From the Guidelines
Effective serum osmolality should be maintained between 275-295 mOsm/kg to prevent cellular damage and ensure proper cellular function, as evidenced by recent studies 1. The calculation of effective serum osmolality is crucial in diagnosing and managing various conditions, including hyponatremia, hypernatremia, and diabetes insipidus. It is calculated as 2 × serum sodium (mEq/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Some key points to consider when managing effective serum osmolality include:
- Normal values range from 275-295 mOsm/kg
- Effective osmolality excludes urea since it freely crosses cell membranes and doesn't contribute to osmotic pressure gradients
- Changes in effective osmolality directly impact cell volume, with increased levels causing cells to shrink and decreased levels causing cells to swell
- Monitoring effective serum osmolality is particularly crucial in neurological conditions, as brain cells are sensitive to volume changes, with rapid fluctuations potentially causing cerebral edema or demyelination syndromes, as noted in studies on the management of intracerebral hemorrhage 1. The most recent and highest quality study on the management of hyponatremia, published in 2018, recommends treating hyponatremia when serum sodium is lower than 130 mmol/L, although there is no good evidence regarding the level of serum sodium at which treatment should be initiated 1. In terms of treatment, the use of hypertonic saline solutions has been shown to reduce intracranial pressure in various conditions, including intracerebral hemorrhage, as noted in a study published in 2007 1. However, the exact mechanism of action, best mode of administration, and concentration to be given remain to be clarified. Overall, maintaining effective serum osmolality within the normal range is crucial to prevent cellular damage and ensure proper cellular function, and treatment should be guided by the most recent and highest quality evidence available 1.
From the FDA Drug Label
Removal of excess free body water increases serum osmolality and serum sodium concentrations.
The FDA drug label indicates that tolvaptan increases serum osmolality by removing excess free body water, which in turn increases serum sodium concentrations.
- Key points:
- Serum osmolality increases as a result of tolvaptan treatment.
- Serum sodium concentrations also increase due to the removal of excess free body water.
- Patients should be monitored to ensure serum sodium remains within normal limits 2.
From the Research
Effective Serum Osmolality
- Effective serum osmolality, also known as blood tonicity, is created by the endogenous (e.g., sodium and glucose) and exogenous (e.g., mannitol) solutes that are capable of creating an osmotic gradient across the membranes 3.
- Under normal conditions, serum sodium concentration is the major determinant of serum osmolality 3.
- The difference between measured osmolality and calculated osmolarity forms the osmolal gap, which can be increased by the presence of solutes that are not included in the osmolarity calculation, such as hypertonic treatments or toxic alcoholic ingestions 3.
Diagnosis and Treatment
- Determination of serum osmolality and osmolal gap is important in the diagnosis of disorders related to sodium, glucose, and water balance, kidney diseases, and small molecule poisonings 3.
- Hyponatremia is the most common electrolyte disorder and can be treated with various options, including fluid restriction, hypertonic saline, urea, demeclocycline, and vasopressin receptor antagonists (vaptans) 4, 5, 6, 7.
- The treatment of hyponatremia should be based on the underlying cause and the severity of symptoms, and should aim to correct serum sodium levels at a rate of less than 8-10 mmol/liter per day to avoid osmotic demyelination 4, 5, 6.
Hyperosmolar States
- Hyperosmolar states, including hypernatremia, diabetic ketoacidosis, and hyperglycemic hyperosmolar syndrome, can have significant clinical consequences, including neurologic symptoms and even death 3.
- Prompt diagnosis and cautious management of hyperosmolar states are essential to prevent these complications 3.
- The use of hypertonic saline, isotonic saline, water restriction, long loops diuretics, urea, or vaptans may be considered in the treatment of hyperosmolar states, depending on the underlying cause and the severity of symptoms 7.