Content area
Full Text
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 dissolved solid components to the unit volume of a specific urine specimen. It reflects the relative degree of concentration or dilution of the urine specimen. Minimal amounts of urine are needed for determination of U-SG, an easy, rapid, non-invasive, and inexpensive way to obtain bedside information on the state of hydration of a patient at any age. Measurement of urine osmolality by freezing point depression in the routine laboratory is not practical for the acute management of fluid and electrolyte abnormalities, certainly not in the very young. It is also a relatively expensive procedure. For these reasons many physicians rely on "dipsticks" (reagent strips) and/or refractometer measurements of U-SG.
Published experience on the use of U-SG measurements with commercially available dipsticks is controversial. 1-5 In the present, prospective study the results of specific dipstick U-SG measurements were compared to other U-SG determinations in the same urine samples.
Methods
Within a few working days, 135 fresh voided urine specimens were collected from children visiting the outpatient renal clinic for various reasons (enuresis, urinary tract infections, malformations of the urinary tract, arterial hypertension, chronic renal insufficiency).
No preservative was used in the collecting bottle, each untreated urine specimen being immediately tested in the laboratory of the paediatric renal unit for osmolality (U-Osm; micro-osmometer, Advanced Instruments, Needham Heights, Massachusetts) and urine pH (U-pH) with a pH meter (TTT- titrator, Radiometer, Copenhagen, Denmark). The U-pH was measured as U-SG determinations may need adjustments according to U-pH...