Monday March 6, 2006
BEDSIDE CREATININE CLEARENCE
The most sensitive measure of changing renal function is not the serum creatinine, but the creatinine clearance. Serum creatinine underestimates the degree of renal insufficiency in many situations like :
- Anyone with a renal insufficiency but a GFR more than 50 ml/min, because serum creatinine does not start to rise until GFR falls below 50 ml/min
- Cachexia – because creatinine production is so low, serum creatinine may rise only when GFR falls below 25 ml/min
- After surgery in patients who have received a lot of fluids. A 10 – 15% increase in total body water results in dilution of serum creatinine by an equivalent amount.
The most useful means of estimating GFR at bedside is a two-hour creatinine clearance. There is nothing about clearance that mandates a 24-hour urine collection, particularly when there is a Foley’s catheter in place, which largely eliminates error due to urine retention. As long as the collection is carefully timed and the urine flow is more than 30 ml/hr, a collection as short as 2-hour will give reasonable data.
Creatinine clearance may be calculated simply by UV/P
where U is the urine creatinine in mg/dl,
V is the urine flow rate in ml/min and
P is the serum creatinine in mg/dl.
The term ‘UV’ i.e is urine creatinine times urine flow rate, represents the creatinine excretion rate. It is THIS that changes rapidly with changing GFR.
1. Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation Classification and Stratification - National Kidney Foundation / Kidney Disease Outcomes Quality Initiative.
2. ESTIMATION OF CREATININE CLEARANCE IN PATIENTS WITH UNSTABLE RENAL FUNCTION (Dr. Roger Jelliffe - School of Medicine at the University of Southern California).