The formula for plasma osmolality is: plasma osmolality = 2*(sodium ion concentration +* potassium ion concentration) + glucose concentration + urea nitrogen concentration, which includes crystal and colloid osmolality. In the formula of plasma osmolality, the unit of concentration of sodium ion, potassium ion, glucose, and urea nitrogen should be mmol/L. The normal range of plasma osmolality is 280~310mOsm/(kg-H2O). Plasma osmolality comes mainly from the crystalline material dissolved in it. The osmolality formed by crystalline material is the crystalline osmolality, 80% of which comes from sodium and chloride ions. The osmolality formed by proteins is called colloid osmolality, because of the high molecular weight of proteins and the small number of protein molecules in the plasma, the osmolality formed by them is low, usually 1.3mOsm/(kg-H2O), which is equivalent to about 3.3kPa or 25mm Hg. 75% to 80% of the colloid osmolality of plasma comes from albumin. When plasma osmolality is higher or lower than the normal range, it should be consulted promptly to identify the cause and standardize treatment.