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why is the first urine discarded while beginning a 24 hrs urine collection?

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"urine specimen collection"[MeSH Terms] AND "24-hour"

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The first urine is discarded because it was made before the collection period began. It holds urine that built up in the bladder overnight, so counting it would add an unknown amount of earlier output to the 24-hour total.
How it works
  • The patient empties the bladder at a set time (for example 7 AM) and throws that sample away. This marks time zero. The bladder is now empty, so everything that follows was made during the timed window.
  • The patient then collects every void for the next 24 hours.
  • The patient also collects the first void the next morning, at the same clock time. This closes the 24 hours.
Campbell-Walsh-Wein Urology gives the reason: "The first morning void is discarded, because this represents urine from the previous night and may not have had a predictable starting point." (Campbell Walsh Wein Urology, 24-Hour Urine Collection section, Chapter 92, p. 2716). The same book repeats this instruction for catecholamine and cortisol testing. Goldman-Cecil Medicine gives the same method for protein excretion: discard the first morning void, collect all urine for 24 hours, and include the first morning void of the next day.
Why it matters
  • Results are reported as amount per 24 hours (protein, creatinine clearance, cortisol, catecholamines, calcium, oxalate, citrate and so on). That only works if the volume and the timing are exact.
  • If the first void were kept, the sample would include urine from before the start time. This would overestimate excretion and the calculated clearance.
  • Excretion of many analytes changes over the day, so a clear 24-hour window also keeps the result comparable between patients.
Checking the collection Urine creatinine is used as an internal check. Expected output is about 20 to 25 mg/kg/day in men and 15 to 20 mg/kg/day in women. A value well outside this range suggests an incomplete collection, an over-collection, or unusual muscle mass (Campbell-Walsh, same section).
I ran a quick PubMed check for recent systematic reviews. The only relevant one is a 2026 review of how 24-hour urine collection is done and reported for kidney stones (PMID 41468103), which notes that methods vary. I did not read it in full, so I can't say whether it changes the standard instruction to discard the first void.

What is the biochemiscal rationale behind discarding the first urine sample in 24 hrs urine sample collection?

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The textbooks give only a short reason for this step. The fuller explanation below is my own synthesis from renal physiology and mass balance, not a quote from the library.
What the textbooks say
  • Campbell-Walsh: the first morning void "represents urine from the previous night and may not have had a predictable starting point" (Campbell Walsh Wein Urology, 24-Hour Urine Collection, p. 2716).
  • Tietz: "The bladder must be emptied at the time the collection is to begin and this urine discarded." The reason given is that "excretion of most compounds varies throughout the day" (Tietz Textbook of Laboratory Medicine, Timed Urine Specimens, p. 135-136).
1. Mass balance: the test measures an amount made in a defined window
  • The kidneys make urine continuously. The bladder only stores it, so the urine in it at the start was made at some earlier, unknown time.
  • A 24-hour test reports the total mass of analyte excreted over exactly 24 hours:
    • Mass excreted = sum of (concentration × volume) for every void between t0 and t24.
  • Emptying the bladder at t0 resets the stored volume to about zero. Everything collected afterward was formed after t0.
  • Keeping the first void would add urine formed before t0, so the collection would cover more than 24 hours of production and the result would be overestimated.
  • The final void at t24 empties the bladder again, so no urine formed inside the window is left behind. This is why the first void of the next morning is included.
2. The error carries into calculated results
  • Creatinine clearance is U × V / P, where V is volume per minute (total volume divided by 1440 min).
  • An extra pre-window volume inflates both V and the total mass of creatinine, so the clearance and eGFR-type calculations come out falsely high.
  • Protein, cortisol, catecholamine and stone-risk results (calcium, oxalate, citrate, uric acid) are also per-day totals and are inflated in the same way.
3. Overnight urine is biochemically atypical
  • Overnight fasting from fluids and ADH action make the first void concentrated, with high osmolality and specific gravity.
  • It is usually more acidic.
  • Many analytes follow circadian rhythms. Examples are cortisol, catecholamines, sodium and creatinine, and calcium, citrate and oxalate depend on urine pH and diet timing.
  • Protein excretion also changes with posture. Orthostatic proteinuria appears upright but not recumbent, so overnight urine is lower in protein.
  • Including this sample would give one overnight, supine, concentrated portion extra weight in a collection meant to represent a typical day.
4. Preservation (my addition, not from the library text)
  • The collection container is kept cold and often holds a preservative. Examples are HCl or acetic acid for catecholamines, metanephrines and 5-HIAA, and boric acid for some other analytes. Tietz lists these preservatives under Urine Preservatives.
  • Urine already sitting in the bladder at 37 °C for hours never had that protection. Catecholamines, for example, degrade at neutral or alkaline pH.
  • Urine voided during the collection also sits in the bladder for a while before it is voided. This is unavoidable.
Practical limits
  • The method assumes complete bladder emptying at both time points. Any residual urine, as in outflow obstruction or neurogenic bladder, adds some error.
  • Check completeness with total urinary creatinine, which should be about 20 to 25 mg/kg/day in men and 15 to 20 mg/kg/day in women (Campbell-Walsh).
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