Kidney Function Tests
Diabetes is the leading cause of chronic kidney failure, or end-stage renal disease (ESRD). Uncontrolled high glucose levels can damage the nephrons — the filtering units of the kidney that remove excess fluids and waste products from the bloodstream. Regular screening for early signs of kidney problems is an essential part of diabetes care.
Testing for Protein
Healthy kidneys should filter and absorb proteins instead of excreting them into the urine. A microalbumin test detects the presence of the protein albumin in the urine — a sign of kidney disease or damage.
Chemical reagent test strips (dipsticks) may be used to screen for protein. They are quick and easy, requiring only a small urine sample and a few minutes for testing. However, they should be performed in conjunction with a urine creatinine test, described later in this chapter, to be clinically accurate in assessing kidney function.
In a 24-hour urine collection, up to 150 milligrams of protein excreted in a 24-hour period is considered normal. Moderate levels of protein (0.5 to 4.0 g/24 hours) are often present in renal disease as a complication of diabetes. High levels (over 4.0 g/24 hours) occur in nephrotic syndrome. Because this test is cumbersome, it is not prescribed often.
The preferred test for microalbuminuria screening is a random spot test taken at your doctor's office or a lab that analyzes both albumin and creatinine levels, called the albumin-to-creatinine ratio. Values of 30 to 299 milligrams signal microalbuminuria, one of the earliest signs of diabetic nephropathy (kidney disease). Levels of 300 milligrams or more indicate macroalbuminuria, which is a hallmark of early end-stage renal disease (ESRD). However, the diagnosis of microalbuminuria or macroalbuminuria should be confirmed in two of three random specimens taken within a three- to six-month timeframe.
Screening for microalbuminuria should be performed annually. If you have type 1 diabetes, current ADA guidelines recommend that testing begin five years after diagnosis. For children, this testing starts five years after diagnosis once the child has reached 10 years of age.
If you have type 2, screening should start at diagnosis and take place every year thereafter. It may be performed more often with those patients at high risk for renal disease. If micro-albumin is present on a test, a repeat test should be performed to confirm the results. Find more on diabetic kidney disease.
If microalbumin is present on several consecutive tests, it usually indicates diabetic nephropathy, or kidney disease due to diabetes. Bladder infection and/or nephritis can also cause an elevated microalbumin level, as can high blood pressure and periods of hyperglycemia.
The ADA recommends that all people with diabetes undergo annual testing of serum (or blood) creatinine levels to assess kidney function. Creatinine is a metabolic by-product of creatine, the acid that supplies energy for muscle contractions. Creatinine is filtered out of the bloodstream by the kidneys. Creatinine blood levels greater than 1.2 mg/dl for women and 1.4 mg/dl for men points to inadequate filtering by the kidneys (renal impairment).
Serum creatinine values help your physician calculate your glomerular filtration rate (GFR), or the rate at which your kidneys are filtering waste and fluids from your body. The GFR is also used to stage kidney disease; a GFR of less than 15 indicates kidney failure (stage 5, or end-stage renal failure). A GFR of less than 60 for three months or more is considered diagnostic of chronic kidney disease. Although “normal” GFR values vary by age, sex, and body type, if your GFR is over 90 and you have no evidence of protein in the urine, then your kidneys are most likely healthy.
Urine Creatinine Clearance
Creatinine clearance measures the kidney's ability to filter creatinine from the blood. Normal kidneys should filter creatinine, a waste product, into the urine at a constant rate.
Before a creatinine clearance urine test is performed, a blood sample is taken to determine the level of creatinine in the bloodstream (serum creatinine, described earlier). A urine specimen is then analyzed for creatinine output, and the creatinine clearance is computed by comparing the urine creatinine to the original blood creatinine levels. If kidney function is impaired, creatinine levels in the urine will be low. Low creatinine clearance levels indicate kidney disease (i.e., polycystic kidney disease, glomerulonephritis), congestive heart failure, and/or severe dehydrationn.
Can't keep your creatinine straight? Just remember that healthy, functioning kidneys will cleanse this waste product from the blood and move it into the urine for disposal — so serum levels should be low and urine levels should be high.
Blood Urea Nitrogen (BUN)
Urea is another waste product that is filtered from the blood by the kidneys. Urea is generated in the liver by metabolized protein. Elevated BUN levels on a blood test indicate a slowdown in kidney function. Normal adult BUN levels are between 7 and 20 mg/dl. BUN will rise as kidney function falls. An elevated BUN is also a sign of several other possible conditions, including dehydration, congestive heart failure, excessive protein intake, internal bleeding, and heart attack.