Diagnostics: 24-Hour Urine Calcium Test
A 24-hour urine calcium test measures how much calcium the kidneys pass into the urine over a full day, and helps evaluate hyperparathyroidism, kidney stones, and bone loss.
What this test is
A 24-hour urine calcium test measures the total amount of calcium the kidneys pass into the urine over a full day. Because the amount of calcium in urine changes from hour to hour depending on meals, activity, and time of day, a single random sample is not reliable. Collecting all urine over 24 hours gives a much more accurate picture.
Calcium is almost always measured together with creatinine, a waste product the muscles produce at a steady rate. Creatinine acts as a built-in quality check: the amount in a complete 24-hour collection falls in a predictable range, so it tells the care team whether the full day's urine was collected or some was missed. The two results are also combined into a number called the calcium-to-creatinine ratio, which is useful in certain diagnoses.
Why a clinician may order it
This test is commonly used to:
Find the cause of high blood calcium or a high parathyroid hormone (PTH) level. It is a key step in evaluating primary hyperparathyroidism, a condition in which one or more parathyroid glands become overactive (hyperparathyroidism).
Tell hyperparathyroidism apart from a harmless inherited condition. A genetic condition called familial hypocalciuric hypercalcemia (FHH) can look almost identical to hyperparathyroidism on blood tests, but people with FHH put very little calcium into their urine. Because FHH does not need parathyroid surgery, making this distinction matters. The urine calcium result, and the calcium-to-creatinine ratio, helps separate the two.
Evaluate kidney stones. Too much calcium in the urine (called hypercalciuria) is the most common identifiable risk factor for calcium kidney stones, and this test is the preferred way to detect it.
Investigate bone loss (osteoporosis) and disorders of calcium metabolism. Either high or low urine calcium can point to problems with how the body absorbs or handles calcium and vitamin D.
How the collection works
The timing is the most important part. The goal is to capture every drop of urine over one continuous 24-hour period, no more and no less.
On waking, empty the bladder into the toilet (do not save it) and write down the exact time. This is the official start time. The urine already in the bladder was made before the test period, so it does not count.
For the next 24 hours, collect ALL urine in the container the lab provides. Use it every single time, day and night.
Exactly 24 hours later, urinate one final time, into the container, to finish. Try to empty the bladder at that final time even without a strong urge, so the last portion is included.
Keep the container cold (usually refrigerated or on ice) during the collection, as instructed by the lab. The container may contain a preservative; if so, do not pour it out and avoid skin contact with it.
Return the container promptly to the lab as directed.
If a collection is accidentally missed, a sample is flushed, or the container is spilled, the lab or clinician should be told. It is usually better to restart than to turn in an incomplete sample, since a missed portion can make the result falsely low.
Preparing for the test
Specific instructions from the care team come first, but in general:
Unless told otherwise, people eat and drink normally during the collection. Urine calcium is strongly affected by diet, so an unusual diet that day can skew the result. In particular, salt, animal protein, and calcium intake all raise urine calcium.
Supplements and medicines should not be changed without asking. Calcium and vitamin D supplements, antacids, water pills (diuretics), and some other drugs can change the result, so the clinician should know about everything being taken, but nothing should be stopped without instruction.
What the results mean
The result is reported as the total milligrams of calcium in the 24-hour sample. Laboratories vary in the exact "normal" range they use, and normal values differ by sex, age, body weight, diet, and other factors, so the result must be interpreted by a clinician in the context of blood tests and the reason for testing. A commonly used threshold for high urine calcium is roughly more than 250–300 mg per day, though the risk of kidney stones rises gradually rather than switching on at a single cutoff.
Higher-than-normal urine calcium can be a sign of overactive parathyroid glands (hyperparathyroidism), too much vitamin D, certain bone conditions, kidney stones, sarcoidosis, or some cancers, or it can simply reflect diet.
Lower-than-normal urine calcium can be a sign of the inherited FHH condition mentioned above, poor calcium absorption, low vitamin D, or other conditions.
An abnormal number does not automatically mean a disease that needs treatment. It is one piece of a larger picture that the clinician puts together along with blood calcium, PTH, vitamin D, and kidney function.
Key points to remember
Collect every urine for the full 24 hours; the completeness of the sample is what makes the test accurate, and creatinine is measured to confirm it.
Eat and drink as usual unless told otherwise.
Tell the care team about all supplements and medicines.
Discuss results with a clinician rather than interpreting the number alone.
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