Knowledge of kidney function is a key component for diagnosing CKD and initiating treatment. The eGFR, which is most commonly used in clinical practice as an approximation of kidney function, is the most practical for everyday use. It has, however, disadvantages because the two most common biomarkers used in eGFR calculations, creatinine and cystatin C, are influenced by patient characteristics such as body composition, inflammation, and many others, leading to significantly inaccurate GFR estimation at times. In patients who are prone to distorted eGFR values, measured GFR (mGFR) should be considered to ensure accurate assessment of kidney function. mGFR is not as influenced by patient characteristics as it uses exogenous biomarkers instead of endogenous ones. It is, however, more complex to perform and has also its limitations. Several different mGFR measurement methods exist. The one that has become most widely accepted and is the only method for which there is an internationally standardized protocol is plasma iohexol clearance. Other mGFR markers include iothalamate, mostly applied as urinary clearance and radioactive markers. Newer techniques that measure GFR transdermally have been developed, although they have yet to be sufficiently externally validated.