Timed Urine and Renal Clearance Calculations

A timed urine result combines the measured concentration with the total collection volume and duration. The calculation must account for the units. The test procedure specifies whether the result describes the full collection, one hour, or 24 hours.

From concentration to total excretion

Urine dipsticks report qualitative or semiquantitative findings. For quantitative follow-up, KDIGO favors an albumin-to-creatinine ratio (ACR) from a first-morning specimen for the initial assessment of albuminuria. A protein-to-creatinine ratio (PCR) is used when total protein rather than albumin is required. A timed collection measures total excretion during a defined interval. ¹

Calculate the total amount in the collected urine first:

total amount in the collection = urine concentration × collection volume

Concentration and volume units must be compatible. If concentration is reported in mg/dL and volume in mL, divide the volume by 100 because 1 dL = 100 mL:

total amount (mg) = concentration (mg/dL) × volume (mL) ÷ 100

To express the result as an hourly rate, divide the total amount by the collection duration:

amount per hour = total amount ÷ collection duration (hours)

A 24-hour equivalent is valid only when the procedure permits time normalization and the analyte is excreted at a sufficiently stable rate during the collection:

amount per 24 hours = total amount × [24 ÷ collection duration (hours)]

These conversions can be combined in one dimensional-analysis chain to report grams per 24 hours:

g/24 h = concentration (mg/dL) × [volume (mL) ÷ 100] × [24 ÷ duration (hours)] × [1 g ÷ 1000 mg]

For a complete 24-hour collection, the time factor is 1 because 24 ÷ 24 = 1. Common reporting units are mg/24 h or g/24 h for mass-based results and mEq/24 h for electrolytes. Some analytes or laboratory procedures specify mmol/24 h instead.

Timed creatinine example

A 10-hour urine collection contains 900 mL at a creatinine concentration of 70 mg/dL.

amount collected = 70 × (900 ÷ 100) = 630 mg

excretion rate = 630 ÷ 10 = 63 mg/h

If the procedure permits time normalization and creatinine excretion is sufficiently stable during the collection:

24-hour equivalent = 630 × (24 ÷ 10) = 1512 mg/24 h, reported as 1.5 × 10³ mg/24 h

The 24-hour result is an estimate because creatinine excretion varies with muscle mass, diet, exercise, illness, kidney function, and collection quality. Keep full precision until the final step, then round as specified by the question or laboratory procedure. ^1,2

Full-day protein example

A complete 24-hour collection contains 2000 mL at a protein concentration of 250 mg/dL.

protein = 250 × (2000 ÷ 100) × (24 ÷ 24) × (1 ÷ 1000) = 5.0 g/24 h

This result exceeds the conventional adult threshold for nephrotic-range proteinuria, which is greater than 3.5 g/24 h. Nephrotic syndrome also includes findings such as hypoalbuminemia and edema. ³

Do not extrapolate a short protein collection to 24 hours. If total daily protein excretion is required, collect the full interval specified by the procedure. When a ratio is appropriate, use a validated spot ACR for albumin or PCR for total protein, preferably from a first-morning specimen. ^1,3

Urine flow and measured creatinine clearance

Urine flow is the collected volume divided by time. Convert the collection duration to minutes first:

urine flow, V (mL/min) = collected volume (mL) ÷ collection duration (min)

Measured creatinine clearance requires urine creatinine, urine flow, and a serum or plasma creatinine obtained within the interval accepted by the laboratory procedure:

creatinine clearance (mL/min) = [urine creatinine, U × urine flow, V] ÷ plasma creatinine, P

Use the same concentration units for urine and plasma creatinine so they cancel, leaving mL/min. Accurate timing and a complete urine collection are required. Because renal tubules secrete some creatinine, measured creatinine clearance usually exceeds true glomerular filtration rate. ²

Clearance example

A 24-hour collection contains 1728 mL. Urine creatinine is 80 mg/dL, plasma creatinine is 1.0 mg/dL, and the patient’s body surface area (BSA) is 1.75 m².

A 24-hour interval contains 1440 minutes:

V = 1728 ÷ 1440 = 1.2 mL/min

measured creatinine clearance = (80 × 1.2) ÷ 1.0 = 96 mL/min

Body-surface-area indexing

Clearance may be indexed to the standard BSA of 1.73 m² to compare results across body sizes:

indexed clearance (mL/min/1.73 m²) = measured clearance × (1.73 m² ÷ patient BSA)

For the clearance example:

indexed clearance = 96 × (1.73 ÷ 1.75) = 94.9, reported as 95 mL/min/1.73 m²

Indexing changes only the reporting scale. The patient’s absolute measured clearance remains 96 mL/min. ²

Cockcroft-Gault estimate and discrepancy review

The original Cockcroft-Gault equation estimates creatinine clearance without a timed urine collection. It uses age, weight, and serum creatinine:

estimated creatinine clearance (mL/min) = [(140 − age in years) × weight (kg)] ÷ [72 × serum creatinine (mg/dL)]

The original publication applies a factor of 0.85 to the calculated result for women. ⁴ Cockcroft-Gault produces an unindexed estimate in mL/min. Drug labels and validated local procedures determine when the equation is required. Current National Kidney Foundation guidance favors race-free estimated glomerular filtration rate equations, adjusted for the patient’s BSA when an absolute mL/min value is needed for a medication decision. ⁵

Before comparing measured and estimated clearances, express both as either indexed or unindexed values. A large discrepancy can have biological, preanalytic, or analytic causes. Review collection timing and completeness, serum sampling time, units, the weight used in the estimate, body size, and serum creatinine stability. Measured clearance and Cockcroft-Gault also have different limitations related to tubular secretion, assay standardization, and weight. ^2,5

References

1. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117-S314. doi:10.1016/j.kint.2023.10.018.
2. Mayo Clinic Laboratories. Creatinine Clearance, Serum and 24-Hour Urine. Accessed August 29, 2026.
3. Kidney Disease: Improving Global Outcomes (KDIGO) Glomerular Diseases Work Group. KDIGO 2021 Clinical Practice Guideline for the Management of Glomerular Diseases. Kidney Int. 2021;100(4S):S1-S276. doi:10.1016/j.kint.2021.05.021.
4. Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41. doi:10.1159/000180580.
5. National Kidney Foundation. Cockcroft-Gault Equation for Estimating Creatinine Clearance. Accessed August 29, 2026.