Dipstick measurements of urine specific gravity are unreliable.

Abstract

Aim--To evaluate the reliability of dipstick measurements of urine specific gravity (U-SG).

Methods--Fresh urine specimens were tested for urine pH and osmolality (U-pH, U-Osm) by a pH meter and an osmometer, and for U-SG by three different methods (refractometry, automatic readout of a dipstick (Clinitek-50), and (visual) change of colour of the dipstick).

Results--The correlations between the visual U-SG dipstick measurements and U-SG determined by a refractometer and the comparison of Clinitek[R]-50 dipstick U-SG measurements with U-Osm were less than optimal, showing very wide scatter of values. Only the U-SG refractometer values and U-Osm had a good linear correlation. The tested dipstick was unreliable for the bedside determination of U-SG, even after correction for U-pH, as recommended by the manufacturer.

Conclusions--Among the bedside determinations, only refractometry gives reliable U-SG results. Dipstick U-SG measurements should be abandoned. (Arch Dis Child 200 1;85:155-157)

Keywords: urine specific gravity; dipstick urinalysis

Evaluation of the urinary concentration is an easy and valuable way of determining the fluid and electrolyte status of a patient. This can help in the day to day care of the critically ill, such as postoperatively and in the intensive care unit, and may be helpful in an outpatient setting. The urinary concentration depends on the presence of small (electrolytes, phosphate, urea, uric acid) and larger particles (proteins, glucose, radiographic contrast media) per unit of urine volume. The latter is obviously related to the state of hydration of the individual and depends largely on the amount of fluid reabsorbed along the renal tubule.

The concentration of solutes in the urine can be assessed by measuring its specific gravity or its osmolality. Urine specific gravity (U-SG) represents the relative proportion of …