1. Our laboratory now reports an estimated GFR without a creatinine clearance test. Can you explain why? I thought the creatinine clearance test was necessary to determine the GFR.
2. Recently I have noticed two new items on my lab listings: GFR (Afr-Am) and GFR (Non Afr), which possibly mean African-American and Non-African American. These are listed on all patients, regardless of race. I looked in a reference book of diagnostic tests and did not find it. Can ADVANCE tell me what it is?
It has been recently recommended that laboratories report the predicted GFR on patients who have their plasma creatinine level determined. The GFR is the rate that blood plasma is filtered by the glomeruli, the tiny capillary blood filters present in our kidneys. A decreased GFR is an indicator of impaired kidney function.
Traditionally, the test most frequently utilized to estimate the GFR is the creatinine clearance test. To perform this test, the laboratory needs a blood sample, a 24-hour urine collection, and the patient's height and weight. The creatinine clearance is reported as the number of milliliters of plasma cleared per minute per an average body surface area of 1.73 m2.
Studies have shown that the greatest source of error in the creatinine clearance test is obtaining an accurate 24-hour urine collection. This is especially true with the outpatient population. Instructions may be misunderstood resulting in the failure to obtain a complete collection. This makes the creatinine clearance test a less reliable method of estimating the GFR when compared to the use of predictive formulas.
Estimation of the GFR using predictive formulas has been shown to be a better method to detect a decline in renal function, as well as eliminate the need for a 24-hour urine collection.
A number of formulas have been proposed and one, the Cockcroft-Gault equation, has been used for decades. The importance of this approach is exemplified by its medical uses.
The estimated GFR is used to:
- detect the onset of renal insufficiency
- adjust drug dosages for drugs excreted by the kidney
- evaluate therapies instituted for patients with chronic renal disease
- document eligibility for Medicare reimbursement in end stage renal disease
- accrue points for patients awaiting cavaderic kidney transplants
One proposed formula is from the MDRD (Modification of Diet in Renal Disease) study.
The formula estimates the GFR by using the plasma creatinine along with mathematically derived factors to account for age, sex, and ethnicity. The use of these factors corrects for variations in the glomerular filtration rate in different populations:
1. Kidney function decreases as one ages
2. Females have less muscle mass than males (creatinine is related to muscle mass)
3. African-Americans have a greater predisposition to hypertension and subsequent renal disease
To demonstrate the variation due to sex in interpretation of the plasma creatinine, a result of 2.0 mg/dl was often used to indicate the presence of kidney disease. Use of the MDRD equation indicates that this threshold is closer to 1.0 mg/dl in women of European-American descent.
The GFR calculated from this plasma creatinine result would be determined for such a woman, taking into account the patient's gender and age, information which immediately available to laboratory information systems.
The laboratory report then provides two possible results, dependent on the patient's ethnicity: African-American or Non-African American. This approach allows clinicians to assess change in kidney function as follows:
- mildly decreased kidney function: GFR, 60-89 ml/mim/1.73 m2
- moderately decreased kidney function: GFR, 30-59 ml/mim/1.73 m2
- severely decreased kidney function: GFR 15-29 ml/mim/1.73 m2
Some predictive GFRs are now reported with a third reference range for Hispanic patients, to account for ethnic variation noted in this population. Other populations are currently being studied to determine if there is a significant difference in GFR as it relates to assessment of renal function.
The National Kidney Foundation's Web site www.kidney.org includes the Health Care Professionals>GFR Calculator which allows you to enter the plasma creatinine result, along with other patient parameters to determine the GFR.
"The Modification of Diet in Renal Disease" study, in addition to introducing an improved approach to estimating the GFR, has demonstrated that identifying a decline in renal function allows for early intervention, which can significantly slow the progression of renal disease in predisposed individuals. These interventions include dietary protein restriction and blood pressure control.
Levey, A.S., Bosch, J.P., Lewis, J.B., et al. (1999). A more accurate method to estimate glomerular filtration rate from serum creatinine, a new prediction equation, Annals of Internal Medicine, 130:461-70.
Coresh, J., Astor, B.C., Greene, T., et al. (2003). Prevalence of chronic kidney disease and decreased kidney function in the adult U.S. population: third national health and nutrition examination survey. American Journal of Kidney Diseases, 41:1-12.
Sally Perry Ball is a Medical Laboratory Science faculty member at Northeastern University and a freelance writer.