Biology of the Tongue Coating and Its Value in Disease Diagnosis

Tongue diagnosis is one of the most important diagnostic tools in traditional Chinese medicine and has been verified for thousands of years. However, its subjectivity and repeatability has been disputed continuously. The tongue coating as the primary coverage of tongue diagnosis provides more objectivity and reproducibility due to its relatively clear molecular basis; it also has a close relationship with many system diseases and may be used as a potentially valuable disease diagnostic tool. This article describes the material basis of the tongue coating, including its biology (epithelial cells, blood cells, vascular endothelial cells, and bacteria) and its metabolites; moreover, we summarize the diseases that are most correlated with the tongue coating. This will be valuable not only for fundamental research of tongue diagnosis but also for the diagnosis and differential diagnosis of disease. We suppose that the tongue coating could serve as a valuable auxiliary diagnosis tool in many diseases, and more research should focus on how to colligate the various information about the tongue and provide useful information for disease diagnosis.