Background/Objectives: Differentiating complicated acute appendicitis (CA) and uncomplicated acute appendicitis (UC) is essential to guide clinical management. While CA requires urgent surgical management, UC can be treated with antibiotic therapy in selected cases. However, accurate identification of CA remains a clinical challenge. This study aimed to identify factors associated with CA and to develop a diagnostic severity scale. Methods: In this retrospective study, we included 132 adult patients (>16 years) with a confirmed postsurgical diagnosis of appendicitis, of whom 52 had CA and 80 had UA. Signs, symptoms, comorbidities, laboratory values, and ultrasonographic findings were evaluated to determine predictive factors and construct a diagnostic scale. Results: The factors most significantly associated with CA were elevated plasma concentrations of C-reactive protein (>7.150 mg/dL), fibrinogen (481.5 mg/dL), International Normalized Ratio (INR) (>1.150), and the presence of free fluid periappendicular. The combination of these factors within one scale showed an area under the curve (AUC) of 0.84, with a sensitivity of 78.75% and a specificity of 82.69%. Conclusions: Serum C-reactive protein concentration, fibrinogen, and INR can be employed individually or as part of a scale as important indicators in diagnosing CA.