Differentiating between acute and chronic kidney disease - Veterinary Medicine
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Differentiating between acute and chronic kidney disease
When dogs and cats have kidney disease, it can be a puzzle determining whether the problem has just developed or has been ongoing. These diagnostics can help you piece the puzzle together to make that distinction so you can provide optimal care and give owners the most accurate prognosis for their pets.


Ultrasonographic examination

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

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.
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.

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 or chronic.

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 hands.


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 sedated.63 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 g 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


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