Measured Creatinine Clearance (24-Hour Urine)
Measured Creatinine Clearance (24-Hour Urine)
Calculate creatinine clearance from a timed 24-hour urine collection and a paired serum creatinine.
Measured Creatinine Clearance (24-Hour Urine)
3 inputs → mL/minUrine creatinine 85 mg/dL, 24-hour volume 1500 mL, serum creatinine 1.3 mg/dL
Formula
- urine creatinine
- mg/dL, from the pooled 24-hour collection
- urine volume
- mL, total volume collected over 24 hours
- serum creatinine
- mg/dL, ideally drawn during the collection period
- 1440
- minutes in 24 hours, converting the volume term to a rate
Worked example
Urine creatinine 85 mg/dL, 24-hour volume 1500 mL, serum creatinine 1.3 mg/dL
85 × 1500 = 127,500
1.3 × 1440 = 1,872
127,500 ÷ 1,872 = 68.1 mL/min
60–89 mL/min → mildly reduced
Interpreting the result
| CrCl | Category |
|---|---|
| < 15 | Severely reduced |
| 15 – 29 | Markedly reduced |
| 30 – 59 | Moderately reduced |
| 60 – 89 | Mildly reduced |
| ≥ 90 | Normal or high |
What the timed collection adds, and where it fails
A 24-hour urine creatinine clearance was for decades the standard way to measure kidney function directly, and it remains useful when eGFR equations are known to be unreliable — at extremes of body size or muscle mass, in amputees, during pregnancy, or in people on markedly unusual protein intakes, all situations where equations built on average body composition perform poorly.
Its first limitation is physiological rather than practical: creatinine clearance systematically overestimates true glomerular filtration rate, because a fraction of urinary creatinine comes from active tubular secretion rather than filtration alone. The overestimate is around 10 to 20% in healthy kidneys and grows larger as GFR falls, since secretion makes up a proportionally bigger share of clearance when filtration is impaired. A measured clearance in someone with advanced CKD can therefore look reassuringly better than their true GFR.
Its second limitation is entirely practical: collection completeness. A short or over-long collection, or one with missed voids, produces an error that can dwarf the physiological one. The standard check is to compare total creatinine excreted against the expected daily output for a person of that build — roughly 15 to 20 mg/kg/day in women and 20 to 25 mg/kg/day in men, since creatinine excretion tracks muscle mass and is relatively stable day to day. A result well outside that range should prompt a repeat collection rather than acceptance at face value.
Frequently asked questions
Why does creatinine clearance overestimate GFR?
Because some urinary creatinine comes from tubular secretion, not just glomerular filtration. The overestimate is roughly 10–20% and grows as true GFR falls, since secretion becomes a larger share of total clearance.
How do I know if the urine collection was complete?
Check total creatinine excreted (urine creatinine × volume) against the expected daily output for the patient’s build — about 15–20 mg/kg/day in women and 20–25 mg/kg/day in men. A large mismatch suggests an incomplete or over-long collection.
When is measured clearance preferred over an eGFR equation?
When body composition falls outside what eGFR equations were built for — very high or low muscle mass, amputation, pregnancy, or extremes of body size — situations where creatinine-based equations systematically mis-estimate GFR.
Should serum creatinine be drawn before, during, or after the collection?
Ideally during the collection period, or at its midpoint, since it should reflect the same steady state as the urine sample being analysed.
Related calculators
References
- Perrone RD, Madias NE, Levey AS. Serum creatinine as an index of renal function: new insights into old concepts. Clin Chem. 1992;38(10):1933–53.
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1–150.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.