The loss of body fluids resulting in a decrease in extracellular fluid is called dehydration. When a person works under high temperature conditions, strenuous exercise or certain diseases (such as violent vomiting, severe diarrhea), a large amount of water and inorganic salts (mainly sodium salts) will be lost, resulting in a drop in the body’s extracellular fluid osmolarity and symptoms such as a drop in blood pressure, increased heart rate, four chills, and even coma in severe cases. Examination items of extracellular fluid reduction due to dehydration: 1. Laboratory tests for isotonic dehydration (1) Blood concentration: red blood cells (RBC), hemoglobin (Hb), hematocrit (HCT), plasma protein concentration increases, or the concentration increases compared to the basal value. However, hemodilution occurs in those who have lost blood. (2) Red blood cell morphology: normal, mean red blood cell volume (MCV), mean red blood cell hemoglobin concentration (MCHC) are normal. (3) Urinalysis: urine sodium, urine chloride concentration and 24h excretion decreased, and urine relative density increased. (2) Hypotonic dehydration blood test indexes (1) Serum sodium concentration: decreased, less than 135 mmol/L. Mostly accompanied by hypochlorhydria, and the degree of decrease of both is generally consistent. Blood potassium concentration may be normal or increased. (2) Plasma crystals: decreased osmolarity. (3) Hemoconcentration: Red blood cell count, hemoglobin, plasma protein and hematocrit are seen to be increased. The value is greater when compared to basal values. (4) Erythrocyte edema: increased intraerythrocyte water, increased mean red blood cell volume, and decreased mean red blood cell hemoglobin concentration. (5) Commonly used urine laboratory indicators: ① Urinary sodium concentration: decreased due to extrarenal factors, mostly less than 15 mmol/L, or even undetectable; increased in patients with renal impairment, abnormal regulatory mechanisms or use of diuretics, mostly greater than 20 mmol/L. Urinary electrolytes should be routinely checked in patients with sodium deficiency, because even patients with low sodium due to extrarenal factors may be accompanied by reduced renal reabsorption. The urine electrolytes should be routinely checked in patients with sodium deficiency because even patients with hyponatremia due to extrarenal factors may be accompanied by decreased renal reabsorption, such as the elderly, chronic hypokalemia, and patients on aminoglycosides. (ii) Urine chloride concentration: consistent with changes in urine sodium concentration. (③) Excretion: decreased due to extrarenal factors, 24h urinary sodium and urinary chloride excretion is significantly reduced, or even not measured. The 24h urinary sodium and urinary chloride excretion is still high or even far above the normal range in patients with renal impairment, abnormal regulatory mechanism or diuretic use, which is one of the reasons for intractable hyponatremia in some patients. ④ Urinary osmolality and urinary relative density: generally consistent with changes in urinary sodium concentration. In patients with hyponatremia caused by extrarenal factors, both osmolality and relative density are very low due to good electrolyte absorption; however, those with low urine tend to be higher (concentrated metabolic waste); while those caused by the kidney itself are more variable, with the level depending mainly on the ratio of water to solute, generally similar to that of plasma.