The duration of a disease is a process of struggle between healthy qi and pathogenic factors, the result of which decides the development of the disease. If healthy qi succeeds, the disease gets improved and gradually heals; if pathogenic factors succeed, the disease becomes worsened or leads to death. In order to enable the disease to develop along a favourable route, it is necessary to support healthy qi and eliminate pathogenic factors. So supporting healthy qi and eliminating pathogenic factors is an important therapeutic principle in clinic treatment.

Strengthening healthy qi usually refers to "nourishing therapy" which includes various methods, such as herbs, acupuncture and moxibustion, tuina therapy, diet regulation and exercise, etc. Nourishing therapy is usually used to treat deficiency syndromes such as qi deficiency, blood deficiency, yin deficiency, body fluid deficiency and kidney-essence deficiency. That is why it is said in TCM that "deficiency syndrome should be treated by nourishing therapy". 

Nourishing therapy can be further divided into different therapeutic methods to deal with different pathogenic factors, such as qi-nourishing therapy, blood-nourishing therapy, yin-nourishing therapy, yang-warming therapy, fluid-generating therapy and essence-enriching therapy, etc.  By means of strengthening healthy qi, deficiency can be improved, body resistance can be reinforced and pathogenic factors can be eliminated. In fact the purpose of strengthening healthy qi is to eliminate pathogenic factors. However, excess syndrome with predominance of pathogenic factors cannot be treated by nourishing therapy because nourishing therapy may retain pathogenic factors in the body.

Eliminating pathogenic factors refers to "purgation therapy" (also known as "attack therapy") with herbs or acupuncture and moxibustion. Purgation therapy is used to treat excess syndromes such as attack of exogenous pathogenic factors, retention of food, internal accumulation of phlegm-dampness, internal retention of water, internal blockage of blood stasis, stagnancy of qi activity, exuberance of fire and stagnation of pathogenic cold, etc. That is why it is said in TCM that "excess syndrome should be treated by purgation therapy.

"Purgation therapy can be further divided into different therapeutic methods to deal with different pathogenic factors, such as diaphoresis therapy, emetic therapy, defecation-promoting therapy, water-draining therapy, blood-activating therapy, dampness-resolving therapy, stasis-breaking therapy, heat clearing therapy and cold-dissipating therapy, etc. Eliminating pathogenic factors is helpful for restoring and strengthening healthy qi. However, deficiency syndrome due to insufficiency of healthy qi cannot be treated by purgation therapy because purgation therapy tends to impair healthy qi.

In applying the principle of strengthening healthy qi and eliminating pathogenic factors, one must carefully analyze the state of pathogenic factors and the condition of healthy qi, differentiate the nature of the syndrome in question and distinguish the relationship between healthy qi and pathogenic factors in order to select the right therapy. Generally speaking, simple deficiency can be treated by strengthening healthy qi and simple excess syndrome can be treated by eliminating pathogenic factors. 

For the treatment of syndrome mixed with both deficiency and excess, the therapy for strengthening healthy qi and the therapy for eliminating pathogenic factors can be used simultaneously. Since the order and urgency of deficiency of healthy qi and excess of pathogenic factors are different, there are three methods for strengthening healthy qi and eliminating pathogenic factors, i.e. purgation prior to tonification, tonification prior to purgation and simultaneous application of purgation and tonification.

Purgation prior to tonification means to eliminate pathogenic factors first arid then strengthen healthy qi. If a syndrome mixed with deficiency of healthy qi and excess of pathogenic factors is marked by predomination of pathogenic factors that must be eliminated immediately and deficiency of healthy qi that still can bear attack, the therapy for strengthening healthy qi may reinforce pathogenic factors instead of strengthening healthy qi. In this case purgation should be used first. After pathogenic factors are reduced, nourishing therapy can be used to strengthen healthy qi. 

Take sudden uterine bleeding due to blood stasis for example. If blood stasis is not eliminated, hemorrhage cannot be stopped. Though hemorrhagia should be treated by nourishing therapy, tonic herbs may make it difficult to eliminate blood stasis. If hemorrhage continues, blood deficiency cannot be rectified. In this case measures should be taken to activate blood to resolve stasis in order to stop bleeding. Then nourishing therapy can be used to invigorate blood.

Tonification prior to purgation means to strengthen healthy qi first and then eliminate pathogenic factors. This therapy can be used to treat syndrome mixed with deficiency of healthy qi and excess of pathogenic factors. Though pathogenic factors should be eliminated, healthy qi is too deficient to bear attack. Early application of purgation may impair healthy qi. 

In this case tonifying therapy should be used first to strengthen healthy qi. Take ascites for example. Since healthy qi is deficient for a long time and cannot bear purgation, measures should be taken to strengthen healthy qi. After healthy qi is reinforced, purging therapy can be used to eliminate pathogenic factors and drain water.

This therapy means to strengthen healthy qi and to eliminate pathogenic factors simultaneously. It can be used to treat syndrome mixed with deficiency of healthy qi and excess of pathogenic factors. Because simple use of tonification may make pathogenic factors linger inside and simple use of purgation may impair healthy qi. Take common cold due to qi deficiency for example. It can be treated by supplementing qi and relieving superficies at the same time.