Tuesday, August 16, 2011
Urinalysis - concentration
Here are some simple facts about urine specific gravity, osmolality and their determination from the urinalysis.
The reference substance for comparative purposes is water, which therefore has a specific gravity of 1.000. The so-called normal ranges are completely dependent on the amount of fluid ingested and solute excreted. Therefore, for example, uncontrolled diabetes mellitus may have a high urine specific gravity (due to the high amounts of glucosuria), as well volume depletion states and proteinuric conditions. Low urine specific gravity may be caused by excessive fluid intake, diabetes insipidus and diuretics, which all cause a relatively dilute urine to be formed.
Proximal tubular injury (e.g. acute tubular necrosis) may interfere with urinary concentrating capabilities, leading to isosthenuria (a specific gravity of 1.007-1.010). This is a set of circumstances whereby the final urine concentration is essentially equal to that of the glomerular filtrate produced at the early proximal tubule.
Note there are potential false elevations in urine specific gravity, many of which are caused by radiographic dyes, which can produce readings >1.03, if measured at a time close to the procedure.
Specific gravity measured by urine dipstick
The reagent strip in the usual urine dipstick actually measures the ionic concentration of urine. The free ions react with a pH indicator in the strip, thereby causing a change in colour, corresponding to the amount of solute present. Obviously, certain molecules may dissociate more freely than others, which can affect how easily they are to detect.
Osmolality differs from the specific gravity in that it depends on the amount of solutes, not on their molecular weight. The range of potential osmolality that can be produced by the normal kidney ranges from around 80-1200 mOsm/Kg. In normal circumstances, the approximate relationship between urine specific gravity and osmolality is 350mOsm/kg per 0.01 unit change in specific gravity (e.g. a specific gravity of 1.01 is equivalent to a urine osmolality of 350mOsm/kg). However, this relationship breaks down in the setting of larger molecular weight compounds, like glucose, certain antibiotics and radiocontrast material.