Calcium Test · 24 Hour Urine

Your blood calcium can look perfectly normal while your body quietly dumps excess calcium through your kidneys, raising your risk for kidney stones and weakening your bones. The only way to catch this is to measure how much calcium actually leaves your body over a full day.

A 24-hour urine calcium test captures that number. It tells you whether your body is holding onto calcium the way it should, or whether something in your diet, your hormones, or your kidney function is pushing calcium out faster than it should go. Because calcium excretion fluctuates throughout the day, a single spot urine check can miss the problem entirely.

What This Test Actually Measures

The test quantifies the total amount of calcium (an electrically charged mineral) that passes through your kidneys and into your urine over 24 hours. About 98% of the calcium your kidneys filter from blood gets pulled back into the body through specialized channels in your kidney tubes. The small fraction that escapes becomes your 24-hour urinary calcium.

That fraction is tightly controlled by a web of hormones: parathyroid hormone (PTH, produced by four small glands behind your thyroid), the active form of vitamin D, and others. When these systems work well, your kidneys reclaim just enough calcium to keep blood levels stable and bones strong. When something goes wrong, either too much or too little calcium ends up in your urine, and this test is the most reliable way to see it.

Kidney Stone Risk

Excess calcium in urine is one of the strongest and most consistent predictors of kidney stone formation. In a large prospective analysis of over 6,200 adults from the Health Professionals Follow-up Study and Nurses' Health Studies, higher 24-hour urinary calcium was identified as one of the three most important urinary risk factors for symptomatic kidney stones. The relationship was nearly linear: the more calcium in the urine, the higher the stone risk, with no single threshold where risk suddenly jumps.

This matters because many people with elevated urinary calcium have no symptoms until they pass a stone. If you have had one kidney stone, your risk of having another within 5 to 10 years is roughly 50%. Knowing your 24-hour calcium level before a second stone hits lets you and a specialist design a targeted prevention plan, whether that means adjusting your sodium intake, your protein consumption, or starting a medication.

Bone Health and Osteoporosis

Calcium that leaves through your kidneys has to come from somewhere. If your dietary intake cannot keep up with the loss, your body pulls calcium from bone. Persistently high urinary calcium (called hypercalciuria) is associated with increased bone turnover markers (blood or urine tests that measure the rate of bone breakdown and rebuilding) and lower bone mineral density. In a study of 230 patients with biopsy-confirmed osteoporosis followed for about 7 years, those with hypercalciuria had greater bone loss at all skeletal sites.

In a separate study of 173 postmenopausal women with osteoporosis, researchers used a simple lab panel that included 24-hour urine calcium, serum calcium, and PTH, and discovered previously unrecognized calcium or hormone disorders in 32% of them. Many of these conditions were treatable. A routine blood panel would have missed most of them.

Hyperparathyroidism and Familial Conditions

When one or more of your parathyroid glands becomes overactive (a condition called primary hyperparathyroidism, or pHPT), it pushes serum calcium up and often increases calcium excretion in urine. The 24-hour calcium test plays a specific role here: it helps distinguish pHPT from a rare inherited condition called familial hypocalciuric hypercalcemia (FHH), where blood calcium is high but urinary calcium is unusually low.

In a study of 150 patients with pHPT who underwent surgery, those with the highest preoperative 24-hour urine calcium (above 400 mg per day, the top quartile) had the greatest gains in bone mineral density at all measured skeletal sites after the overactive gland was removed. This association held after adjusting for age, sex, kidney function, and other factors, suggesting that the preoperative urinary calcium level is a useful predictor of how much bone will recover after surgery.

Kidney Function

Your kidneys are the gatekeepers for calcium. When kidney function declines, urinary calcium drops because the kidneys filter less blood overall. In a study of nearly 5,000 hospitalized patients (3,815 with chronic kidney disease, or CKD, and 1,133 without), lower 24-hour urinary calcium was independently associated with a higher risk of kidney function decline. For every additional 1 mmol per day of calcium excreted (roughly 40 mg), the risk of CKD progression was about 15% lower.

A separate study in the CRIC cohort (Chronic Renal Insufficiency Cohort, a large group of about 3,768 adults with CKD followed for roughly 10 years) initially found a similar pattern, but the association mostly disappeared after accounting for how much kidney function patients had at baseline. This suggests that very low urinary calcium in CKD is partly a marker of how damaged the kidneys already are, rather than an independent cause of further decline. Either way, a very low reading on this test in someone with known or suspected kidney disease is a signal worth investigating.

Blood Pressure

In a large international study of nearly 15,000 adults across 17 countries, higher 24-hour urinary calcium excretion was directly associated with higher blood pressure levels. The relationship was consistent across populations and independent of other mineral excretion patterns. While this does not prove that calcium in urine causes high blood pressure, it does mean that elevated readings may warrant a closer look at cardiovascular risk.

Reference Ranges

Reference ranges for 24-hour urinary calcium depend heavily on your age, sex, and racial background. Many labs still report a generic "normal" of 100 to 300 mg per day, but research shows this range is too broad and can miss real problems in some groups while overdiagnosing others. The ranges below come from a study of 959 healthy US women measured by automated analyzer, and a multicenter study of 1,239 Chinese adults. Your lab may report different numbers, so always compare your results within the same lab over time.

Black women consistently excrete less calcium than White women across all age groups. Using a single 100 to 300 mg range for everyone means Black women with genuine hypercalciuria at 230 mg per day might be told they are "normal," and older White women with low excretion reflecting poor calcium intake might be overlooked. Ask your lab for age- and sex-specific ranges, and if they do not provide them, use these research-based values as orientation.

When Results Can Be Misleading

The biggest source of error is an incomplete collection. If you miss even a few hours of urine, the result will underestimate your true 24-hour calcium. Labs check for this by measuring creatinine (a waste product your muscles produce at a steady rate) in the same sample. If your 24-hour creatinine is unusually low for your body size, the collection is probably incomplete, and the calcium number cannot be trusted.

• Thiazide diuretics (such as hydrochlorothiazide or chlorthalidone): These medications reduce urinary calcium by increasing how much your kidneys reabsorb. If you are taking one, your result may look deceptively low and could mask true hypercalciuria. In some studies, chlorthalidone reduced urinary calcium by roughly 40 to 45%.
• Loop diuretics (such as furosemide): These do the opposite, increasing calcium excretion. A result obtained while on a loop diuretic may overestimate your usual calcium loss.
• High sodium or high protein intake the day before: Both increase urinary calcium acutely. An unusually salty or protein-heavy day can push results 20 to 70 mg higher than your typical baseline.
• Acute kidney injury or sudden changes in kidney function: A temporary drop in kidney filtration (from dehydration, illness, or surgery) will lower urinary calcium and could be misread as normal when it is not.