An abdominal ultrasonographic examination allows for the evaluation of kidney size, shape, and architecture. A normal feline
kidney is about 3.8 to 4.4 cm in length and has no pelvic dilation.56 Ultrasonographic determination of renal volume (by measuring renal length, width, and depth) and the ratio of renal length
to aortic diameter have been evaluated to determine normal renal size parameters in dogs.57 Although both the renal volume and the ratio methods provide some utility, the amount of normal variation is large, especially
in very small or very large dogs, and these techniques are not commonly used.54,57
During an ultrasonographic examination, CKD will frequently appear as small irregular kidneys with hyperechoic cortices and
decreased corticomedullary distinction (Figure 3). Pelvic dilation, renal mineralization (nephrocalcinosis), or nephroliths may also be present. Renal asymmetry, with one
small and one normal or large kidney, suggests chronic disease of the small kidney, but the contralateral kidney may have
chronic disease (e.g. compensatory hypertrophy, hydronephrosis from obstruction) or acute disease (acute obstruction, pyelonephritis, ischemic or
toxic insult). Renal cysts typically denote chronic disease.58 The kidneys may appear structurally normal in the presence of CKD.
Figure 3. Ultrasonographic images of cats with diseased kidneys. Panel A: Normal renal shape and internal architecture with
good corticomedullary distinction in a cat with AKI. Panels B and C: Reduced corticomedullary distinction (arrowheads), renal
asymmetry (right larger than the left), irregular renal contour bilaterally, and a caudal infarct on the left kidney (flattened
caudal pole, arrow) in a cat with CKD.
In the presence of AKI, the kidneys may appear normal or may be enlarged, hydronephrotic, and hyperechoic on ultrasonographic
examination. Perinephric fluid generally is associated with acute disease.59
Enhanced corticomedullary distinction does not distinguish between acute or chronic disease, nor does the presence of a medullary
rim sign (a hyperechoic band at the corticomedullary junction), which has also been seen in normal animals.56 Decreased cortical echogenicity can be seen in both acute and chronic renal disease.56 Varying degrees of renal pelvic dilation or ureteral dilation occur with partial or complete obstruction, which can be acute
Ultrasonographic examination can also be used to evaluate the size and echogenicity of the parathyroid glands. In CKD, chronically
increased phosphorus and decreased calcium concentrations stimulate chronic increases in PTH production. This renal secondary
hyperparathyroidism leads to parathyroid gland hyperplasia. In one study, parathyroid glands in dogs with CKD were larger
(2.8 to 7.1 mm) than parathyroid glands in healthy control dogs or dogs with AKI (1 to 3.5 mm).60
Parathyroid gland size varies with patient size. Although ultrasonographic examination of the parathyroid gland is not part
of the routine evaluation of patients with kidney disease in most practices, it is helpful to determine chronicity in experienced
Fibrosis, sclerosis, and atrophy are histologic indicators of chronicity of injury.61 Because these changes are irreversible, renal biopsy rarely affects the management or outcome of CKD.62 Biopsy is typically reserved for suspected cases of AKI when a diagnosis cannot be confirmed by less invasive testing, and
it is used to attempt to definitively diagnose histologic lesions causing protein-losing nephropathy, providing appropriate
technique is used. Fibrosis and other evidence of chronicity may not be uniformly distributed throughout the kidney, so biopsies
may miss the lesion. It is prudent to inspect a core biopsy with a dissecting microscope to ensure that at least 10 glomeruli
have been collected. If the sample is insufficient, additional tissue should be collected.
Good-quality kidney biopsy samples are more likely to be obtained in anesthetized patients compared with those that are only
A complication rate of 13.4% in dogs and 18.5% of cats was noted in one large study.62 Ten percent of dogs and 17% of cats required transfusion because of post-biopsy hemorrhage, and 2.5 % of dogs and 3% of
cats died.62 Renal function does not decrease in healthy dogs and cats after unilateral renal biopsy.64,65
Various other tests have been investigated to help distinguish between AKI and CKD but are not commonly used in veterinary
medicine. An elevated urea concentration causes hemoglobin to become carbamylated, and this abnormal hemoglobin accumulates
over time within red blood cells. The carbamylated hemoglobin concentration can be correlated to the duration of azotemia.
In one study, CKD was accurately diagnosed in dogs with a carbamylated hemoglobin concentration exceeding 108 µg of valine
hydantoin per gram of hemoglobin with a positive predictive value of 71.2%.66 If the blood urea nitrogen concentration is not elevated, this test will not be predictive.
The creatinine concentration at the distal end of the fingernails is another method for differentiating AKI and CKD in people.
Nails do not change in their chemical makeup after they leave the cuticle.67 Fingernail growth from proximal to distal end takes about three months.67 Therefore, the creatinine concentration at the distal tip indicates the creatinine concentration three months before, with
a false positive result rate of 6.12% with no false negative test results.67 To our knowledge, no veterinary study has evaluated toenail creatinine concentrations.
Identifying whether a patient's kidney disease is acute or chronic in origin is not always easy. All the clinical evidence
must be evaluated. In our experience, a long-standing history of clinical signs consistent with kidney disease, poor body
condition, and identification of small, irregular kidneys are the most useful in confirming that kidney disease is chronic.
In the absence of these findings, evaluation of other parameters such as PTH concentration or parathyroid gland size, presence
of anemia, presence of urinary casts, presence of uroliths, renal ultrasonographic architecture, and renal biopsy may be used
to further characterize the disease and may, in aggregate, help differentiate acute from chronic disease. Differentiating
between AKI and CKD in each patient will ensure that the patient is getting optimal care and that clients are well-informed
about their pets' condition to make optimal decisions.
Meghan Myott, DVM
Cathy Langston, DVM, DACVIM
The Animal Medical Center
510 East 62nd St.
New York, NY 10065