TCM, on the other hand, is characterized by TDS. Differentiation means comprehensive analysis, while syndrome refere to symptoms and signs. So differentiation of syndromes implies that the patient's symptoms and signs collected by the four diagnostic methods are analyzed and summarized so as to identify the etiology, nature and location of a disease, and the relation between vital qi and pathogens, thereby determining what syndrome the disease belongs to. By treatment is meant selecting the corresponding therapy according to the outcome of differentiating syndromes. Taken as a whole, TDS means diagnosis and treatment based on overall analysis of symptoms and signs.

The eight principal syndromes

The eight principal syndromes, also known as "eight principles" (ba gang), refer to yin, yang, exterior, interior, cold, heat, deficiency and excess. The differentiation of syndromes according to the eight principles is the general guideline of TDS and important components of TCM diagnostics. Complicated as the clinical manifestations of diseases may be, they are classified under the eight principles. For example, the classification of syndromes-yin or yang; the location of disease-exterior or interior; the nature of disease-cold or heat; and the preponderance or discomfiture of vital qi and pathogens-deficiency of vital qi or excess of pathogen. Differentiation of syndromes according to the eight principles generalizes the intricate and volatile syndromes into four pairs: exterior and interior, cold and heat, deficiency and excess, and yin and yang. Therefore, in the diagnostic process, the eight principles can play a role in bring out the essentials.

Although this differentiating method classifies various syndromes into eight categories, they are interrelated and inseparable from each other. For instance, the exterior syndrome and interior syndrome are related to the cold, heat, deficiency and excess syndromes; the cold syndrome and heat syndrome are related to the exterior, interior deficiency and excess syndromes; the deficiency syndrome and excess syndrome are related to cold, heat, exterior and interior syndromes. These pathologic changes do not occur singly, the exterior and interior, cold and heat, deficiency and excess syndromes are usually found simultaneously. For instance, the disease with both the exterior and interior syndromes; the deficiency syndrome mingling with excess syndromes; intertwinement of the cold and heat syndromes. Under certain conditions these syndromes are often transformed into one another. For example, the exogenous pathogens may invade the interior and vice versa the cold syndrome may be transformed into heat syndrome and vice versa; the deficiency syndrome may be transformed into the excess syndrome and vice versa. With the progress of disease, some false appearances contrary to its nature may appear. e. g. ,cold syndrome with pseudo-heat symptoms and vice versa, deficiency syndrome with pseudo-excess symptoms and vice versa. Thus, when applying differentiation of syndrome according to the eight principles, physicians are required not only to have a masterly command of the characteristics of each syndrome, but also to pay attention to their coexisting, interlacing, transforming, and true or false conditions, so and so only can the disease be understood in an all-round way. Thereby, providing reliable basis for treatment.

Yin and yang

Yin and yang are a pair of principles used to summarize the other three pairs of principles and are also the key principles in the eight principles. So the other three pairs of principles are classified under either yin or yang. Exterior, heat and excess syndromes are classified into the category of yang, while interior, cold and deficiency syndromes fall into the category of yin. Yin syndrome is characterized by deficiency of yang-qi and excess of yin in the body. Yang syndrome in characterized by the hyperactivity of yang-qi and hyperfunctions of the zang fu-organs, resulting from excess of yang-heat in the body. Yin and yang are also used to explain the pathological changes of the zang-fu organ, e.g. yin depletion, yang depletion, yin deficiency and yang deficiency, etc.

Qi, blood and body fluid

Differentiation of syndromes according to the theory of qi, blood and body fluid is a differentiating method to analyse and identify the pathological changes of qi, blood and body fluid according to the theory of qi, blood and body fluid. Qi, blood, and body fluid, are the material basis for the functional activities of the zang-fu organs, their formation and circulation depend upon the normal functions of the zang fu organs. Therefore the pathological changes of qi, blood and body fluid may bring about the dysfunction of the zang-fu organs, and the dysfunction of the zang-fu organs will be bound to cause the pathological changes of qi, blood and body fluid. Hence, both of them should closely coordinate and complement each other. Qi has many syndromes, which are usually classified into four classes: qi deficiency, qi sinking, qi stagnation and reversed flow of qi. As concerns blood syndromes, TCM tends to group them under four heads: blood deficiency, blood stasis, heat in blood and cold in blood. " Physiologically, they complement each other and, pathologically, affect each other. Thereby, forming differentiation of syndromes of the same disease of qi and blood. Clinically, there is qi stagnation and blood stasis, qi deficiency and blood loss, deficiency of both qi and blood, qi deficiency and blood stasis, and qi prostration resulting from hemorrhage. Syndromes of body fluid may be classified into two categories: insufficiency of body fluid and water retention. Insufficiency of body fluid may cause the clinical manifestations: dryness of the mouth and throat, dry lips and tongue, subsidence of eyes, dry skin, scanty urine, constipation, a red tongue with scanty saliva and thready, rapid pulse. Whereas, retention of water may form such pathological substances as water, dampness and phlegm retention. Usually seen in edema, tympanites and phlegm-retention.

Wei (defensive), qi (vital-qi), ying (nutrient) and xue (blood)

Differentiation of syndromes according to the theory of wei (defensive), qi (vital-qi), ying (nutrient) and xue (blood) is a differentiating method applied to  analysing and differentiating exogenous febrile diseases. It was developed and created by YeTianshi in the Qing Dynasty. It supplements the differentiation of syndromes by the theory of the six meridians and enriches the differentiation of syndromes and treatment for exogenous febrile diseases in TCM. This theory is the basis used to classify syndromes, to identify transformation and determine treatment. The theory of wei, qi, ying and xue is of great practical value because, for one thing, it generalizes the pathological changes of febrile dis-eases as the four kinds of syndromes: weifen, qifen, yingfen and xuefen, for another, it represents the four different stages: superficial or deep and mild or serious in the development of exogenous febrile diseases. It is also believed as a law of the development and changes of exogenous febrile diseases. Diseases of the wei and qi stages are mild and superficial, whereas those of the ying and xue stages are deep and serious.