A multiple linear regression analysis was performed to evaluate the effect of BMI, age, sex, nicotine abuse, the season of sampling, history of medication (glucocorticoids, L-thyroxine, proton-pump inhibitors, and aromatase inhibitors), and secondary diagnoses (diabetes, rheumatic disease, COPD, and carcinoma) on their potential influence as predictive variables for preoperative vitamin D serum levels. The gene discussed is CYP19A1; the disease is chronic obstructive pulmonary disease.