Development Process and Recommendations for National Reference Standards of Tongue Body and Tongue CoatingColors

This study describes the development process and results of national reference standards for the color of tongue body and tongue coating by sex and age, based on tongue diagnosis, a traditional diagnostic method in Korean medicine. Standard operating procedures for tongue image measurement established in previous research were used to develop protocols for data production and reference standard evaluation, ensuring traceability and measurement uncertainty. Using a dataset of 2,158 tongue images from healthy Koreans, 967 images were selected for analysis. The tongue body and coating were further subdivided into tip, edge, center, and root regions, and reference standards were established for each subregion. The measurement uncertainty was derived by calculating and combining the uncertainties fromrepeated measurements (five or more times) of the same tongue for each specific area and the uncertainty arising from the color calibration process. Reference standards and expanded uncertainties (k=2) were established for 12 groups, consisting of six age groups (20–29, 30–39, 40–49, 50–59, 60–69, 70–79 years) by sex. The smallest sample size and highest uncertainty were found in the 70–79 age group, indicating the need for additional data collection in this demographic. The CIE L* and b* values for tongue coating were higher than those for the tongue body, reflecting the brighter and less reddish nature of the typically translucent white coating. The CIE a* value was highest in the tip region of the tongue body, likely due to anatomical and physiological characteristics. The CIE b* values for both tongue body and coating were higher in women than in men, indicating that women’s tongues tend to appear less red. By developing national reference standards for tongue body and coating color in healthy Koreans, this study establishes criteria for normal tongues and aims to enhance the reliability of tongue diagnosis measurements. These standards can serve as normal reference values for various tongue imaging systems and are expected to be useful in education, clinical research, and mobile health management. Further data collection in older adults and the development of reference standards for other tongue diagnostic indices beyond color are suggested for future research.