How is serum osmolality measured

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Email Address. Password Show. Jump to content. A serum osmolality test measures the amount of chemicals dissolved in the liquid part serum of the blood. Chemicals that affect serum osmolality include sodium, chloride, bicarbonate, proteins, and sugar glucose. A substance called antidiuretic hormone ADH partly controls serum osmolality.

Water constantly leaves your body as you breathe, sweat, and urinate. If you do not drink enough water, the concentration of chemicals in your blood serum osmolality increases.

When serum osmolality increases, your body releases ADH. This keeps water from leaving in the urine, and it increases the amount of water in the blood. The ADH helps restore serum osmolality to normal levels. If you drink too much water, the concentration of chemicals in your blood decreases. When serum osmolality decreases, your body stops releasing ADH.

This increases the amount of water in your urine. It keeps too much water from building up in your body overhydration. When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch. There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site. Each lab has a different range for what's normal.

Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Aldosterone is a hormone related to renin-angiotensinaldosterone system RAAS , synthesized by the enzymatic cascade beginning with the conversion of cholesterol into pregnenolone.

The increased circulating concentrations of aldosterone result in the reabsorption of sodium and excretion of potassium, directly influencing serum osmolality 31 31 Knepper MA, Kwon TH, Nielsen S. Disorders of hydration status may present of several forms, corresponding to an overall variation of the amount of water or abnormal distribution between the extra- and intracellular compartments. The loss of hypotonic liquids eliminated at the expense of extracellular liquids cause the increase of serum osmolality, being a parameter to evaluate dehydration 38 38 Hooper L, Abdelhamid A, Attreed NJ, et al.

In the disorders related to potassium handling, for example in cases of renal tubular acidosis, the diagnostic approach is the calculation of transtubular potassium gradient TTKG , in which knowledge of serum and urine osmolality values is necessary 39 39 Choi MJ, Ziyadeh FN. The utility of the transtubular potassium gradient in the evaluation of hyperkalemia. J Am Soc Nephrol. Renal tubular acidosis: the clinical entity.

Hypernatremia is associated with an elevated mortality rate, generally due to the deficit of fluid intake, such as in severe hydric restriction, excessive sodium intake, or iatrogenic administration of hyperosmolar solution 32 32 Muhsin SA, Mount DB. Management of hyperosmolar coma. J R Soc Med. Hypernatremia also occurs in renal losses of water or hypotonic fluids, in which there is inadequate ADH secretion pituitary diabetes insipidus , due to the lack of sensitivity to ADH in the nephron nephrogenic diabetes insipidus , by the use of ADH-antagonist drugs, in the presence of osmotic diuresis important hyperglycemias, hypercalcemias or by extrarenal losses of water or hypotonic fluids as in the cases of prolonged vomiting, pulmonary hyperventilation, and prolonged diarrhea.

Measuring serum osmolality contributes to the elucidation of etiology in these circumstances, as it translates the increase of sodium in extracellular fluids 32 32 Muhsin SA, Mount DB.

The decreased plasma sodium concentration or the increased amount of water causes hyponatremia by dilution. In the guidelines proposed by the investigation of hyponatremias, serum osmolality is the main parameter in the differentiation of possible etiologies.

Diagnosis and treatment of hyponatremia: a systematic review of clinical practice guidelines and consensus statements.

BMC Med. Hyponatremia with increased serum osmolality is mainly caused by hyperglycemia, hypertonic mannitol, sucrose, maltose, and low-molecular-weight dextran. In patients with diabetes, the increased concentration of plasma glucose moves water from the intracellular space to the extracellular space, diluting the extracellular sodium concentration. Pseudohyponatremias co-occur with normal serum osmolality.

This can happen in cases of multiple myeloma with hyperproteinemia, severe hyperlipoproteinemia and in the measurements by means of flame photometry. Nowadays, in most services, sodium is measured using the principle of ion selective electrode ISE , in which the electrode is immersed in a biological sample, with the activity of sodium measured depending on the difference of electric potential 18 18 Argyropoulos C, Rondon-Berrios H, Raj DS, et al.

Extreme hypercholesterolemia presenting with pseudohyponatremia - a case report and review of the literature. J Clin Lipidol. Hyponatremia: a review. J Intensive Care Med. Hyponatremias caused by depletion with decreased osmolality occur because of prolonged and excessive low-sodium diets, digestive losses excessive vomiting, prolonged diarrhea, and digestive fistulas or renal losses treatment with thiazide diuretics, and adrenal insufficiency 42 42 Nagler EV, Vanmassenhove J, Van Der Veer SN, et al.

Diagnosis and treatment of hyponatraemia. Hyponatremia with decreased serum osmolality can also occur in situations of excessive fluid intake or water retention higher than electrolyte retention, as observed in the presence of heart failure of low cardiac output, hepatic cirrhosis, hypothyroidism, and severe renal failure 42 42 Nagler EV, Vanmassenhove J, Van Der Veer SN, et al.

In the high anion gap metabolic acidosis, serum osmolality and principally calculation of osmolal gap are valuable tools in the presence of intoxication by alcohol and other drugs 2 2 Glaser DS. Utility of the serum osmol gap in the diagnosis of methanol or ethylene glycol ingestion. Ann Emerg Med. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap.

Clinical problem-solving. Mind the gap. New Engl J Med ; Measurement of methanol and ethylene glycol is not available in most hospitals. Measurement of serum osmolality, along with increased osmolal gap calculation, can contribute to clinical suspicion, as a screening method in these patients 2 2 Glaser DS.

Adrenal insufficiency is a condition characterized by the decreased production or reduction of glucocorticoid action, with or without mineralocorticoid deficiency. Adrenal insufficiency. In patients with neurological lesions to whom osmotherapy with mannitol or hypertonic saline is indicated, measuring serum osmolality can be used as a laboratory parameter in the follow-up of these patients 48 48 Raghava A, Bidkar PU, Prakash MV, Hemavathy B.

Comparison of equiosmolar concentrations of hypertonic saline and mannitol for intraoperative lax brain in patients undergoing craniotomy. Surg Neurol Int. Blood and urinary measures of hydration status during progressive acute dehydration. Med Sci Spo Exer. Dehydration and rehydration in competitive sport. Scand J Med Sci Sports. Measuring serum osmolality is greatly useful in the management of hydroelectrolytic disorders associated with sodium, appearing in several guidelines 51 51 Braun MM, Barstow CH, Pyzocha NJ.

Diagnosis and management of sodium disorders: hyponatremia and hypernatremia. Am Fam Physician. It helps in the diagnosis of hyperglycemias, adrenal insufficiency, therapies with hypertonic solutions in neurological lesions, and in physical exercise. In exogenous intoxications, the osmolal gap has high sensitivity, principally in the detection of alcohol intoxication.

The pieces of equipment currently available in the market are based on principles of colligative properties of solutions: osmotic-pressure elevation, vapor-pressure lowering, and freezing-point depression. When there is no possibility of measuring serum osmolality in the laboratory, formulas can be applied that have good correlation with the clinical picture and the effective measure.

This study tries to demystify the measurement of serum osmolality, a procedure widely available in clinical laboratories, but so little understood regarding its uses and limitations. Abrir menu Brasil. Jornal Brasileiro de Patologia e Medicina Laboratorial. Abrir menu. Daniel K. ABSTRACT Introduction: Serum osmolality is an essential laboratory parameter to understand several clinical disorders such as electrolyte disturbances, exogenous intoxication and hydration status.

Objective: This study aims to update knowledge about the osmolality examination through research papers published to date. Materials and methods: The survey was conducted on PubMed database. Discussion: The measurement of serum osmolality is relevant in changes in intracellular and extracellular balance, as a trusted and valuable indicator of solute concentration in the blood.

Conclusion: The measurement of serum osmolality is useful in cases of dehydration, sodium and potassium disorders, glucose alteration, exogenous poisoning, adrenal insufficiency, neurological injury, physical exercise and others.

Glaser DS. Kruse JA, Cadnapaphornchaihai P.