Clinical Lab Handbook

Container/Tube: SST/Red Top

Transport Temperature: Refrigerated

Collection Instructions: Separate serum from cellular material within two hours from the time of collection. Evacuated tubes containing gel for serum separation is acceptable.

Stability:

1. Samples can be stored at room temperature for up to 24 hours.
2. Samples can be stored refrigerated for 48 hours.

Advanced Osmometers use the technique of freezing-point depression to measure osmolality. Osmolality is the total solute concentration of an aqueous solution or the measure of the number of dissolved solute particles in solution. Osmometers measure the number of solute particles irrespective of molecular weight or ionic charge. Advanced Osmometers utilize high precision thermistors to sense the sample temperature, to control the degree of supercooling and freeze induction, and to measure the freezing point of the sample. They can routinely determine differences of ±1 mOsm/kg H2O.

Osmolality can assist in the proper diagnosis and treatment of patients with disorders involving water and electrolyte imbalances. It is also useful in the evaluation of patients suspected of having ingested a toxin such as the alcohols methanol, ethanol, and isopropanol; ethylene and propylene glycol; salicylic acid; and paraldehyde.

The hypothalamus (at the base of the brain) responds to an increase in osmolality (usually representing an increase in Na+ of less than 1%, activating two types of protective responses:

Thirst sensors respond to an increase in osmotic pressure, increasing water intake, lowering osmolality, and returning the system to normal. To a much lesser degree, thirst receptors respond to a decrease in intravascular volume. Intake of water triggered by thirst is the most important factor in maintenance of normal water and electrolyte status. Patients with neurologic disorders, the elderly, newborns, and those without access to water (including infants) often cannot respond to this signal; thus, they are prone to dehydration. Antidiuretic hormone (ADH). The hypothalamus also produces this hormone in response to an increase in osmolality. ADH causes an increase in permeability of the collecting ducts of the kidney to water, increasing urine osmolality and attempting to return plasma osmolality to normal. While ADH can decrease water loss in the urine, it can only reduce total water losses from the body to a minimum of about 1-1.5 L daily.

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