Serum electrolyte test includes serum cation and serum anion test, cation includes blood potassium, blood sodium and blood calcium test, and anion includes blood chloride and blood phosphorus test. The reference range of serum potassium is 3.5-5.5 mmol/L. Serum potassium higher than 5.5 mmol/L is considered hyperkalemia and lower than 3.5 mmol/L is considered hypokalemia. Input of large amount of stored blood, high potassium diet, long-term use of spironolactone, and hypoadrenocorticism can cause hyperkalemia, and application of large amount of insulin, frequent vomiting, and long-term application of furosemide can cause hypokalemia. The reference range of serum sodium is 135-145 mmol/L, more than 145 mmol/L is hypernatremia and less than 135 mmol/L is hyponatremia. Heavy sweating, burns, and hyperadrenocorticism cause hypernatremia, and starvation, malnutrition, recurrent vomiting and diarrhea, and uremia cause hyponatremia. Total serum calcium is 2.25～2.58mmol/L. Total serum calcium over 2.58mmol/L is hypercalcemia, which may be caused by primary hyperparathyroidism, massive application of vitamin D. Total serum calcium less than 2.25mmol/L is hypocalcemia, which may be caused by rickets, long-term hypocalcemic diet. The reference range of serum chlorine is 95～105mmol/L. Severe vomiting, respiratory alkalosis and excessive respiration can cause abnormalities. The normal range of serum phosphorus is 0.97～1.61mmol/L. Malignant disease and ethanol poisoning can cause abnormalities.