Important to determine whether serum is hypo-osmolar, iso-osmolar, or hyperosmolar.
If hypo-osmolar, determine overall fluid status.
Using urine osmolality and urine sodium can assist with history and physical to determine etiology
Patients with significant vomiting can present with very low chloride levels (<60) due to high concentration of chloride within gastric lumen. Parietal cells secrete HCl into stomach lumen so excessive vomiting can significantly decrease serum chloride levels.
Metabolic alkalosis and hypokalemia can result from excessive vomiting. Hypokalemia could also be worsened with intracellular shift due to alkaline conditions.
Main trigger for renin aldosterone system is low volume status.
Main trigger for ADH secretion is elevated osmolality. Second trigger is low volume status.
Low volume status results in a higher urine osmolality and low urine sodium level, in the setting of proper renal function.
Low volume status with low solute intake (as in alcoholics) may display lower values of urine osmolality in addition to low urine sodium level.
May check urine chloride in a patient with hyponatremia, hypokalemia, and alkalosis:
If urine chloride low, suggestive of excessive vomiting.
If urine chloride high, suggestive of diuretic use.
If urine chloride normal, suggestive of possible Gitelman/Barttner syndrome