This diagnostic method was first proposed by Ye Gui (1667-1746 AD), a renowned physician in the Qing Dynasty. Guided by the theories of the Yellow Emperor's Classic of Internal Medicine (huang di nei jing), he synthesized the knowledge of earlier physicians with his own clinical experience to develop a diagnostic framework for acute febrile diseases caused by external pathogenic factors. He classified the progression of febrile diseases into four pathological levels: defensive (wei), qi, nutritive (ying), and blood (xue). These four levels represent the depth and severity of pathological changes within the body, and serve as an important guide for the diagnosis and treatment of febrile diseases. This method not only characterizes the various patterns of febrile disease but also reflects the progressive stages of severity as the disease develops and penetrates from the exterior to the interior.

• Defensive (wei) phase refers to disorders in the outer defensive forces of the body protective qi. It usually occurs at the onset of a febrile disease due to external contraction, the pathological changes are present in the lung, skin and hair.

• Qi phase refers to disorders in the body's inner defensive forces, which manifests as an interior heat disharmony caused by excessive heat evils. In this phase, both the body's defense forces and pathogenic factors are very strong, resulting in intense struggling. The affected lesions are usually located in the lung, spleen, stomach, intestines and gall bladder.

• Nutrient (ying) phase occurs when pathogenic heat is transmitted into the qi of the blood. Ying is a component of the blood, so the disease is actually located in the heart and pericardium.

• Blood (xue) phase represents further invasion of the ying system by the evils. It results in excessive heat stirring the blood and further disturbing the spirit. The pathological changes are mainly in the liver and kidney.

 Typical  in the Defensive Phase 

Disharmonies in the four phases
Phase	Disharmony	Main features
Wei	Heat invading exterior body	Fever, slight aversion to wind and cold, very little or no sweating, cough, runny nose, headache, sore throat, slightly thirsty and body aches. On examination, the tongue tip and margin are red and covered with a thin white or little yellow coating, and the pulse is floating and rapid.
Qi	Heat retention in lungs	Fever, sweating, thirst, cough with yellow sticky sputum, asthma, chest pain, chest fullness and sore throat. On examination there is a red tongue with yellow coating, and a rapid pulse.
	Excess heat in qi system	High fever, irritability, extreme thirst with preference for cold drinks, profuse sweating, flushed face and coarse breathing. On examination there is a dry and yellow tongue coating and surging pulse.
	Heat retention in stomach and intestine	Thirst with preference for cold drinks, anxiety, delirium, fever in afternoon, constipation or diarrhea and distention and pain in abdomen, which feels better when pressed. On examination, there is a dry and yellow tongue coating, and a deep and forceful pulse.
	Damp heat  retention in spleen	Slight fever, thirst but without desire to drink, chest fullness, nausea, heavy sensation in body and limbs, sleepiness, perspiration with sticky sweat, diarrhea and yellow and scanty urine. On examination, the tongue is red with a yellow thick and greasy coating and the pulse is hesitant and rapid.
Ying	Heat damaging nutrient yin	Fever (worse at night), chest fullness, insomnia, or even coma and delirium and transient rashes. On examination, there is a deep red tongue and thready and rapid pulse.
	Heat attacking pericardium	High fever, irritability or apathy, stupor, delirium, tongue stiffness and cold limbs. On examination there is a deep red tongue and rolling and rapid pulse.
Xue	Blood heat lead to circulation disorder	High fever, irritability, insomnia, mania, transient rashes, feverish sensation of the palms and soles, and bleeding disorders. On examination, the tongue is dry, deep red or purple color and the pulse is rapid.
	Liver heat stir up winds	High fever, headache, dizziness, stupor or irritability, upward turning of eyes, tightly closed jaw, a stiffly back and neck and agitation or convulsions. On examination, the tongue is deep red and the pulse is taut and rapid.