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Clinical Laboratory

Osmometer

An osmometer is a device used in clinical laboratories for measuring the concentration of particles in a solution, known as the osmolar concentration. This quantity can be expressed as osmolality (in mmol/kg) or osmolarity (in mmol/L). Clinical laboratories typically measure osmolality, which is considered more precise as weight is temperature-independent. In laboratory analysis, a dissolved substance is referred to as a solute, and the substance in which the solute is dissolved is referred to a

v1 · Reviewed Nov 7, 2026 by Automated Clinical Reviewer (Strict Verbatim v1)
  • Hyponatremia in the context of elevated serum osmolality (>295 mOsm/kg) is most commonly caused by elevated levels of serum glucose, or it may also occur when osmotically active substances such as mannitol, radiographic contrast agents, and glycine (surgical irrigant solutions) are administered.
  • Ingestion of toxic alcohols such as ethanol, methanol, propylene glycol, and acetone can cause elevated osmol gap.
  • Hypernatremia and hyperosmolar plasma in the context of hypoosmolar urine (<300 mOsm/kg) are suggestive of diabetes insipidus.
  • An elevated serum osmol gap can be found in the following clinical scenarios or disease states:
  • An osmometer is a device used in clinical laboratories for measuring the concentration of particles in a solution, known as the osmolar concentration.
  • Clinical laboratories typically measure osmolality, which is considered more precise as weight is temperature-independent.
  • The freezing point depression method is commonly employed in clinical laboratories due to its high accuracy and ease of execution.
  • Vapor pressure osmometers determine the osmolality of a solution by measuring the voltage difference between 2 thermistors.
  • membrane osmometers measure the flow of solvent, often water, from a pure solvent container across a semipermeable membrane into a solution containing a solute and the same solvent.
  • Alternative methods to osmometers can be used to measure osmolality, including electrical conductivity and specific gravity.
  • Acceptable specimens, in general, include serum, heparinized plasma, tears, sweat, and urine.
  • Serum osmolality is an important initial test employed in investigating hyponatremia.
Calculated Serum Osmolality
Osmolality (mOsm/kg) = 2×Na (mmol/L) + glucose (mg/dL)/18 + urea (mg/dL)/2.8
Osmol Gap
The difference between the calculated and measured osmolality is known as the osmol gap, also known as the osmolal or osmolar gap.
Normal Osmol Gap
A normal osmol gap is 2 or less, although according to some sources, it is greater than 10 mOsm/kg in the clinical environment.
Stool Osmotic Gap
The stool osmotic gap is defined as the difference between the measured osmolality and a calculated osmolality (2*([fecal Na+] + [fecal K+]).
Stool Osmotic Gap Interpretation
A stool osmotic gap <50 mOsm/kg implies that the diarrhea is secretory.
Stool Osmotic Gap Interpretation
In patients with osmotic diarrhea, the stool osmotic gap is typically >75 mOsm/kg.
  • Specimens should undergo centrifugation before analysis to eliminate any particulate matter.
  • A full calibration of osmometers is required at least every 6 months.
  • For QC purposes, laboratories are typically mandated to conduct 2 controls at 2 different concentrations daily or with each batch of samples.
  • Acceptable controls are generally within ±2 SD from the mean.
  • Enter or scan the sample ID.
  • Load the sample to be tested into the osmometer.
  • Record the osmolality of the sample.
  • Alternative methods to osmometers can be used to measure osmolality, including electrical conductivity and specific gravity.
  • Specific gravity was utilized before osmometers were practical to measure the osmolality of urine, utilizing various instruments such as refractometers, which measure the refraction of light through a fluid, or hydrometers.
  • The applicability and accuracy of these methods besides osmometry for determining a sample's osmolality depend on the specific method and the sample.
  • In patients with low serum osmolality and a clinically normal extracellular fluid volume, urine osmolality can aid in distinguishing between various etiologies.
  • In these cases, a urine osmolality >100 mOsm/kg is indicative of the syndrome of inappropriate antidiuretic hormone, hypothyroidism, or glucocorticoid deficiency.
  • On the other hand, a urine osmolality <100 mOsm/kg is observed in patients with primary polydipsia or beer potomania.
  • The stability of different samples varies, with some general guidelines as follows:
  • Unspun serum/plasma (room temperature or refrigerated at 2 to 8 °C): 3 hours
  • Separated serum/plasma (room temperature or refrigerated at 2 to 8 °C): 48 hours
  • Urine (room temperature): 24 hours
  • Urine (refrigerated or frozen): 7 days
  • Particulate matter can cause premature crystallization of samples, which can be avoided by centrifuging blood and urine samples or filtering urine samples.
  • The presence of air bubbles in a sample may also result in the inadequate freezing of samples.
  • Test samples allowed to evaporate, non-aqueous, or highly viscous may not freeze well and yield an inaccurate result.
  • review
    Osmometer. StatPearls [Internet] — Michael C. Larkins, Muhammad Zubair, Aparna Thombare — StatPearls Publishing, 2024

Decision support only. Verify against local protocols and current labeling. Source: www.ncbi.nlm.nih.gov