Tongue reading is one of the most important method of Traditional Chinese Medicine diagnosis. Through the tongue diagnosis, a experienced acupuncturist can get more details about your conditions. And more experiences he has, more details he can get.
Before you start reading your tongue, a very important viewpoint of Chinese medicine must be accepted. That is: any diagnosis methods of TCM, including Tongue Reading, is a dynamic observation process. There is no particular standards of one-size-fits-all. There is only a rough standard: light red tongue with thin white coat. However, everyone's tongue is different. Someone's tongue is a little bit fat, whereas some others' may be a little bit thin. If someone's tongue is not as same as others, you can't say that one of them is sick on that condition.
We should learn our body's condition from reading our tongues' changes. For example, the coating of the tongue used to be very thin. And recently, it has become thicker. Or, the tongue is supposed to be light red, but it becomes very red recently. All of those could make diagnostic sense.
In what conditions that you can't recognize your body's physical state from reading your tongue?
First, be careful not to let your tongue be stained by foods, drinks, drugs and so on. When we observe the tongue, we must avoid artificially changing its color. Drinking orange juice, strong tea, coffee or some other colored things, may mislead the observer's diagnose.
Second, don't check your tongue in the morning. Many books say that looking at tongue images in the morning can get the most accurate diagnose. But in my experience, reading the tongue images in the morning uaually leads to inaccurate conclusion. When you get up in the morning, your tongue tends to be light in color and the coating of the tongue tends to be thick and dry. That can not represent your physical state during the day.
Third, try not to observe a person's tongue within half an hour after his/her meal. That is because the tongue is fully involved in food stirring and chewing. The increasing of blood circulation makes the tongue red. On the other hand, during the course of eating, some foods, like celery, can thins the coating of the tongue, and some foods, like mild, can thicken the coating on the tongue. These can affect the observation of tongue images. Therefore, try not to see a Chinese medicine doctor within half an hour after a meal.
In addition, there are some certain physiological stages such as a woman's menstrual period, when the tip of her tongue turns red. So when a woman see a TCM doctor, she has to tell the doctor whether it is during the menstruation or one or two days before or after the menstruation. If it is not, then the doctor can judge the tongue as a performance of the heart fire.
Once all these special factors above are excluded, the observation of the tongue can be more accurate for a diagnose of the illness.