Analysis of Caffeine Detection Threshold in Hair: Evidence from Northern China

The optimal diagnostic threshold was determined by maximizing the Youden index and further validated independently using eight volunteers with documented caffeine-intake histories. The results showed that the mean caffeine concentration in hair of the CNB abuse group was 15.48 ng/mg (range: 0.05~206.35 ng/mg), which was significantly higher than that of the healthy control group. The area under the ROC curve (AUC) was 0.828, and the optimal threshold was determined as 2.01 ng/mg, with a sensitivity of 74.9%, specificity of 80.8%, positive predictive value of 81.5% and negative predictive value of 73.9%.

The correct classification rate of the validation cohort reached 87.5%. This study established a population-specific forensic threshold for caffeine in hair in Northern China. The method can be directly applied to public security forensic identification and judicial trials.