Osmolality, Serum and Urine

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Subject: Osmolality, Serum and Urine

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Definition

  • Osmolality refers to the osmotic concentration of a fluid. The osmolality of serum, urine, or any other body fluid depends on the number of active ions or molecules in a solution and yield important information about a patient's ability to maintain a normal fluid balance status. Osmolality is measured with an osmometer by freezing point depression or vapor pressure elevation techniques, or it can be calculated from a formula.

  • Osmolarity is the osmotic concentration of solution expressed as osmoles of solute per liter of solution or the property of solution that depends on the concentration of solute per unit of total volume of solvent.

  • Serum osmolality measures the amount of chemicals dissolved in the blood. Chemicals that affect serum osmolality include sodium, chloride, bicarbonate, proteins, and glucose. A serum osmolality test is done to evaluate electrolyte and water balance. Serum osmolality is controlled partly by ADH or vasopressin. ADH is produced by the hypothalamus and is released by the pituitary gland into the blood.

  • Urine osmolality reflects the total number of osmotically active particles in the urine, without regard to the size or weight of the particles. Substances such as glucose, proteins, or dyes increase the urine specific gravity. Therefore, urine osmolality is a more accurate measurement of urine concentration than specific gravity, and urine osmolality can be compared with the serum osmolality to obtain an accurate picture of a patient's fluid balance.

  • Normal range: see Table 16.59.

 
TABLE 16–59
Normal Ranges for Osmolality

Use

  • Evaluate the balance between the water and the chemicals dissolved in blood.

  • Determine whether severe dehydration or overhydration is present.

  • Help determine if the hypothalamus is producing ADH normally.

  • Help determine the cause of seizures or coma. In severe cases, an imbalance between water and electrolytes in the body can cause seizures or coma.

  • Screen for the ingestion of certain poisons, such as isopropanol, methanol, or ethylene glycol.

  • Evaluate concentrating ability of the kidneys.

  • Evaluate electrolyte and water balance.

  • Used in workup for renal disease, SIADH, and diabetes insipidus.

  • May be used with urinalysis when patient has had radiopaque substances, has glycosuria, or proteinuria

  • Evaluate dehydration, amyloidosis. Osmolality is desirable in examination of neonatal urine when protein or glucose is present.

Interpretation

Increased In

  • Hyperglycemia

  • DKA (osmolality should be determined routinely in grossly unbalanced diabetic patients)

  • Nonketotic hyperglycemic coma

  • Hypernatremia with dehydration

    • Diarrhea, vomiting, fever, hyperventilation, inadequate water intake

    • Diabetes insipidus—central

    • Nephrogenic diabetes insipidus—congenital or acquired (e.g., hypercalcemia, hypokalemia, chronic renal disease, sickle cell disease, effect of some drugs)

    • Osmotic diuresis—hyperglycemia, administration of urea or mannitol

  • Hypernatremia with normal hydration—caused by hypothalamic disorders

    • Insensitivity of osmoreceptors (essential hypernatremia)—water loading does not return serum osmolality to normal; chlorpropamide may lower serum sodium toward normal

    • Defect in thirst (hypodipsia)—forced water intake returns serum osmolality to normal

  • Hypernatremia with overhydration—iatrogenic or accidental (e.g., infants given feedings with high sodium concentrations or given NaHCO3 for respiratory distress or cardiopulmonary arrest)

  • Alcohol ingestion, which is the most common cause of hyperosmolar state and of coexisting coma and hyperosmolar state

Decreased In

  • Hyponatremia with hypovolemia (urine sodium is usually >20 mmol/L)

    • Adrenal insufficiency (e.g., salt-losing form of CAH, congenital adrenal hypoplasia, hemorrhage into adrenals, inadequate replacement of corticosteroids, inappropriate tapering of steroids)

    • Renal losses (e.g., osmotic diuresis; proximal RTA; salt-losing nephropathies, usually tubulointerstitial diseases such as GU tract obstruction; pyelonephritis; medullary cystic disease; polycystic kidneys)

    • GI tract loss (e.g., vomiting, diarrhea)

    • Other losses (e.g., burns, peritonitis, pancreatitis)

  • Hyponatremia with normal volume or hypervolemia (dilutional syndromes)

    • CHF, cirrhosis, nephrotic syndrome

    • SIADH

Limitations

Variations in the urine osmolality play a central role in the regulation of the plasma osmolality and Na+ concentration. This response is mediated by osmoreceptors in the hypothalamus that influence both thirst and the secretion of ADH. 
The relationship between serum and urine osmolality and the clinical significance of laboratory values are shown in Table 16.60.
 
TABLE 16–60
The Relationship Between Serum and Urine Osmolality and the Clinical Significance of Laboratory Values
 
or 
in SI units: = (1.86 × serum Na) + serum glucose (mmol/L) + BUN (mmol/L) + 9 
  • More simply: NA+ + K+ + (BUN ÷ 28) + (glucose ÷ 18). Because K+ is relatively small, and BUN has no influence on water distribution, the formula can be simplified to 2Na+ + (glucose ÷ 18).

 
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