Physical Evaluation of Urine

Physical aspects of the urine, e.g. color and clarity, are noted. After recording the reagent strip results (see Chemical Evaluation below), the urine is centrifuged and the specific gravity is determined on the supernatant using a refractometer. Protein and glucose in the urine raise the urine specific gravity (USG), though their concentrations generally have to be high before interpretation of the USG is affected. For example, 1+ protein (0.3 g/L or 30 mg/dL) increases the USG by about 0.0001 to 0.0002, and 3+ protein (5 g/L or 500 mg/dL) increases the USG by about 0.0015 to 0.0025. Also, 1+ glucose (2.8 mmol/L or 50 mg/dL) raises the USG by about 0.0002 to 0.00025, and 4+ glucose (55 mmol/L or 1000 mg/dL) raises the USG by about 0.004 to 0.005.

Normal urine is usually pale to dark yellow due to various pigments, commonly called urochromes. Although more concentrated urine is often darker yellow to orange due to these pigments, it cannot be assumed that dark urine is necessarily concentrated and pale urine is necessarily more dilute. USG should always be measured to assess degree of concentration. In most species urine is clear. Turbidity or cloudiness can be due to the presence of crystals, cells, casts, bacteria, mucus, and fluids from the reproductive tract. Normal horses often have cloudy urine due to the presence of calcium carbonate crystals and mucus. Normal feline kidneys contain abundant fat, therefore, lipid droplets can appear in the urine causing it to be cloudy.

The USG, in isolation, provides limited information, with the exception that highly concentrated urine indicates adequate renal tubular function regardless of other findings. Normal, healthy individuals can have urine that is dilute/hyposthenuric (USG ≤1.007), isosthenuric (1.008 to 1.012), or concentrated (>1.030 in dogs; >1.035 in cats; >1.025 in cattle and horses). Therefore, there is no “normal” USG; interpretation of a given USG depends on hydration status, serum urea and creatinine concentrations, and historical details of water consumption and drug and fluid administration. For example, a very dehydrated cat with normally functioning kidneys is likely to be azotemic but will have concentrated urine. Therefore, the USG will be at least 1.035 (often higher) under these circumstances. An azotemic cat with a USG of 1.030 has strong evidence of renal disease, whereas an azotemic cow with a USG of 1.030 is likely to have a nonrenal cause of the azotemia.

An animal that is isosthenuric or hyposthenuric with a history of polydipsia and polyuria, but that is not dehydrated or azotemic, may be a candidate for a water deprivation test. Animals with these findings may have psychogenic polydipsia, renal diabetes insipidus, or central diabetes insipidus. The water deprivation test or modified water deprivation test will challenge the kidneys to conserve water in order to determine if urine concentrating ability has been lost. As the endpoint of these tests is dehydration, azotemia, or both, close monitoring of the patient is very important and, of course, these tests should never been performed if dehydration, azotemia, or both are pre-existing. Details about these tests are provided in Chapter 10: Endocrine System.