Clinical Overview Emergency Medicine Internal Medicine/Endocrinology Obstetrics/Gynecology Urology/Renal Function Surgery/Critical Care

Emergency Medicine
The speed and simplicity of testing with a modern osmometer makes it a very useful first step in the evaluation of cases presented to the emergency room, in monitoring response to fluid therapy and in monitoring the recovery from surgical procedures. Freezing point is the method of choice for osmometers in this field, because they are sensitive to all types of solutes. Vapor pressure osmometers will not read volatile solutes, which may deprive the clinician of significant information in cases of toxic ingestion of methanol or ethylene glycol, as well as the necessary ability to monitor therapeutic infusions of mannitol.

Drug Intoxication Screening: The ingestion of toxic substances, often as a cheap alternative to ethanol, is a frequent occurrence and not limited to inner city hospitals. Differentiation from alcohols, by enzymatic means, must be confirmed, but in all instances the course of treatment is most easily monitored by routine osmolality.

Head Injury: Mannitol, an intracranial pressure (ICP) lowering agent is sometimes dramatically effective in reversing acute brain swelling. There is evidence that in prolonged dosage, mannitol may pass from the blood into the brain, where it might cause reverse osmotic shifts that increase intracranial pressure. Having a freezing point osmometer available to rapidly assess these osmotic shifts is valuable.

Monitoring Mannitol Therapy: Monitoring the concentration of osmotically active agents with freezing point osmometry is an important advantage of freezing point over other methods.

In the case of mannitol, an osmotically active agent used by neurologists and neurosurgeons to control cerebral edema, freezing-point osmometry is one of the few easy methods to determine the concentration of mannitol.

The goal is to maintain an increase in osmotic gap of 10 mOsm/Kg , though increments as low as 5 mOsm have been shown to be effective in lowering intracranial pressure .

It is important to keep the increase at as small a level as possible because too high sustained elevation of serum osmolality appears to increase the risk of rebound elevations of intracranial pressure and hyperosmolar encephalopathy . If the gap were to reach 50 mOsm/Kg, renal damage is likely

An important use of mannitol in treating increases in intracranial pressure, is in the treatment of Reye's Syndrome in children. Suggestions are at least 12 determinations per day be used to make sure that serum osmolality does not go above 340 mOsm/Kg . Of course, being a volatile, mannitol will not be detected by vapor pressure osmometry and caution should be used when employing this method or calculated osmometry alone

Coma: In cases where patients are non-responsive or demonstrate erratic behavior, serum osmolality is a valuable tool to assess mental status state, patient state of hydration and osmolality.

Burns: Osmolality offers a rapid assessment of the burn patient’s state of hydration and helps to monitor proper fluid therapy.

Quality Control: Pharmacy: Hospital admixtures, pharmaceutical
manufacturers, and nutritional support formulary can use routine osmolalities. Benefits include monitoring for consistency and proper formulation, minimizing infusion trauma, and optimizing uptake and reaction kinetics.

Emergency Medicine: The speed and simplicity of testing with a modern osmometer makes it a very useful first step in the evaluation of cases presented to the emergency room, in monitoring response to fluid therapy and in monitoring the recovery from surgical procedures. Freezing point is the method of choice for osmometers in this field, because they are sensitive to all types of solutes. Vapor pressure osmometers will not read volatile solutes, which may deprive the clinician of significant information in cases of toxic ingestion of methanol or ethylene glycol, as well as the necessary ability to monitor therapeutic infusions of mannitol.

Alcohol Intoxication: Ethanol ingestion will elevate serum osmolality by 23 mOsm for each 0.1%. This benchmark has been used by laboratories that don't have a stat ETOH method. The Dorwart and Chalmers formula is well recognized, and subsequent studies have helped to understand previously unexplained osmolal gaps (UEOGs). With the availability of enzymatic assays for ethanol, and the legal issues involved with testing, osmometers have a secure place in the quality control of standards. (Redetzki)

Differential Toxicology: Probing the osmolal gap, one might find a variety of small MW solutes, at toxic levels, that are most easily detected by an osmometer. Methanol, ethylene glycol, and salycylates are the usual suspects in the emergency room setting.

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