Osmolality, Serum

OSMO

GOLD (SST)

1 mL blood (0.5 ml Serum)

Serum (Red) or Gold (SST) or Orange (OJ) or Tiger Top Serum
GREEN (Li Heparin Plasma)

Allow serum tubes to clot adequately before centrifugation.
Keep tubes stoppered and upright at all times.

24x7

No interferences noted.
Analytical Range:  0-2000 mOsm/kg

282-300 mOsm/kg

Serum Osmolality determinations aid in the detection and monitoring of Hypernatremia, Hyponatremia, False hyponatremia, Hyperglycemia, Dehydration, Renal Disease, Drug Poisoning, and Non-ketotic Diabetic Acidosis. When used with sodium concentration, osmolality is useful in distinguishing between true sodium depletion and dilution, and between primary water loss and false hyponatremia. Dialyzer efficacy can also be determined using osmolality values. Whole blood osmolality measurements are a rapid way of determining the effect of administration of sodium bicarbonate during CPR. Values obtained using whole blood have been shown to be comparable to those using serum. When the serum osmolality exceeds 350 mOsm/kg during CPR, a disproportionate increase in mortality is seen.
 
Screening for Toxin ingestion: In the evaluation of patients suspected of having ingested a toxin, if the poison is present in millimolar concentrations, then the osmolality will be increased. The osmotic gap gives a reliable estimate of the molar concentration of additional substances present such as alcohols methanol, ethanol, and Isopropanol; ethylene glycol and propylene glycol; salicylic acid; and paraldehyde. If the compound can be identified, then the concentration in mg/dL can be calculated by multiplying the osmotic gap by the factor (molecular weight/10). This is one argument for using Freezing point depression for osmolality measurements, which detects both volatile and non-volatile toxins, as opposed to vapor pressure osmometry which is not sensitive to volatile toxins.
 
Monitoring the concentration of osmotically active agents: In the treatment of patients with cerebral edema (swelling of the brain), osmotically active compounds such as mannitol are often given in an attempt to draw water out of cells, thereby reducing the amount of edema present. Because there is no easy way to monitor the concentration of the mannitol, using the osmotic gap is a way to estimate mannitol concentration. The goal of the therapy is to maintain an increase in osmotic gap of 10 mosm/kg. If the osmotic gap reaches 50 mosm/kg, then renal damage from the mannitol will occur. This shows the necessity of good precision for measuring osmolality and electrolytes.
 
Evaluation of hyponatremia: Sodium is the major contributor to serum osmolality, therefore most patients with hyponatremia also have a decreased serum osmolality.
 

1. Occasionally, a low serum sodium is not associated with hypo-osmolality. This can occur in patients with an increased serum glucose, since glucose is capable of forcing water to leave the intracellular fluid, thus diluting serum electrolytes. Patients with diabetes usually have an increased osmolality, with an occasional patient falling within the normal range. For approximately every 100 mg/dL increase in glucose, sodium concentration should fall by between 1.6-2.0 mmol/L. Because this increases osmolality 5.6 mosmol/kg (for the glucose) while decreasing it 3.2-4.0 mosmol/kg (for the sodium), it follows that osmolality will increase 1.5-2.5 mosmol/kg for each 100 mg/dL increase in glucose. A greater increase implies significant loss of water relative to sodium or the presence of other osmotically active molecules; a lower increase implies excess sodium loss.
2. Other osmotically active substances, such as mannitol (as used in "2." above) and glycine may produce the same phenomenon of falling sodium levels, though with a normal serum osmolality. Glycine is used by urologists in irrigation fluids while performing a transurethral resection of the prostate (TURP). In up to 10% of cases the fluid is absorbed enough to significantly dilute the serum sodium. The amount of fluid absorbed and the rate of glycine metabolism can be estimated from the osmolal gap.
3. Patients whose water content of plasma is lower than normal will have falsely low serum sodium. This phenomenon occurs in the presence of markedly increased total protein or lipids when sodium is measured by ion selective electrodes using diluted specimens. Since osmolality is related to solute concentration in water, these patients will have a normal osmolality.