24-hour urine collection

Principle

• Many substances are excreted at rates that vary during the day; a timed 24-hour collection gives the total daily excretion.

Specimen

• All urine passed over exactly 24 hours in a large labeled container, with or without preservative as required by the test.

Reagents and equipment

• Clean 2–4 L container; preservatives such as hydrochloric acid, boric acid, acetic acid or sodium carbonate depending on the analyte (lab-specific).
• Graduated cylinder or balance to measure total volume.

Procedure

• Start at a set time (e.g. 7 am): the patient empties the bladder into the toilet and discards this urine; record the start time.
• Collect every void for the next 24 hours into the container.
• At exactly the same time the next day, void one last time and add this urine to the container; record the end time.
• Keep the container refrigerated or on ice during collection unless the test states otherwise.
• In the laboratory, record the total volume, mix well and take an aliquot.

Results

• Report results as amount per 24 h (concentration × total volume).
• Creatinine clearance (mL/min) = (urine creatinine × urine volume in mL) ÷ (plasma creatinine × 1,440 min), often corrected to 1.73 m² body surface area.
• Typical 24-h protein excretion in adults is below about 150 mg/day; local ranges apply.

Quality control and pitfalls

• Missed voids (under-collection) are the most common error; keeping the first void causes over-collection.
• Check completeness using total creatinine: typical excretion is about 14–26 mg/kg/day in men and 11–20 mg/kg/day in women (varies with muscle mass).
• Wrong or missing preservative invalidates tests such as catecholamines or calcium.
• Acid preservatives are corrosive; warn patients not to urinate directly into acid-containing containers.

Clinical use

• Protein and creatinine clearance in kidney disease; catecholamines/metanephrines (pheochromocytoma), cortisol (Cushing syndrome), calcium, oxalate, citrate and uric acid for stone risk, and electrolytes.