, area under the curve 0.81. Conclusions. The WBCC is helpful in diagnosing simple AA and CRP in diagnosing complicated AA. If both are normal, AA is very unlikely. Together the WBCC and CRP are useful tools in diagnosing and staging AA. RESEARCHPostoperatively, all patients included in the study were divided into three groups according to the intraoperative macroscopic findings, as follows:• Normal appendix (no AA). All patients operated on with the preoperative diagnosis of AA but in whom the appendix was found to be macroscopically normal. The findings on histological analysis of the appendix in this group were recorded. • Simple AA. All non-perforated AA.• Complicated AA. Perforated appendix, peritonitis, appendicular abscess and appendicular mass. Statistical analysisMeans (standard deviations (SDs)) were used to describe the numerical variables. Statistical analysis was performed with parametric tests (Student's t-test or analysis of variance). A cross-sectional analysis of the diagnostic tests was performed where sensitivity, specificity and likelihood ratios were calculated. Receiver operating characteristic (ROC) curves were also elaborated for each test and prediction, calculating the area under the curve with a confidence interval of 95%. The level of significance was set at p<0....

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