Sorted By Test Name - Mayo Medical Laboratories

UOSMS
9340
UOSMU
9260
If sodium concentration or 24-hour sodium excretion rate is 2 to 3 times normal and osmotic gap >30
mOsm/kg, secretory diarrhea may be the cause. Agents such as phenolphthalein, bisacodyl, or cholera
toxin should be suspected. An osmotic gap >100 mOsm/kg indicates factitial diarrhea, likely due to
magnesium or phenolphthalein consumption. For very low stool osmolality, consider factitial diarrhea.
Reference Values:
0-15 years: not established
> or =16 years: 220-280 mOsm/kg
Clinical References: 1. Phillips S, Donaldson L, Geisler K, et al: Stool composition in factitial
diarrhea: a 6-year experience with stool analysis. Ann Intern Med 1995;123:97-100 2. Ho J, Moyer T,
Phillips S: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092
Osmolality, Serum
Clinical Information: Osmolality is a measure of the number of dissolved solute particles in solution.
It is determined by the number and not by the nature of the particles in solution. Dissolved solutes change
the physical properties of solutions, increasing the osmotic pressure and boiling point and decreasing the
vapor pressure and freezing point. The osmolality of serum increases with dehydration and decreases with
overhydration. The patient receiving intravenous fluids should have a normal osmolality. If the osmolality
rises, the fluids contain relatively more electrolytes than water. If the osmolality falls, relatively more
water than electrolytes is being administered. Normally, the ratio of serum sodium, in mEq/L, to serum
osmolality, in mOsm/kg, is between 0.43 and 0.5. The ratio may be distorted in drug intoxication.
Generally, the same conditions that decrease or increase the serum sodium concentration affect the
osmolality. A comparison of measured and calculated serum osmolality produces a delta-osmolality. If
this is >40 mOsm/kg a H2O in a critically ill patient, the prognosis is poor. An easy formula to calculate
osmolality is: Osmolality (mOsm/kg H2O)=2 NA+ Glucose + BUN 20 3
Useful For: Evaluating acutely ill or comatose patients
Interpretation: An increased gap between measured and calculated osmolality may indicate ingestion
of poison, ethylene glycol, methanol, or isopropanol.
Reference Values:
275-295 mOsm/kg
Clinical References: Murphy JE, Henry JB: Evaluation of renal function, and water, and electrolyte,
and acid base balance. In Todd-Sanford-Davidsohn Clinical Diagnosis and Management by Laboratory
Methods. 19th edition. Edited by JB Henry. Philadelphia, PA, WB Saunders Company, 2006
Osmolality, Urine
Clinical Information: Osmolality is an index of the solute concentration. Urine osmolality is a
measure of the concentration of osmotically active particles, principally sodium, chloride, potassium, and
urea; glucose can contribute significantly to the osmolality when present in substantial amounts in urine.
Urinary osmolality corresponds to urine specific gravity in nondisease states. The ability of the kidney to
maintain both tonicity and water balance of the extracellular fluid can be evaluated by measuring the
osmolality of the urine either routinely or under artificial conditions. More information concerning the
state of renal water handling, or abnormalities of urine dilution or concentration can be obtained if urinary
osmolality is compared to serum osmolality and if urine electrolyte studies are performed. Normally, the
ratio of urine osmolality to serum osmolality is 1.0-3.0, reflecting a wide range of urine osmolality.
Useful For: Assessing the concentrating and diluting ability of the kidney
Interpretation: With normal fluid intake and normal diet, a patient will produce a urine of about
500-850 mosmol/kg water. Above age of 20 years there is an age dependent decline in the upper reference
range of approximately 5 mOsm/kg/year. The normal kidney can concentrate a urine to 800-1,400
mosmol/kg and with excess fluid intake, a minimal osmolality of 40-80 mosmol/kg can be obtained. With
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