Lab Tests for Evaluating Fluid Status



Lab Test

Normal Value


Serum osmolality

  • Determined mainly by serum Na+ concentration; one of the most reliable measures of hydration.

275 to 295 mOsm/kg

·Increases in dehydration

·Decreases with water excess

BUN (blood urea nitrogen)

  • Reflects difference between rates of urea synthesis in liver and its excretion by the kidneys.  (Urea is main end product of protein catabolism).

10 to 20 mg/dL

·Increases with decreased renal blood flow or decreased urine production (causing reduced urea clearance), dehydration, some neoplasms, and certain antibiotics (less specific for renal failure than creatinine)

Decreases in pregnancy, overhydration, severe liver disease and low protein intake.


  • Measures portion of blood volume occupied by RBCs

Female:  37-47 ml/100 ml (%)

Male:  40-54 ml/100 ml (%)

Increases in fluid volume deficit.

·Decreases with low RBCs or with normal hemoglobin in the presence of fluid volume excess.

Creatinine (serum)

  • Product of muscle metabolism

0.5 to 1.5 mg/100 ml

·Elevated when 50% or more of the nephrons are destroyed

Serum glucose

70-110 mg/dL

Markedly elevated glucose in blood stream causes osmotic diuresis and fluid volume deficit.

Urine osmolality

  • Measures number of solute particles per unit of water in urine; determines diluting and concentrating ability of kidneys.

50 to 1200 mOsm/L (depends upon the circulating titer of ADH and the rate of urinary solute excretion.

Reflects changes in urine contents more accurately than specific gravity, but depends on the prior state of hydration.  It should be 12 times that of serum osmolality.  Conc. urine has osmolality > 1000.

Urine Specific Gravity (S.G.)

  • Measures degree of concentration of urine; determined by number and weight of solute particles in urine.

1.010 to 1.030

Increases with any condition causing hypoperfusion of kidneys leading to oliguria, i.e., dehydration, shock.

Decreases when renal tubules lose their ability to reabsorb water and concentrate urine as in early pyelonephritis.