Osmolality is a measure of the total number of molecules dissolved in a liquid, regardless of their nature. Ions (potassium, sodium, phosphates, etc.) urea and creatinine (two waste products excreted by the kidneys) are the main contributors to urinary osmolality. Urine osmolality varies considerably throughout the day depending on the amount of water ingested in previous hours. Osmolality will be very high in morning concentrated urine, upon waking up, and will be lower in the pale urine of the afternoon. Generally speaking, urinary osmolality measurement complements the measurement of osmolality and blood electrolytes collected simultaneously, and measures the body’s ability to maintain its water balance.
Urinary osmolality increases with dehydration, heart failure, hypernatremia (high blood sodium levels), improper secretion of the antidiuretic hormone, insufficiency of the adrenal glands (Addison’s disease), liver damage and circulatory shock. Low urinary osmolality may indicate diabetes insipidus (no antidiuretic hormone effect), excessive water intake, high blood calcium, low blood potassium or kidney damage that prevents normal water reabsorption (kidney failure, kidney tubule damage).