Measurement of Renal Function by Assessment of GFR
Measurement of GFR using exogenous markers may be required in special situations such as when dosing toxic drugs with a narrow therapeutic window, and in the assessment of living renal donors prior to kidney donation. Radioisotope markers such as 99m Tc-DTPA are commonly used. In practice, the usual way of assessing renal function is by estimating GFR from serum creatinine using a prediction equa
Measurement of GFR using exogenous markers may be required in special situations such as when dosing toxic drugs with a narrow therapeutic window, and in the assessment of living renal donors prior to kidney donation. Radioisotope markers such as 99mTc-DTPA are commonly used.
In practice, the usual way of assessing renal function is by estimating GFR from serum creatinine using a prediction equation like the Modification of Diet in Renal Disease (MDRD) Study and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations.
Serum creatinine can only be used to estimate GFR in individuals with stable kidney function. Other limitations include variations in the production, secretion and extrarenal excretion of creatinine, and issues associ- ated with creatinine measurement. Creatinine as a marker is less reliable with extremes of body size or muscle mass (e.g. amputation), unusual nutritional states (e.g. vegan diet) and pregnancy.
To reduce the variation in measurement, most laboratories now use creatinine assays which are traceable to reference materials. They therefore report “standardised creatinine” which has important implications for estimating GFR using the prediction equations.
Note 1:
Screening for kidney disease should involve blood pressure measurement and urine dipstick testing. If any abnormalities are found then the estimated GFR should be determined by measuring serum creatinine concentration and the protein excretion quantified by measuring the protein/creatinine ratio or albumin/creatinine ratio on a random urine sample. The preferred method of estimating GFR is the CKD-EPI equation.
Note 2:
The Cockcroft-Gault equation has not been revised for use with standardised creatinine and will substantially overestimate GFR.
Note 3:
The 4-variable MDRD equation has been reexpressed to work with standardised creatinine and is still widely used, performing well at GFR values under 60 ml/min. Laboratories and clinicians should be sure to use the re-expressed MDRD equation when estimating GFR from standardised creatinine. It is not necessary to use the adjustment factor for black race in South African patients.
Note 4:
Prediction equations should be validated in local populations. Measurement of GFR using exogenous markers can be considered when there is doubt about the validity of prediction equations.
Note 5:
The use of 24-hour urine collection to measure creatinine clearance is limited by frequent errors in urine collection and increasing creatinine secretion as the GFR decreases. This latter factor leads to overestimation of the GFR.
Paget G, Naicker S, et al. Guideline for the optimal care of patients on chronic dialysis in South Africa. South African Renal Society; 2015.