The two most common tests for renal function include blood urea nitrogen (BUN) and serum creatinine (Cr). In additon, the creatinine clearance can be measured by collection of a 24-hour urine sample. However, properly and accurately collecting urine over 24 hours is hard to do.
The creatinine clearance is a close estimate of glomerular filtration rate (GFR) that gives an indication of renal function. Normally, the GFR for a healthy young adult exceeds 100 mL/min.
Chronic renal disease is characterized by diminished creatinine clearance (CrCl), typically with elevations in BUN and Cr. The degree of renal impairment can be categorized by reduction in GFR (assuming standard adult size of 1.73 square meters body surface area):
Mild reduction GFR 60 to 89 mL/min
Moderate reduction GFR 30 to 59 mL/min
Severe reduction GFR 15 to 29 mL/min
Renal failure GFR <15 mL/min
The Cockcroft-Gault formula for estimating CrCl may be employed to estimate residual renal function in patients with chronic kidney disease. The formulas are as follows:
CrCl (male) = ([140-age in years] X weight in kg)/(serum creatinine X 72)
CrCl (female) = CrCl (male) X 0.85
Thus, for a 70-year-old man weighing 70 kg with serum creatinine of 1 mg/dL, the estimated Cockcroft-Gault CrCl is 68 mL/min.
As the degree of renal impairment increases, involving all parts of the kidney, and functional renal mass diminishes, then it is likely that homeostasis of fluid and solutes is impaired. With chronic renal failure:
The ability to excrete phosphate diminishes, and the increasing serum PO4 leads to a decrease in serum calcium, with increase in parathyroid hormone.
Concentrating ability is diminished, so if water intake is restricted, then the kidney does not respond to antidiuretic hormone, and the urine specific gravity does not increase. It may be fixed at sp gr of 1.010.
Vitamin D3 (calcitriol) decreases
Erythropoietin production decreases
Autoregulation of blood flow and pressure is impaired, and hypertension occurs