Specific Gravity (sp gr) and pH parameters in Urinalysis

Specific gravity (directly proportional to urine osmolality which measures solute concentration) measures urine density or the ability of the kidney to concentrate or dilute the urine over that of plasma. Dipsticks are available that also measure specific gravity in approximations. Most laboratories measure specific gravity with a refractometer.

Specific gravity between (1.002 - 1.035) on a random sample should be considered normal if kidney function is normal. Since the sp gr of the glomerular filtrate in Bowman's space ranges from (1.007 - 1.010), any measurement below this range indicates hydration and any measurement above it indicates relative dehydration.

If sp gr is not > 1.022 after a 12 hour period without food or water, renal concentrating ability is impaired and the patient either has generaliz ed renal impairment or nephrogenic diabetes insipidus. In end-stage renal disease, sp gr tends to become 1.007 to 1.010. Any urine having a specific gravity over 1.035 is either contaminated contains very high levels of glucose, or the patient may have recently received high density radiopaque dyes intravenously for radiographic studies or low molecular weight dextran solutions. Subtract 0.004 for every 1% glucose to determine non-glucose solute concentration.

pH

The glomerular filtrate of blood plasma is usually acidified by renal tubules and collecting ducts from a pH of (7.4 - 6) in the final urine. However, depending on the acid-base status, urinary pH may range from as low as 4.5 to as high as 8.0. Th e change to the acid side of 7.4 is accomplished in the distal convoluted tubule and the collecting duct.

Urinalysis Reagent Strips