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When a sore or ulcer (Yang illness) suppurates, it produces foul matter and emits a putrid odor. Those who are fastidious naturally recoil from it, fearing contamination, and it might seem unnecessary to record in writing or analyze its color and form. Nevertheless, observing color to differentiate syndromes is essential among the Four Diagnostic Methods (Inspection, Auscultation-Olfaction, Inquiry, Palpation). Both pus and serous fluid are brewed from flesh and blood; they can be used to assess the strength or decline of the constitution and to determine whether the disease condition is mild or severe. With experience and insight, one can judge by a single glance, so how can one ignore discussing this and proceed muddledly? Therefore, regarding the consistency of pus, it should be thick rather than thin. Thick and dense pus indicates that the patient's Yuan Qi (Original Qi) must be robust; thin and watery pus indicates that the patient's Ben Zhen (Fundamental Essence) must be weak.

If pus forms internally and remains un-drained for a long time, especially if the opening is deep and large with prolonged brewing, its quality will often not be thick. Once drained, if it flows freely like water, with a clear luster and no offensive odor, it remains a standard, favorable presentation, not a sign of collapse. In cases where the cavity is deep and pus abundant, there will inevitably be interspersed necrotic tissue, resembling pig fat membranes or clumps of dregs. This represents the blood collaterals and fascia within the muscle that have not completely decomposed, flowing out with the pus.

After the rupture of major lesions, for two to three days, there will typically be a continuous flow of clear, thin pus. In severe cases, necrotic tissue may occasionally discharge spontaneously. (If the necrotic tissue is massive and blocks the rupture site, pus drainage will be obstructed, swelling will not subside, and pain will persist; measures must be taken to ensure complete removal.) Although the opening is large and pus volume is high, after rupture, pain ceases and toxins are expelled. If the Stomach Qi is strong and appetite is good, recovery is often achieved in less than ten days. Cases that do not heal after ten days or more are mostly due to mismanagement leading to deteriorated conditions, not necessarily because the disease itself is difficult to treat.

However, at the moment of initial rupture, if the pus is scant yet clear as water, or turbid, greasy, and dull like black bean juice or muddy slurry, it indicates long-term deficiency of Qi and Blood, where Zheng Qi (Upright Qi) cannot overcome the pathogen, lacking the power to generate pus. Correlating with the patient's appearance, one invariably sees emaciation, loss of luster, wasting of muscles, and a fine, faint pulse. If Stomach Qi remains adequate, tonifying treatments may be administered, offering hope for turning defeat into victory. But if pathogens remain uncleared, or Stomach intake is exhausted, making supplementation difficult, then even masters like Lu Dan and Bian Que would find it hard to restore Yuan Qi from nothingness.

If the pus is turbid, thick, and bright in color, it indicates excess Qi and Fire, requiring clearing and regulating therapies. Even if the pus is not thick, but white or yellow, pure and translucent, it is still a favorable progression. If the pus is greenish or bluish and thin, it results from prolonged brewing causing the quality to become thin. If pus contains mixed blood stasis appearing as purple clots, it indicates corrosion of blood collaterals, with blood leaking out and accumulating as stasis. If fresh blood is mixed with pus, it signifies that blood from the collaterals is discharging together with the pus. If pus and blood are indistinguishable and impure in form and color, there is concern for deficiency of Zheng Qi and excess of pathogens. If pus and blood are not thick, with mixed colors—either pale white like flour water or filthy like sewage—it indicates insufficient Zheng Qi, unable to push out toxins. Prolonged delay often leads to changes. Severe cases present as dark purple and dull, like ground agarwood juice mixed with mud, or like rotten liver pounded into paste. The odor may be fishy, foul, and repulsive, while the color is black and white beyond description, like well-bottom silt or ditch filth, indistinguishable as pus, blood, water, or serum. Such indescribable states indicate the absence of Zheng Qi, where flesh and blood have transformed into foreign matter; these are incurable conditions.