A more accurate, but still not perfect, estimate of kidney function is a calculation of your estimated glomerular filtration rate (GFR). GFR is generally accepted as the best overall index of kidney function and is one that the NKF encourages clinicians to use. Studies suggest GFR identifies CKD in its early stages better than other creatinine-based tests.
In a nutshell, GFR is the rate at which all of the nephrons in your kidneys filter a substance from your blood. The more functioning, healthy nephrons you have, the better the per-minute filtration rate and the higher the GFR. The lower the GFR, the poorer the kidney function.
Normal kidneys filter out most of the creatinine in the blood at a fixed rate:
- In young adults, GFR hovers around 120-130 ml/min/1.73m2 and declines with age.
- In women with normal kidneys, GFR is about 105 ml/min/1.73m2.
- In men, GFR is about 125 ml/min/1.73m2.
In 2002, the NKF proposed a five-stage classification of CKD based upon GFR. The five stages reflect that CKD is progressive, moving from mild to moderate to severe reductions in GFR. The lower the GFR, the more advanced the CKD. The CKD stages are:
Stage One: GFR 90 or above. An estimated GFR of 90 or above is considered normal. However, some people, even with normal GFR, have other signs of possible kidney damage, such as protein or blood in the urine. Patients remain in Stage One indefinitely or move on to Stage Two.
Stage Two: GFR 60 to 89. Stage Two is kidney damage with a mild decrease in GFR. The glomeruli show damage, and small amounts of blood protein such as albumin leak into the urine, a condition known as microalbuminuria. Normal albumin loss in the urine is less than 5 micrograms per minute, but the rate of albumin loss in Stage Two might increase to 200 micrograms per minute. People with diabetes should have the urine microalbuminuria test to detect protein spillage. With good control over blood pressure and blood glucose levels and with exercise, diabetics could remain in Stage Two without advancing.
Stage Three: GFR 30 to 59. Stage Three is considered moderate kidney failure. When a patient presents GFR of less than 60 ml/min/1.73m2 for 3 or more months, the patient is diagnosed with CKD. When CKD has advanced to this stage, signs of bone deterioration, heart and vessel disease, and anemia are common.
Stage Four: GFR 15 to 29. Stage Four constitutes severe CKD. Large amounts of protein leak into the urine and high blood pressure usually exists. Cardiovascular issues commonly develop, as well as anemia and chemical imbalances requiring diet modifications and drug intervention. During Stage Four, the patient must consider options such as dialysis or kidney transplantation.
Stage Five: GFR less than 15. Stage Five is considered end-stage renal disease (ESRD), and the kidneys do not work well enough to keep the patient alive without dialysis or a transplant.
The NKF’s most recent definition for CKD is either a GFR of less than 60 (Stage III) or the presence of protein in the urine for at least three months. For more on the five kidney disease stages, see the KidneySteps book.
Blood in the Urine
While a consistent finding of proteinuria indicates kidney disease, the presence of blood in the urine also may be a sign. The abnormal presence of red blood cells (RBC) in urine is called hematuria (hee-muh-TOOR-ee-uh).
Hematuria at low levels requires a microscopic exam to detect. At higher levels, the urine may appear red or smoky-brown.
In healthy individuals, RBCs occasionally may appear in the urine after vigorous exercise, so-called joggers’ nephritis. Occasional appearances of RBCs do not indicate kidney disease. Also, hematuria may be unrelated to the kidneys; but, the RBCs may indicate a urinary tract infection, kidney stones, urinary tract cancer, or may even result from the consumption of large amounts of vitamin C.