Finding the right tests…
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Finding the right tests…
Biochemistry
Most of the calcium in your body sits in bone, but a small, steady amount passes through your kidneys and leaves in urine every day. This test collects every urine you pass over 24 hours and adds up how much calcium left your body over that stretch. It is ordered far more often for kidney stone risk than for bone disease, since most kidney stones in adults are calcium-based, and how much calcium your kidneys excrete is a direct clue to that risk. On a normal diet, expected daily output falls in a defined range; a result well above it points toward stone prevention, while a result well below it points toward a parathyroid or absorption problem instead.
The lab reports total calcium excreted over the full 24-hour collection, in milligrams per day. Because diet strongly affects how much calcium reaches your urine, the result is read in the context of what you were eating during the collection, not as a fixed number that applies regardless. Some reports also give urine volume and creatinine from the same collection, since creatinine output is used to sense-check whether the sample was gathered completely rather than left with a missed void.
Urine calcium sits at the intersection of three organ systems: the gut, which absorbs calcium from food; bone, which stores and releases it; and the kidneys, which decide how much filtered calcium gets reabsorbed versus excreted. A 24-hour collection is used, rather than a single sample, because calcium excretion varies through the day, and only a full-day total gives a stable number to compare against a reference range.
Calcium oxalate and calcium phosphate stones make up most kidney stones in adults, so this test is often ordered after a stone has passed, or for someone at higher risk of one, to check whether high urinary calcium is part of the picture. A high result, hypercalciuria, is one of several correctable risk factors doctors weigh when planning how to lower the chance of another stone.
When a blood calcium result comes back high or low, urine calcium shows whether the kidneys are responding appropriately, excreting more when blood levels are high, holding on to more when they're low, or whether an overactive parathyroid gland is driving it instead. It also helps judge whether calcium and vitamin D supplementation is adequate in people with reduced fat absorption, since poor absorption shows up as low urinary calcium.
A laxative used during the collection period can distort the result, so mention any medicines you're taking when you drop off the sample.
Blood calcium and urine calcium answer different questions. Blood calcium shows the level circulating right now; urine calcium shows how much your kidneys are excreting over a full day, which is what matters for stone risk.
It suggests hypercalciuria — more calcium passing into your urine than expected. That raises the risk of calcium-based kidney stones and is one of the factors doctors look at when planning stone prevention.
Yes. Because a normal diet includes dietary calcium, how much you're eating and absorbing directly affects how much shows up in a 24-hour urine collection.
No. It measures calcium leaving your body through urine, not how much calcium is stored in your bones. Bone density needs a separate scan, such as a DEXA scan.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
An overactive parathyroid gland raises blood calcium and typically increases how much calcium the kidneys excrete too, so the two tests together help confirm that pattern.
Yes. Unexpectedly low urinary calcium can point toward poor calcium or vitamin D absorption, or toward a parathyroid gland that isn't working enough, and is followed up differently from a high result.
Tell the centre about any laxatives or supplements you're taking, since laxative use during the collection period can distort the result. Ask your doctor or the centre for full collection instructions.