Density (specific gravity)

The determination of urinary density is a valuable tool to evaluate the kidney concentration and dilution capacity, which plays a crucial role in the maintenance of the hydro-osmotic balance (homoeostasis). This measurement reflects the amount of constituants dissolved in urine, i.e. their concentration relative to the sample volume. The measurement is temperature dependent and should be performed at 20 ° C. The specific gravity units are expressed in g/mL; in SI units it is expressed in kg/m3.

The teststrip determination is practical and simple to use as a screening and routine method . It gives a rough estimate of urine concentration. However significant discrepancies in precision were observed when comparing repeated measurements.

The test measures the urinary ion concentration and shows a good correlation with the results obtained by the refractometric method. It is based on proton release and complex formation in presence of cations, which leads to a change in color of the Bromothymol Blue indicator which turns from blue to turquoise, then to yellow.

Urinary teststrip: reading scale (left) , then a 1.020 value (right)

 

Causes of error

Density is overestimated by the teststrip in presence of protein (100-500 mg/l), and also in case of ceto-acidosis. An increase in urinary density due to a high glucose concentration > 56 mmol/L (>1000mg/dL) is not detected. For pH values of 7 and obove 7,0.005 must be added to the result.

Reference values : 1.003-1.030 g/mL (possibly higher)

Density values :

  • Eusthenuria: 1.010-1.030 g/mL.
    This corresponds to the normal reference range.Density of normal urine sample is 1.020 g/mL (or more in case of fluid restriction during the night).
  • Hyposthenuria : < 1.010 g/mL
  • Hypersthenuria: > 1.030 g/mL
  • Isosthenuria: Density of urine is constant at about 1.010 g/mL and does not vary as a function of urinary volume. A severe renal affection is the cause of this loss of the dilution or concentration capacity.

Specific gravity measurement allows also to estimate or to calculate precisely the amount of fluid intake in case of treatment of renal calculi, or in case of drug treatment, or in case of drug abuse follow-up. It cans this detect a possible fraudulent dilution of urine.

In parallel, it is recommended to measure also creatinin !

Clinical relevance

Decrease in specific gravity

  • Diabetes insipidus and nephrogenic diabetes insipidus: in case of antidiuretic hormone (ADH) insuffiicency or in case of ADH renal receptor defects, the distal and collecting tubule water permeability increases. Large volumes of urine are passed with a specific gravity of 1.001 à 1.003 g/mL.
  • Diseases of the renal parnchyma , with severe damage of the urinary tract such as pyelonephritis, polykystic kidney and hydronephrosis.
  • Severe potassium deficiency
  • Hypocalcelmia of various origin

Increase in specific gravity

  • Massive fluid loss (perspiration, fever, vomiting, diarrhea)
  • Adrenal insufficiency
  • Hepatic disease
  • Cardiac insufficiency

See also chapter "Physical examination ".