Limitations of measured creatinine clearance
What are limitations to use of measured creatinine clearance?
Several problems can compromise the utility of creatinine clearance. Firstly, accurate measurements of creatinine clearance require complete and carefully timed urine collections; inadequate urine collections yield spurious results. Secondly, because creatinine is secreted by the kidney tubules, the creatinine clearance systematically overestimates GFR.
Between 10% and 20% of urinary creatinine is secreted rather than filtered, so the creatinine clearance will overestimate the GFR by a similar percentage. Cimetidine, the over-the-counter H2-blocker, competitively inhibits creatinine secretion. Thus a 24-hour urine creatinine clearance while a patient is on cimetidine is theoretically closer to the actual GFR. However, there the extent to which cimetidine blocks secretion occurs variable among individuals. Drugs that block creatinine secretion will also cause a slight elevation in serum creatinine that does not reflect a change in GFR, just a loss of tubular creatinine secretion.
Drugs that block creatinine secretion in the proximal tubule
• Cimetidine
• Trimethoprim
• Ranolazone
• Pyrimethamine
• Dronedarone
• Tenofovir
