Metabolic assessment of kidney stones (blood and 24-hour urine)

Blood and 24-hour urine test to investigate nephrolithiasis, that is, stones in the kidney. It measures calcium, phosphorus, uric acid, PTH and kidney function in blood, and in urine it quantifies substances that promote or inhibit crystal formation, such as calcium, oxalate, uric acid, citrate, sodium, magnesium and pH. It is designed mainly for people with recurrent kidney stones or a high risk of them coming back.

No prescription or appointment needed

Results in 5-7 working days

Blood and 24-hour urine sample. Includes: Full blood count · Urea, creatinine and eGFR · Sodium and potassium · Uric acid · Total calcium · Albumin · Albumin-corrected calcium · Phosphorus. 20 in total. See what each one measures

What it is for

It is used to investigate why kidney stones form or keep coming back, and which factors can be changed to reduce the risk of recurrence. It answers two questions. First, blood makes it possible to look for abnormalities of kidney function, calcium, uric acid or PTH that may encourage stones. Second, 24-hour urine shows how much of the substances that form crystals, and of those that inhibit their formation, is being excreted.

With these results, the doctor managing the case can guide prevention measures and assess whether a specific treatment is needed. This blood test does not locate a stone, does not report its size and does not replace analysis of a passed stone. Imaging is required to find out whether there is an obstruction or where the stone is.

When it is recommended

More than one episode of renal colic or more than one kidney stone
A stone that has come back after treatment
A first stone in a young person, in someone with a family history or in someone with a single kidney
Stones in both kidneys, large stones or fast-growing stones
Stones associated with gout, bowel disease, bariatric surgery or other high-risk factors
Before starting long-term preventive treatment or to review a known metabolic abnormality

Clinical comment

Not everyone who has had a kidney stone requires a full metabolic assessment. After a single first stone with no risk factors, guidelines recommend starting with a basic assessment and analysing the composition of the stone if it has been recovered. The blood and 24-hour urine test adds more when stones recur or there is a high risk of recurrence.

The main limitation of this profile is that it includes one 24-hour urine collection. The European guideline recommends two consecutive collections, because the excretion of calcium, oxalate, citrate and other substances changes from one day to the next. A first collection can guide the assessment, but it is worth repeating if the result is borderline, does not fit the history or is going to be used to adjust preventive treatment.

The guideline recommends measuring PTH when blood calcium is raised. We have included it in the same blood draw so that, if that abnormality appears, it can be assessed without having to repeat the blood test. This does not mean that PTH is the usual cause of all stones.

Bear in mind that this profile does not measure urine cystine or include a culture. If a cystine stone is known or suspected, or if there are signs of urinary infection, additional specific tests are required.

Dr. Antonio Mas

What is tested

Full blood count: Checks red blood cells, white blood cells and platelets as part of the general context.
Urea, creatinine and eGFR: Assess kidney function, which is needed to interpret the rest of the assessment.
Sodium and potassium: Measure the main electrolytes in the blood and complete the kidney and metabolic assessment.
Uric acid: Shows whether it is raised in the blood. The risk of forming stones also depends on how much is excreted in the urine and on its pH.
Total calcium: Measures the calcium circulating in the blood, both bound to proteins and free.
Albumin: The main protein that calcium binds to, and it makes total calcium easier to interpret.
Albumin-corrected calcium (calculated): Approximately adjusts total calcium according to albumin. It is not the same as a direct measurement of ionised calcium.
Phosphorus: Interpreted together with calcium, PTH and kidney function to assess mineral metabolism.
Magnesium: Completes the assessment of mineral balance and helps interpret the magnesium excreted in the urine.
Intact PTH: The parathyroid hormone that regulates calcium. It is particularly relevant if blood calcium is raised.
Calcium in 24-hour urine: Quantifies how much calcium is excreted over a full day and can show hypercalciuria.
Oxalate in 24-hour urine: Measures oxalate excretion, one of the most common components of calcium stones.
Citrate in 24-hour urine: Citrate inhibits calcium crystallisation. Low excretion can encourage stone formation.
Uric acid in 24-hour urine: Measures its daily excretion. An excess or a low urine pH can encourage certain stones.
Sodium in 24-hour urine: Helps estimate usual salt intake, which influences calcium excretion in the urine.
Magnesium in 24-hour urine: Assesses the amount excreted, another figure related to the balance between substances that promote or inhibit crystallisation.
Creatinine in 24-hour urine: Helps check whether the collection was complete, although it must be interpreted according to each person's characteristics.
24-hour urine pH: Measures how acidic the urine is. A low pH encourages uric acid stones and a high one can encourage calcium phosphate stones, among other causes.
Urine crystals: Identifies the crystals present in the sample. Their absence does not exclude stones, and their presence does not on its own determine the composition of a stone.
Urinalysis and sediment: Looks for blood, leucocytes, nitrites and other abnormalities. If there are signs of infection, a urine culture may be required, which is not included.

How to prepare

Fast for 8 to 12 hours for the blood sample. You can drink water. Keep to your usual diet, fluid intake and supplements in the days beforehand, because the aim is to see what your urine is like in your daily life. Do not change a prescribed treatment on your own.

Collect all your urine over 24 hours. Discard the first void of the morning and note the time. From then on, keep all your voids, including the first one the next day at the same time. Ask the partner centre for the container a few days beforehand, follow its storage instructions and hand in the urine on the same day as your blood sample.

If you have recently had renal colic, wait at least three weeks and do the collection once you have recovered, there is no active urinary infection and you are back on your usual diet and fluid intake. The European guideline places the preferred time at around three months after the acute episode.

You can add

Without another needle. It is offered when you order, so you do not have to decide now.

General blood testA general blood test with full blood count, glucose, cholesterol and kidney and liver function: the basic parameters included in any general check-up.+€59.00
General + nutritional blood testEverything in the general blood test (anaemia, kidneys, liver, cholesterol and glucose) plus iron stores, vitamin D, vitamin B12 and folic acid. It is the broadest add-on: it covers the general and the nutritional in one go.+€129.00

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What it is based on

European Association of Urology · capítulo «Metabolic Evaluation and Recurrence Prevention»
EAU Guidelines on Urolithiasis
This is the protocol this profile follows. In blood it asks for creatinine, sodium, potassium, calcium with albumin, phosphate and uric acid, and PTH if calcium is high; in 24-hour urine, volume, pH, specific gravity, calcium, oxalate, uric acid, citrate, sodium and magnesium. It recommends collecting urine on your usual diet and fluid intake, no sooner than three weeks after the colic, and says that two collections are ideal: this profile does one, and it is repeated if the result calls for it.

Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.

This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.