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Timed urine and creatinine clearance

15 min

Skills you'll build3 skills
  • Calculate total excretion from concentration and a complete timed collection
  • Calculate measured creatinine clearance from timed urine flow
  • Convert a measured clearance between absolute and BSA-indexed reporting

Try first

Try first

A complete 24-hour urine collection has a volume of 1,800 mL, and its calcium is 12 mg/dL. How much calcium did the patient excrete in the 24 hours?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Calculating total excretion

A timed urine result combines three measurements: the concentration, the collected volume and the collection time.

Amount collected (mg) = urine concentration (mg/dL) × collection volume (mL) ÷ 100. Divide by collection duration (h) for mg/h. A full-day amount requires the specified complete collection or permitted time normalization. A deciliter is 100 mL, which is why the volume in milliliters is divided by 100. For a complete 24-hour collection, the amount collected is the amount per 24 hours. A shorter protein collection is not extrapolated to 24 hours.1,2

Every calculation depends on a complete collection. The patient empties the bladder at the start time and discards that urine, then saves every void through the final one at the end time.3 Total creatinine excretion is used to check completeness. Adults excrete about 800 to 2,000 mg/day of creatinine in men and 600 to 1,800 mg/day in women, so a much lower total points to missed urine. When the total is low, check the recorded start and end times and ask about missed voids before reporting.3

Urine flow and clearance

Urine flow V (mL/min) = total collected urine volume (mL) ÷ collection duration (min). A 24-hour collection lasts 1,440 minutes. Draw the plasma creatinine during the collection, near its midpoint when possible, so that both creatinines reflect the same period.3

Measured creatinine clearance (mL/min) = urine creatinine × urine flow (mL/min) ÷ plasma creatinine, from a complete, accurately timed collection with plasma creatinine drawn within the procedure's accepted interval. Urine and plasma creatinine use the same concentration units. The tubules secrete some creatinine, so measured creatinine clearance overestimates the true GFR.2 A clearance calculated from a collection with missed urine is falsely low.

Indexing to body size

Indexed clearance (mL/min/1.73 m²) = absolute clearance (mL/min) × 1.73 m² ÷ patient BSA (m²). Reverse the BSA ratio to recover the absolute value. Indexing only changes the reporting scale. A patient with a BSA above 1.73 m² gets an indexed value lower than the absolute one, and a smaller patient gets a higher one. The absolute clearance is unchanged. Compare an indexed value only with another indexed value, and an absolute value with an absolute threshold, such as a drug label written in mL/min.2,4

Follow the bench demonstration

Sources for this step
References
  1. Kidney Disease: Improving Global Outcomes 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 September 27, 2026. https://www.mayocliniclabs.com/test-catalog/overview/615813
  3. Bishop ML, Fody EP, Van Siclen C, Mistler JM, Moy M. Clinical Chemistry: Principles, Techniques, and Correlations. 9th ed. Jones & Bartlett Learning; 2023.
  4. National Kidney Foundation. Cockcroft-Gault equation for estimating creatinine clearance. Accessed September 27, 2026. https://www.kidney.org/professionals/kdoqi/cockcroft-gault-equation-estimating-creatinine-clearance

Watch one

Read each decision and its explanation.

A 54-year-old man turns in a 24-hour urine collection for creatinine clearance. A plasma creatinine is drawn during the collection.

What is his measured creatinine clearance?

TestResultReference intervalFlag
Urine creatinine96 mg/dL
Plasma creatinine1.6 mg/dL High0.7–1.3 mg/dL0.7–1.3High

Specimen: H 3, L 6, I 1. Complete 24-hour urine, 1,872 mL, and plasma drawn during the collection

  1. Check completeness. 96 mg/dL × 1,872 mL ÷ 100 = 1,797 mg of creatinine, which is within the 800 to 2,000 mg/day expected for a man.

    An incomplete collection makes every later calculation wrong, so completeness is checked first.

  2. Urine flow: 1,872 mL ÷ 1,440 min = 1.3 mL/min.

    Clearance needs a flow per minute, and 24 hours is 1,440 minutes.

  3. Check the units. Both creatinines are in mg/dL.

    Urine and plasma creatinine must share units so that they cancel and leave mL/min.

  4. Clearance: 96 mg/dL × 1.3 mL/min ÷ 1.6 mg/dL = 78 mL/min.

    Clearance is the plasma volume that would supply the creatinine excreted each minute.

The measured creatinine clearance is 78 mL/min, an absolute value before any body-surface-area indexing.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A complete 24-hour urine collection contains 1,500 mL with protein 60 mg/dL. What is the total protein excretion?

Incorrect. Multiplying 60 by 1.5 pairs a volume in liters with a concentration in mg/dL, which loses a factor of 10. A volume of 1,500 mL is 15 dL.

Incorrect. Multiplying mg/dL by milliliters without converting, as 60 × 1,500, makes the result 100 times too large.

Correct. The 1,500-mL collection is 15 dL, and 60 mg/dL × 15 dL = 900 mg in the complete 24-hour collection.

Hint
  1. Put the volume in the same unit as the concentration's denominator.
  2. How many deciliters are in 1,500 mL?

Review From concentration to total excretion

Problem 2 of 3 · Hints available

Measured creatinine clearance is 80 mL/min and body surface area (BSA) is 2.00 m². What is the clearance indexed to 1.73 m², rounded to one decimal place?

Incorrect. Multiplying by 2.00 ÷ 1.73 inverts the indexing factor, so a body larger than 1.73 m² raises the result when indexing should lower it.

Correct. The indexed value is 80 mL/min × (1.73 m² ÷ 2.00 m²) = 69.2 mL/min/1.73 m². The absolute measured clearance stays 80 mL/min.

Incorrect. That value is the absolute clearance with the indexed unit attached. Indexing multiplies it by 1.73 m² ÷ 2.00 m².

Hint
  1. Indexing scales the clearance to a body of 1.73 m².
  2. This patient is larger than 1.73 m². Decide whether the indexed value should come out higher or lower.

Review Body-surface-area indexing

Problem 3 of 3 · On your own

A complete 24-hour collection has a volume of 2,160 mL. Urine creatinine is 60 mg/dL, and plasma creatinine drawn during the collection is 0.9 mg/dL. What is the measured creatinine clearance before indexing?

Show the answer

100 mL/min

Urine flow = 2,160 mL ÷ 1,440 min = 1.5 mL/min. Clearance = 60 mg/dL × 1.5 mL/min ÷ 0.9 mg/dL = 100 mL/min.

Review Urine flow and measured creatinine clearance

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Marisol Stirling, 52, turns in a 24-hour urine for creatinine clearance.
  • The collection volume is 820 mL.
  • On the requisition she wrote that she forgot to save one void during the night.
  • A plasma creatinine was drawn during the collection. Her BSA is 1.62 m².
TestResultReference intervalFlag
Urine creatinine58 mg/dL
Plasma creatinine1.1 mg/dL High0.5–1.0 mg/dL0.5–1.0High

Specimen: H 2, L 5, I 1. 24-hour urine, 820 mL, and plasma drawn during the collection

Decision 1 of 3

How much creatinine is in the collection?

This multiplies mg/dL by milliliters. The 820 mL is 8.2 dL, which gives a result 100 times smaller.

Multiplied mg/dL by mL without converting volume

Concentration in mg/dL times volume in mL gives a number 100 times too large, because 1 dL is 100 mL. Skipping the conversion reports 90,000 mg for a urine protein of 60 mg/dL in a 1,500-mL 24-hour collection, which holds 900 mg per 24 hours.

58 mg/dL × 820 mL ÷ 100 = 476 mg of creatinine in the container.

This divides the volume by 10. A deciliter is 100 mL, so the volume is divided by 100.

Review From concentration to total excretion

Decision 2 of 3

The calculation gives a clearance of 30 mL/min. How reliable is it?

Accurate measurements cannot correct for missing urine. The flow calculation includes only the urine in the container.

A woman excretes about 600 to 1,800 mg of creatinine a day, and this collection contains 476 mg. With the missed void, the flow and the clearance both come out too low.

Indexing rescales the same number to a 1.73 m² body. Indexing does not recover the missed void, and the indexed value, about 32 mL/min/1.73 m², has the same error.

Review Timed collections

Decision 3 of 3

What do you do with the clearance?

The result would come from a collection known to be incomplete, and it would understate her kidney function.

A comment cannot tell the reader how large the error is. The number itself is not reportable as a clearance.

The volume of the missed void is unknown. A guessed volume would give a clearance with an unknown error.

Only a complete collection can give a reportable clearance. Record the missed void as the reason and follow your procedure for the recollection.

Review Timed collections

The container held only 476 mg of creatinine, well below the 600 mg/day lower limit expected for a woman, and the requisition records the missed void. The clearance from that collection is falsely low, so a new complete collection is needed for a reportable result.

For reference

Sources checked