Laboratory Tests for Evaluating Fluid Status

LABORATORY TESTS FOR EVALUATING FLUID STATUS

Osmolality reflects the concentration of fluid that affects themovement of water between fluid compartments by osmosis. Osmolality measures the solute concentration per kilogram in blood and urine. 

It is also a measure of a solution’s ability to create osmotic pressure and affect the movement of water. Serum os-molality primarily reflects the concentration of sodium. Urine osmolality is determined by urea, creatinine, and uric acid. When measured with serum osmolality, urine osmolality is the most re-liable indicator of urine concentration. Osmolality is reported as milliosmoles per kilogram of water (mOsm/kg).

Osmolarity, another term that describes the concentration ofsolutions, is measured in milliosmoles per liter (mOsm/L). The term “osmolality,” however, is used more often in clinical practice. Normal serum osmolality is 280 to 300 mOsm/kg, and normal urine osmolality is 250 to 900 mOsm/kg. Sodium predominates in ECF osmolality and holds water in this compartment.

Factors that increase and decrease serum and urine osmolality are identified in Table 14-3. Serum osmolality may be measured directly through laboratory tests or estimated at the bedside by doubling the serum sodium level or by using the following formula:

The calculated value usually is within 10 mOsm of the mea-sured osmolality.

Urine specific gravity measures the kidneys’ ability to excrete or conserve water. The specific gravity of urine is compared to the weight of distilled water, which has a specific gravity of 1.000. The normal range of specific gravity is 1.010 to 1.025. Urine spe-cific gravity can be measured at the bedside by placing a calibrated hydrometer or urinometer in a cylinder of approximately 20 mL of urine. 

Specific gravity can also be assessed with a refractometer or dipstick with a reagent for this purpose. Specific gravity varies inversely with urine volume; normally, the larger the volume of urine, the lower the specific gravity. Specific gravity is a less reli-able indicator of concentration than urine osmolality; increased glucose or protein in urine can cause a falsely high specific grav-ity. Factors that increase or decrease urine osmolality are the same for urine specific gravity.

Blood urea nitrogen (BUN) is made up of urea, an end prod-uct of metabolism of protein (from both muscle and dietary in-take) by the liver. Amino acid breakdown produces large amounts of ammonia molecules, which are absorbed into the bloodstream. Ammonia molecules are converted to urea and excreted in the urine. The normal BUN is 10 to 20 mg/dL (3.5–7 mmol/L). The BUN level varies with urine output. Factors that increase BUN include decreased renal function, GI bleeding, dehydra-tion, increased protein intake, fever, and sepsis. Those that de-crease BUN include end-stage liver disease, a low-protein diet, starvation, and any condition that results in expanded fluid vol-ume (eg, pregnancy).

Creatinine is the end product of muscle metabolism. It is a bet-ter indicator of renal function than BUN because it does not vary with protein intake and metabolic state. The normal serum crea-tinine is approximately 0.7 to 1.5 mg/dL (SI: 60–130 mmol/L); however, its concentration depends on lean body mass and varies from person to person. Serum creatinine levels increase when renal function decreases.

Hematocrit measures the volume percentage of red blood cells (erythrocytes) in whole blood and normally ranges from 44% to 52% for males and 39% to 47% for females. Conditions that in-crease the hematocrit value are dehydration and polycythemia; those that decrease hematocrit are overhydration and anemia.

Urine sodium values change with sodium intake and the status of fluid volume (as sodium intake increases, excretion increases; as the circulating fluid volume decreases, sodium is conserved). Normal urine sodium levels range from 50 to 220 mEq/24 h (50–220 mmol/24 h). A random specimen usually contains more than 40 mEq/L of sodium. Urine sodium levels are used to assess volume status and are useful in the diagnosis of hyponatremia and acute renal failure.