Osmolality, plasma or serum

Clinical uses of serum osmolality include screening for toxin ingestion (alcohols, glycols), monitoring concentration of osmotically active agents (eg., mannitol), and evaluation of hyponatremia (exclusion of pseudohyponatremia, presence of other osmotic agents such as glycine and glucose). Clinical uses of urine osmolality include evaluation of increased urine output (as in primary polydipsia, diabetes insipidus, diabetes mellitus), evaluation of decreased urine output (dehydration, acute tubular injury), and evaluation of renal acidification defects. Stool osmolality may be used to separate secretory diarrhea (due to agents damaging the mucosa) from osmotic diarrhea (due to the presence of unabsorbed solutes, laxative abuse, malabsorption).