Background and aims Diagnosis of acute appendicitis is not always straightforward; even the most experienced surgeon may remove normal appendix or sit on the perforated one. The aim of this study is to investigate the role of preoperative laboratory tests in the prediction of complicated appendicitis (CA). Patients and methods A total of 126 patients presented with right iliac fossa pain (appendicitis) were included between March 2018 and January 2019 in the Department of General Surgery, Zagazig University Hospitals. All patients underwent blood sample test, surgical exploration, and management according to operative findings. Results According to the operative and histopathological findings, the participants were divided into group A which included 72 (57%) patients with non-CA and group B which included 54 (43%) patients with CA. The preoperative laboratory predictors such as white blood cells (WBCs), international normalized ratio (INR), serum Creactive protein (CRP), and total bilirubin (TB) were highly significantly higher in group B (P<0.0001) than in group A. Serum WBCs and CRP showed the highest sensitivity for defining cases with CA followed by INR and TB (92.6, 92.6, 83.4, 68.75%), respectively, while serum TB showed the highest specificity for defining cases with CA followed by INR, CRP, and WBCs (89.7, 74.5, 67.8, 67.8%), respectively. Conclusion The diagnostic accuracy of WBCs, INR, TB, and CRP were between 68% and up to 93% indicating that these preoperative laboratory tests were valid for early detection of CA. Further studies evaluating these laboratory predictors are recommended.