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Problems with creatinine measurement:
Although commonly used, serum creatinine (sCr) is not an ideal marker of kidney function since it is affected by age, sex and body mass. In fact, creatinine levels often remain within the normal range until kidney function is significantly impaired, especially in older individuals or those with low muscle mass.
Alternatives to creatinine measurement:
Glomerular filtration rate (GFR) is the gold standard measure of kidney function, and, in adults, can be easily and accurately estimated using the abbreviated MDRD GFR equation (Estimation of GFR). Although creatinine clearance measurements, as measured using a 24 hour urine collection, also estimate kidney function, they are no more accurate than estimated GFR and are cumbersome to obtain. Among adults, the MDRD Study equation provides a clinically useful estimate of GFR (up to approximately 90 mL/min/1.73 m2). The MDRD Study equation has the advantages of having been derived based on:
- GFR measured directly by urinary clearance of 125I-Iothalamate;
- A large sample of >500 individuals with a wide range of kidney diseases;
- Inclusion of both European-American and African-American participants;
- Validated in a large (n > 500) separate group of individuals as part of its development.
This equation provides estimates of GFR standardized for body surface area. The abbreviated version is easy to implement since it requires only serum creatinine, age, sex, and race (adjusted for African American status only; given the small proportion of African American patients living in Alberta, this variable is not considered). The calculation of the GFR is made automatically by your local laboratory according to the following abbreviated MDRD Study equation has two equivalent forms (Abbreviated MDRD Study Equation).
Why monitor GFR?
A decrease in GFR precedes kidney failure in all forms of progressive kidney disease. Monitoring changes in GFR can delineate the stage of kidney disease and whether it is progressive or not. Estimation of GFR in clinical practice allows proper dosing of drugs excreted by glomerular filtration to avoid potential drug toxicity. The level of GFR is a strong predictor of the time to onset of kidney failure as well as the risk of complications of chronic kidney disease. To enable appropriate assessment and treatment of complications associated with CKD, CKD has recently been classified, irrespective of diagnosis, based on the level of kidney function according to the Stages of Chronic Kidney Disease.
Complications associated with CKD
Many of the complications of chronic kidney disease can be prevented or delayed by early detection and treatment. Hypertension, anemia, bone disease and disorders of mineral metabolism become common in Stage 3, and very common in Stage 4 CKD (Association of level of GFR with complications in adults). Patients with Stage 5 kidney disease commonly have uremic symptoms (anorexia, nausea, and poor energy).
K/DOQI CKD Classification Figure 15: Estimated prevalence of selected complications, by category of estimated GFR, among participants age > 20 years in NHANES III, 1988 to 1994.
Click here to view Figure 15
K/DOQI CKD Classification Figure 16: Estimated distribution of the number of complications, by category of estimated GFR among participants age > 20 years in NHANES III, 1988 to 1994.
Click here to view Figure 16 |
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