Renal Function Estimation
Basic, Organ-Based and Clinical Sciences
Inulin clearance is the gold standard for measuring GFR. It is given exogenously followed by multiple timed urine and blood samples in order to accurately measure GFR. As such, it is not a useful test. Other exogenous markers such as radioisotopes or iohexol can be administered as well however each comes with inherent pitfalls.
Multiple other equations have been developed to take into account different demographic data such as age, sex, and race. These include Cockcroft-Gault, Modification of Diet in Renal Disease (MDRD), and CKD-EPI (CKD epidemiology collaboration). The CKD-EPI equation has been found to be more accurate at normal and mildly reduced GFR than MDRD. CKD-EPI was also found to show a lower prevalence of CKD and was better at predicting mortality risk.
The above equations based on creatinine become less accurate in certain ethnic groups, pregnant women, unusual body mass, morbid obesity, and amputees. Therefore, other compounds should be considered. Cystatin C is one of these other endogenous markers. The CKD-EPI equation can be adjusted to utilize cystatin C instead of creatinine.