Differentiating between acute and chronic kidney disease can be a complicated task. The recommended tests, treatments, and short- and long-term prognoses differ depending on whether the patient has acute or chronic disease.
Illustration by Kip Carter
In this article, we review important components of the diagnostic process that aid in differentiating acute kidney injury (AKI) and chronic kidney disease (CKD). We discuss historical, physical examination, laboratory, and histologic findings and diagnostic imaging results. Common clinical findings and diagnostic test results in cases of AKI or CKD are summarized in Table 1. We also briefly discuss a few less common diagnostic techniques.
Table 1: Common Clinical Findings and Diagnostic Test Results in Cases of Acute Kidney Injury or Chronic Kidney Disease*
OVERVIEW OF AKI VS. CKD
Multiple definitions of AKI are used in the human medical and veterinary literature. Specific criteria for the severity of damage range from an increase in creatinine concentration of 0.3 mg/dl (compared with a previous reading) to the need for renal replacement therapy.1-5 The time frame for increased creatinine concentrations varies from hours to weeks.6 The term acute kidney injury has been adopted in lieu of acute renal failure to accentuate that damage to the kidney is a continuum—even mild decreases in glomerular filtration rate, even if they do not lead to overt azotemia, are associated with adverse clinical outcomes.6,7 AKI may be reversible.
Most definitions of CKD specify that renal disease has been present for at least three months,8,9 although some sources suggest that four to eight weeks may be sufficient time for stabilization after an acute insult to allow accurate categorization.10 After an acute insult to the kidney, compensatory hypertrophy may gradually improve renal function, but this adaptation is generally maximal within three months. Renal dysfunction that persists after three months is typically not reversible. The term chronic kidney disease is preferred to chronic renal failure to accentuate the concept that renal disease may be present in the absence of azotemia. For azotemia to develop, over 75% of the nephrons must be lost.11
In this article, AKI and CKD are used to include pre-azotemic (disease) and azotemic (failure) conditions. Dogs and, especially, cats may have compensated CKD in which they exhibit no clinical signs until an acute uremic crisis is superimposed. This "acute-on-chronic" kidney disease may have clinical features of both AKI and CKD.
The diagnostic and therapeutic approach to AKI differs from the approach to CKD. With AKI, an aggressive diagnostic plan is recommended to uncover any ongoing process that requires a specific treatment in addition to supportive therapy. AKI frequently requires aggressive treatment in the hospital, whereas CKD may be treated on an outpatient basis in many cases.
Prognosis varies considerably between acute, chronic, and acute-on-chronic kidney disease. Giving clients prognostic information allows them to make informed decisions about their pets' care. In one study, 53% of cats survived an episode of AKI.12 Of these surviving patients, 47% were discharged from the hospital with a normal serum creatinine concentration, whereas the remaining 53% of surviving cats had persistent azotemia.12 Therefore, 25% of the total study population were discharged without azotemia.12 A similar study in dogs with AKI revealed comparable results in dogs, with 44% of the study population surviving to be discharged from the hospital.13 Nineteen percent of the total study population had creatinine concentrations that returned to normal.13
Although cure is impossible with CKD, long-term survival (i.e. years) is possible in many patients. Patients with acute-on-chronic disease may require aggressive treatment in the hospital but will have residual CKD requiring some degree of long-term management.