Plasma creatinine and urea are relatively insensitive assays of renal function since more than 60% of the kidney can be destroyed before they are significantly affected.
Urea and creatinine clearance tests measure glomerular filtration and are commonly used to assess renal function. Neither give an absolutely accurate value. Urea gives a slightly low value since it diffuses back into the bloodstream from the proximal tubule, whilst creatinine gives a slightly high value because it is secreted from the proximal tubule.
Inulin would give a more accurate estimate of glomerular function but is rarely used since it has to be injected.
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