URINE SPECIFIC GRAVITY
Urine osmolality and urine specific gravity (USG) are used to assess the kidneys' ability to concentrate or dilute urine.
Although urine osmolality is the most accurate method to detect urine solute concentration, USG is an easier, cheaper, and
more practical measurement in a clinical setting. A USG > 1.030 in dogs and > 1.035 in cats indicates adequate urine concentration.
Generally, primary renal disease can be excluded if azotemia is seen with adequate concentrating ability; however, a small
percentage of cats develop azotemia and clinical signs of CKD yet maintain urine-concentrating ability.18,19
Always interpret USG in light of an animal's drinking habits in the context of its physiologic state.20 Hyposthenuric urine may be seen with primary (psychogenic) polydipsia and central diabetes insipidus, but it can also occur
secondary to conditions that cause nephrogenic diabetes insipidus such as hypercalcemia, pyometra, heavy metal toxicosis,
and hyperadrenocorticism. Medications such as glucocorticoids, phenobarbital, and diuretics may also cause dilute urine.21 Use caution when interpreting USG after hetastarch administration as it may lead to an overestimation of urine concentration.22 Kidney failure is commonly misdiagnosed in dogs with hypoadrenocorticism. Mineralocorticoid deficiency and subsequent hyponatremia
lead to decreased tonicity of the renal medullary interstitium, leading to inappropriate USG in the face of azotemia (secondary
Proteinuria may be divided into three main categories: prerenal, renal, and postrenal (Table 2). Primary renal causes can be further subdivided into functional and pathological.
Table 2: Causes of Proteinuria Based on the Site or Mechanism of the Underlying Abnormality*
In dogs, persistent renal proteinuria with urine protein:creatinine (UPC) values ≥ 2 is usually due to glomerular disease.
UPC values ≥ 0.5 are evidence of persistent renal proteinuria when they are found repeatedly in three or more samples obtained
two or more weeks apart and cannot be attributed to prerenal or postrenal causes. In dogs, relative risks of development of
uremic crises and death were three times higher in dogs with UPC ≥ 1 compared with dogs with UPC < 1.23
In cats, UPC values ≥ 1 should prompt suspicion of glomerular disease; however, values ≥ 1 may be seen with progressive tubular
disease and end-stage CKD. Proteinuria is highly related to survival in cats with CKD. The current conventional definition
of persistent proteinuria in cats is > 0.4, which is associated with reduced survival. Even cats with mild proteinuria of
0.2 to 0.4 have been shown to have decreased survival when compared with cats with UPC < 0.2.24
When elevated UPC is treated and monitored in proteinuric animals, the UPC must change by at least 35% at high UPC values
(near 12) and 80% at low UPC values (near 0.5) to demonstrate a significant difference between serial values.25
Microalbuminuria testing is generally reserved as an adjunctive test when albuminuria is suspected despite previous negative
test results, in cases of renal disease or hypertension, or as an additional screening test. In-house semiquantitative test
kits such as the E.R.D. HealthScreen (Heska) are species-specific for dogs and cats. In cats, microalbuminuria and UPC do
not always correlate, and repeatability is an issue with semiquantitative testing.26