Quantifying the therapeutic index of acupoints using measurements of effective and hazardous depths

Background: Acupuncture needles are widely used to treat various medical conditions, but can occasionally lead to serious adverse events. The therapeutic index (TI), originally developed in pharmacology to quantify the safety margin between effective and toxic doses, provides a valuable potential framework for evaluating anatomical safety in acupuncture. Methods: We applied the TI concept to two commonly used acupoints—GB21 and ST36—by quantifying the TI value, defined as the ratio of the median hazardous depth (HD₅₀) to the median effective depth (ED₅₀), based on ultrasound-guided measurement and de-qi responses. Using ultrasound-guided measurements in 39 participants, we calculated the TI for each point and constructed cumulative distribution functions to visualize the therapeutic window. Results: GB21 demonstrated a TI of 1.54, indicating a relatively wide safety margin, whereas ST36 had a TI of 1.09, reflecting a narrow and potentially risk-prone margin. Conclusions: These findings emphasize the impact of anatomical variability and highlight the need for personalized depth control in acupuncture practice. Our study provides preliminary evidence that integrating the TI concept into acupuncture safety assessment may support more individualized, data-driven needling strategies. These findings should be interpreted as exploratory and require confirmation in larger, demographically diverse populations and at additional acupoints before being generalized to routine clinical practice.