Advanced interpretation of the urine dipstick (Proceedings)
Indications for performing a urinalysis
In addition to the CBC and chemistry panel, the urinalysis is the third component of the minimum database. In addition to helping in the evaluation and monitoring of diseases of the kidneys and the lower urinary tract, a urinalysis provides information on the function of a number of other organs. In general, a urinalysis should be performed whenever as part of any 'metabolic screen' or 'healthy animal exam,' or when a clinician is investigating any systemic clinical sign or disease. The urinalysis is also be an essential part of monitoring specific diseases or therapies, such as with diabetes mellitus or administration of some antibiotics. Familiarity with how varopis diseases can affect the urinalysis and how to further work-up abnormalities are necessary for a proper appreciation of this diagnostic test.
Urinalysis methodologyThe dipstick portion of the urinalysis should be performed within 30 minutes of sample collection, ideally prior to refrigeration. If refrigeration prior to analysis cannot be avoided, urine should be warmed to room temperature and mixed well prior to examination; if urine is opaque or turbid then gently centrifuge the sample as described for preparation of urine sediment examination and test only the supernatant.
Dipsticks are very prone to interference from a variety of chemicals. The pads should never be touched, exposed to room air for extended periods, or allowed to come into contact with moisture. Never use expired strips. Avoid using an excess amount of urine for the dipsticks; excess urine should be removed by tapping the strip on a paper towel or dragging the edge across a clean surface.
Specific gravity estimates urine osmolarity, which is an indirect measure of renal function. In normal animals specific gravity can vary greatly; this author uses 1.040 in cats and 1.030 in dogs as cut-offs for 'almost 100% guaranteed to be normal.' However, in the absence of any clinical signs (for example, during a routine health exam), lower values can also be acceptable. Conversely, some animals (especially cats with kidney disease or dogs with glomerular disease) may still be able to concentrate their urine despite being in renal failure. When urine is persistently isosthenuric or minimally concentrated (i.e. below 1.020), animals are polyuric even if not reported by the owner. However, some dogs will have urine specific gravities this low due to increased water consumption during times of stress (such as hospitalization or following car rides), and therefore having owners collect a urine sample at home for measurement may be preferred if there is any doubt.
Inability to concentrate urine does not automatically imply renal disease, just as azotemia does not always imply renal failure. In fact, hyposthenuria (specific gravity below 1.008) implies active urine dilution by the kidneys. One tricky situation is the animal that has a non-renal disease that causes minimally to non-concentrated urine, and then is denied access to water or is unable to drink. These animals will rapidly develop pre-renal azotemia because of dehydration, but their urine will remain dilute. In other words, isosthenuria + azotemia in these patients does not equal renal failure. These animals should have rapid resolution of azotemia with fluid administration; hypoadrenocorticism is the classic disease where this combination of findings may occur.
Color is not a reliable method for estimating specific gravity. Hematuria and bilirubin may change urine color without significantly changing specific gravity. Puppies less than 3 weeks of age usually have more dilute urine than dogs 4 weeks of age or older.
Methodology tips: Refractometers are the best balance between cost and accuracy for measuring urine specific gravity. The best refractometers have different scales for dogs and cats; however, if these are not available, the differences are rarely clinically significant. Urine dipsticks are too inaccurate to be clinically reliable for measurement of specific gravity.