Osmolality, Urine

UOSMO

URINE RANDOM (URO)
U RED/CLR Vacutainer
 

1 mL urine

Collect urine samples in an appropriate urine container with no preservatives. 
 

24x7

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

38-1400 mOsm/kg

Urine Osmolality is a measure of total urine solute concentration, it provides the best estimate in the evaluation of the kidney's concentrating ability, and changes in renal function. In normal patients the urine is composed waste products such as creatinine and urea, approximately 80% of the total solute in normal urine. In patients with kidney disease, electrolytes make up an increasing percentage of total solute, while persons with very high blood levels of other solutes, such as glucose or ethanol, may have over 30% of urine solute composed of these substances. Urine osmolality is usually interpreted along with measured urine electrolytes and creatinine. It is used primarily to evaluate Increased Urine Output, Decreased Urine Output, and Renal Acidification Defects.
Increased Urine Output is generally evaluated as one of three major causes.
 

1. Most common is an increased ingestion of water (Polydipsia) due to either a compulsion to drink due to a psychlogical disorder (psychogenic polydipsia) or dry mouth causing the perception of thirst (primary polydipsia). Urine osmolality will be at maximal diluting capacity, which is typically below 100 mosm/kg. (Low serum, Low urine osmolality)
2. Increased water ingestion due to an absence of Antidiuretic hormone or ADH (Central Diabetes Insipidus) or an inability of the kidney to respond to ADH possibly due to medications such as lithium (Nephrogenic Diabetes Insipidus). (High serum, Low urine osmolality)
3. An increased concentration of glucose (Diabetes Mellitus) in the urine causes an increase in loss of water. (High serum, High urine osmolality)

 
Decreased Urine Output is generally due to intrinsic kidney problems or an appropriate attempt by the body to conserve water and electrolytes.
 

1. With kidney disease (generally acute damage to the tubules of the kidney) the urine osmolality approaches that of serum (approximately 290 mosm/kg) and the free water clearance* will approach zero.
2. With dehydration the urine osmolality is extremely high and the free water clearance* will be negative. Patients that are suspected of having inappropriate ADH production, the urine osmolality tends to be high, but it does not rise with fluid restriction or fall when fluids are administered.

 
Renal Acidification Defects are found in some cases of kidney disease, when there is an inability of the kidney to maximally excrete acid or reabsorb bicarbonate. Normally the main form of acid excreted by the kidney is NH4⁺. These patients tend to have a reduction in ammonium ion excretion.