The human kidney is responsible for many vital homeostatic processes throughout the body. Since proper kidney function is essential to life, the anesthesiologist must be able to recognize, diagnose, and properly treat kidney dysfunction. An important step in managing perioperative renal physiology is familiarity with basic renal function tests (RFTs).
The gold standard for measuring glomerular filtration rate (GFR) is the inulin clearance test, which involves intravenous injection of inulin (a polyfructose sugar), and measurement of urinary inulin excretion over time. Inulin is completely filtered from the blood by the glomerulus, and is not secreted or reabsorbed by the renal tubules. Therefore, its clearance from the body into the urine is an accurate indicator of GFR and renal function. However, its use is limited in clinical practice because it is labor intensive and requires strict attention to detail. Thus, other methods for assessing renal function are generally used.
Creatinine is an end product of skeletal muscle ATP energy production. It is cleared from the blood by the kidneys through glomerular filtration, and then excreted in the urine. Creatinine production in the body depends on many factors, such as skeletal muscle mass, dietary protein intake, physical activity, and catabolism, and can vary from one person to another, but is usually stable on an individual basis.
A common pitfall in interpreting creatinine levels is not accounting for muscle mass. Although a creatinine of 1.2 mg/dL may be normal in a muscular 25 year-old man, it likely indicates significant renal dysfunction in a frail, elderly woman. The serum creatinine level represents the balance between muscle creatinine production and creatinine excretion by the kidneys. Thus, all else being equal, a change in an individual’s serum creatinine level reflects a linear change in GFR and proper kidney function. For example, an increase in creatinine from 0.8 to 1.6 mg/dL indicates a 50% reduction in GFR.
The Acute Kidney Injury Network defined acute kidney injury (AKI) as one or more of the following occurring within a 48-hour time period:
An absolute increase in the serum creatinine of 0.3 mg/dL or more.
A 50% or more increase in serum creatinine.
A reduction in urine output to less than 0.5 mL/kg/h (for more than 6 hours).
However, monitoring serum creatinine levels will not detect acute changes in GFR because it takes hours for serum creatinine levels to rise in response to decreased GFR. Conversely, serum creatinine levels may be elevated for a time even though GFR is recovering or has normalized.
Much like the clearance of inulin, creatinine clearance (CrCl) can be used to measure GFR by comparing urinary and plasma creatinine levels over time (usually 24 hours). However, in addition to filtration through the glomeruli, some creatinine is also ...