A 24 hour urine collection starts with a void you throw away. Every guide we write here starts from the step that goes wrong most often, and here that step is the first void. If you keep it, every milligram figure the laboratory reports afterwards is too high.
The Verdict
The Collection Step by Step
MedlinePlus sets out the sequence the same way most laboratories do. Five steps cover the whole day.
- Urinate into the toilet as usual and do not collect it. Write down the time. That timestamp is both the start of the window and the moment you will stop tomorrow.
- Collect every void for the next 24 hours in the container the laboratory gave you. Every void, including the 3am one and the one just before you leave the house.
- Store the container in a refrigerator, or in a cooler with ice, for the whole period. ARUP requires refrigeration during collection.
- At the 24 hour mark, urinate once more if you can and add it. That is the last collection of the test.
- Record the total volume and return the container. ARUP asks for total volume, the collection time interval and the pH to be written on the transport tube and the request form.
Pick the day deliberately. A collection needs you within reach of a fridge and a bathroom you control for a full 24 hours. A working day away from home is the usual reason a first attempt fails.
Why the pH Rule Exists
Uric acid is poorly soluble in acidic urine, so it precipitates out of the sample and no longer shows up in the aliquot the analyser reads. ARUP handles this by requiring urine pH above 8.0, and by adding 5 percent sodium hydroxide to raise it when the sample arrives at or below that. Some kits ship with alkali already in the container.
Two rules follow. Do not empty a container that arrived with liquid in it. Sodium hydroxide is caustic, so do not urinate directly into a container holding preservative unless the laboratory told you to.
If your kit came with no instruction sheet, ask the ordering laboratory which of its collection protocols applies before you start rather than after.
How the Number Is Worked Out
The reported result is a concentration multiplied by your total volume. The laboratory measures urate in a small aliquot drawn from the mixed collection, in milligrams per decilitre, then scales it by how many decilitres you produced.
A 1,800 mL collection is 18 dL. At an aliquot concentration of 35 mg/dL that gives 630 mg per 24 hours, inside ARUP's reference interval of 250 to 750 mg per day. Drink more water and the concentration falls while the milligrams per day stay where they were.
The volume you write on the form therefore does as much work as the chemistry. An unrecorded or guessed volume makes the result uncomputable, and a volume written down wrong scales the whole answer by the same error.
The Creatinine Check That Catches a Bad Collection
A laboratory judges completeness from the creatinine on the same collection. Daily creatinine output tracks muscle mass and stays fairly steady for a given person, so it works as a yardstick urate cannot provide for itself. ARUP publishes age-specific and sex-specific 24 hour intervals, billed under Current Procedural Terminology (CPT) code 82570.
| Age | Male | Female |
|---|---|---|
| 13 to 17 years | 500 to 2,300 mg per day | 400 to 1,600 mg per day |
| 18 to 50 years | 1,000 to 2,500 mg per day | 700 to 1,600 mg per day |
| 51 to 80 years | 800 to 2,100 mg per day | 500 to 1,400 mg per day |
| 81 years and over | 600 to 2,000 mg per day | 400 to 1,300 mg per day |
A creatinine result well below your band usually means urine that never reached the container. The consequence matters more on this test than on most, because a missed void drags the urate figure down and a low urate reads as good news. The American Urological Association includes creatinine among the minimum nine analytes for stone testing, alongside total volume, pH, calcium, oxalate, uric acid, citrate, sodium and potassium.
Six Mistakes That Cost You a Repeat Collection
| Mistake | What it does to the result | What to do instead |
|---|---|---|
| Keeping the first morning void | Adds urine your kidneys made before the clock started | Discard it and write down the time. |
| One missed void | Lowers the reported milligrams per day, which reads as a reassuring result | Tell the laboratory. A collection with a known gap is usually restarted rather than reported. |
| Storing the container at room temperature | ARUP requires refrigeration throughout collection | Use a fridge shelf or a cooler with ice, and plan the day around being near one. |
| Eating unusually during the collection day | A purine-free day produces a number describing a day you do not normally live | The AUA asks for a random diet. Eat as you usually do, including the beer or the steak if that is normal for you. |
| Not recording the total volume | The result is concentration times volume, so a missing volume makes it uncomputable | ARUP asks for total volume, collection time interval and pH written on the transport tube and the request form. |
| Not mentioning your medicines | Diuretics, allopurinol and salicylates all change urate handling directly | List everything, including over-the-counter painkillers, when the collection is ordered. |
Who Should Not Start a Collection Yet
- Anyone whose question is gout. The 2020 American College of Rheumatology guideline conditionally recommends against checking urinary uric acid in patients considered for or receiving uricosuric treatment. Ask instead about a serum urate and joint aspiration.
- Anyone handed only a urate order. A lone urine uric acid line is hard to interpret. Ask the ordering clinician whether the full AUA stone panel is intended, since the urate figure is read against volume, pH and calcium from the same jug.
- Anyone facing a day away from a fridge. Move the collection. A warm container is one you will be asked to fill again, and rebooking costs a phone call rather than a day.
- Anyone mid-course on a new diuretic or urate-lowering drug. The result will describe the drug as much as you. Ask whether to collect before starting, or after the dose has settled.
What Would Change Our Answer
A validated spot urine urate-to-creatinine ratio would replace most of these steps, and it is not in the AUA stone guideline today. Until then, a full 24 hour collection is the route the AUA stone guideline recognises.
Before you start, call the laboratory listed on your order. Ask which collection container and preservative its 24 hour urine uric acid protocol uses, and what time window it accepts the container back in.
Frequently Asked Questions
How do you collect a 24 hour urine sample for uric acid?
You urinate into the toilet as usual to start, note the time, and do not collect that one. For the next 24 hours you collect every void in the container the laboratory gave you. MedlinePlus asks for the container to be kept refrigerated, or in a cooler with ice, the whole time. At the 24 hour mark you urinate once more if you can, and that final void goes into the container. Then you record the total volume and return it within the laboratory's stated window.
Why do you throw away the first urine sample?
Because that first void contains urine your kidneys produced before the collection period started. Uric acid is reported as a mass per 24 hours, so the container has to hold exactly the urine made during a defined 24 hour window and nothing else. Discard the first void and note its time. That time starts the window, and the final void at the 24 hour mark closes it.
Does a 24 hour urine sample need to be refrigerated?
Yes. ARUP Laboratories specifies that the urine uric acid specimen be refrigerated during collection. A cooler with ice works when a fridge is not available. Plan the collection for a day you will be near one, because a container left warm for hours is a collection you will be asked to repeat.
What preservative is used for a 24 hour urine uric acid test?
ARUP Laboratories requires the urine pH be greater than 8.0, and if it is at or below 8.0 the laboratory adjusts it upward by adding 5 percent sodium hydroxide. The reason is chemical. Urate is poorly soluble in acidic urine and precipitates out, so an unadjusted acidic sample under-reports the urate the sample contained. Some collection kits arrive with alkali already in the container. That is why laboratories tell you not to pour it out, and not to urinate directly into a container holding preservative unless told to.
How is 24 hour urine uric acid calculated?
The laboratory multiplies the urate concentration in a well-mixed aliquot by the total volume you collected. If a collection measures 1,800 mL and the aliquot reads 35 mg/dL, that is 18 dL times 35 mg/dL, or 630 mg per 24 hours. That result sits inside ARUP's reference interval of 250 to 750 mg per day. The concentration alone tells you nothing, since drinking more water lowers it without changing how much urate left your body.
How do you know if a 24 hour urine collection was complete?
Creatinine on the same collection is the check. Daily creatinine output is fairly stable for a given person because it tracks muscle mass. ARUP publishes age-specific and sex-specific 24 hour intervals. For adults aged 18 to 50 the band is 1,000 to 2,500 mg per day in men and 700 to 1,600 in women. A creatinine well under your band usually means urine that never reached the jug rather than a real finding. The American Urological Association includes creatinine in the minimum nine analytes for stone testing for this reason among others.
Can you eat normally during a 24 hour urine collection?
Yes, and for stone testing you should. The American Urological Association specifies that the one or two 24 hour collections be obtained on a random diet, which means your ordinary eating rather than a prepared low-purine day. A collection done on an unusually restricted day produces a low number that will not repeat. Follow any separate instruction about medicines, since several of them move urate handling directly.
What happens if you miss a urination during the collection?
Tell the laboratory before you return the container. A missed void makes the reported milligrams per day lower than the truth. A low urate result looks reassuring, so the number itself rarely exposes the error. Most laboratories restart the collection rather than report a known-incomplete one. If the creatinine comes back well below your age and sex band, expect to be asked to repeat it anyway.