Acute appendicitis is one of the most common causes of acute abdominal diseases seen between the ages of 10 and 19, mostly seen in males. The lifetime risk of developing acute appendicitis is 8.6% for males and 6.7% for females. We aimed to investigate the efficacy of the complete blood count parameters, C-reactive protein, and Lymphocyte-C-reactive Protein Ratio laboratory tests in the diagnosis of acute appendicitis, as well as their relationship with appendix diameter. We retrospectively examined all patients who underwent appendectomy between 1 January 2012 and 30 June 2019 in the General Surgery Clinic of Gaziosmanpasa University Faculty of Medicine. Laboratory tests, imaging findings, age, and gender were recorded. Lymphoid hyperplasia is considered as normal appendix-in other words, as negative appendicitis. The distribution of Lymphoid hyperplasia and appendicitis rates were statistically different in the groups formed according to appendix diameter (<= 6 and >6 mm) (p < 0.001). We found a significant correlation between appendix diameter and WBC (White blood count), Lymphocyte, Neutrophil, RDW(Red blood cell distribution width), NLR(Neutrophil to lymphocyte ratio), and PLT/L (Platelet to lymphocyte ratio), MPV (Mean platelet volume) and RDW were significantly different in patients with an appendix diameter of <= 6 mm (p = 0.007, p = 0.006, respectively). WBC, Neutrophil, PDW, and NLR values were significantly different between appendicitis and hyperplasia groups in patients with an appendix diameter of >6 mm. The sensitivity of the NLR score (cutoff = 2.6057) in the diagnosis of appendicitis was 86.1% and selectivity was 50% in these patients. Complete blood count parameters evaluation with the clinical findings revealed that NLR is an important parameter that may help the diagnosis of acute appendicitis with an appendix diameter of >6 mm. In patients whose pathological results indicated acute appendicitis but who had a diameter of <= 6 mm, we found an elevated MPV and low RDW values.