IMG_2303Jeff, a reader of this site, suggested this topic and I realized it was one that I should have written long ago.

You can make sense of all those numbers in the 24 hour kidney stone reports.

You can use those numbers to understand how things stand with you.

Reading the numbers can help you achieve your best possible chance at stone prevention – which is the only reason all those lab tests were done in the first place.

This article deals with calcium stones. A second one will cover uric acid stones.

Be Sure You Have Been Screened For Systemic Diseases

Your physician does this part. But even for systemic diseases stone risk is gauged in the urine so you can read along and assess how prevention is going.

Units of 24 hour Urine Measurements


Because excretions of stone risk factors are in amount per 24 hours, we need the time of the collection. You provide the beginning and end times for the collection, in minutes and hours, and the laboratory corrects all excretion values to amounts in 24 hours.

For example, you start a urine collection at 7 am and complete it at 5 am the next morning. The measured total time is 22 hours. If the volume is 1.5 liters, the volume ‘corrected’ to 24 hours is 1.5 liters x 24 /22 (1.09) or 1.636 liters/24 hours.

It is obvious that the less ‘correcting’ the better, because the ‘correction’ assumes that every hour is the same as every other hour, which is not likely. So you want to collect as close to 24 hours as possible. If your time is below 22 or above 26 hours throw it away – no cost – and do another.

I am sure this is an unnecessary comment but sometimes people forget that a urine collection begins by emptying the bladder, writing down the time, and discarding the urine. Here and there people add it, making the real collection from whenever the bladder had last been emptied.

Volume per 24 Hours

Urine volume is measured in liters (L) or milliliters (ml). A milliliter is 1/1,000 of a liter, so there are 1,000 ml in a liter. One liter is 1.06 quarts. The laboratory measures the total volume of urine if you send or bring it. Often you will read the volume yourself, off of a collection container.

Because excretion rates like calcium, oxalate, citrate, and uric acid are calculated by multiplying their concentrations by the 24 hour volume, mistakes in timing, collection, or measurement of volume will make the results less valid. You provide the collection times, assure all the urine has been collected, and even may measure the volume, so quality is under your control.

Excretion Rates

Urine creatinine, calcium, oxalate, phosphate, and citrate excretions, are in milligrams (mg) or grams (gm) per 24 hours. A mg is 1/1,000 of a gram. An ounce contains 28.3 gm.

Urine sodium is measured in millimoles (mmol) or milliequivalents (mEq)/24 hours. Atoms, like sodium, each have a weight made up mainly of their constituent protons and neutrons. One mmol of sodium is 23 mg.

Some atoms like calcium have two charged sites which can bind to other molecules or atoms. They are often measured in milliequivalents (mEq) which is the weight times the number of sites. Sodium has only one, so mmol and mEq are the same.

Molecules like creatinine and oxalate have weights made up of their constituent atoms.

Conversion of Units

Some laboratories report in mmol or mEq. You can convert your lab report if it differs from mine. I will be using L for urine volume, mg for urine creatinine, calcium, oxalate, and citrate, and mmol for urine sodium. Here are the conversions:

Creatinine; 113 mg/mmol

Calcium: 40 mg/mmol; 20 mg/mEq

Sodium: 23 mg/mmol or mEq

Oxalate (oxalic acid): 88 mg/mmol; 44 mg/mEq

Citrate (citric acid): 189 mg/mmol

Direct Measurements

Acidity or alkalinity are measured in pH – no units. It is a logarithm to the base 10 so a change from 6 to 5 means a 10 fold increase in acidity, from 6 to 7 a ten fold increase in alkalinity.

Supersaturations are calculated from urine concentrations. The one we use here is the ratio of the concentration dissolved in urine of each of the three important stone forming salts, calcium oxalate, calcium phosphate, and uric acid divided by their individual solubilities at body temperature. Values below 1 mean crystals will dissolve. Values at one mean crystals will neither grow nor shrink nor form. Values above 1 mean crystals can form and grow. Being a ratio it has no units.

Is The 24 Hour Urine Valid?

Urine creatinine

Like all 24 hour excretion rates, that for creatinine is calculated from the concentration, measured volume, and collection time.

Because muscle cells make almost all the creatinine lost in urine any two urines you collect will have about the same amount of creatinine in them. A more than 20% variation between two urines suggests an error in one collection. If there are many collections, most creatinine excretions will line up pretty well, and an outlier – too high or low by more than 20% of the average for all the collections will stand out.

When a urine stands out as different from prior collections, I say it is best to repeat it. If there are two, use the one that seems more correct.

‘More correct’ is judged by expected amounts of urine creatinine. For men, one expects about 18 – 24 mg/kg body weight; for women, 16 – 22. A urine far out of those ranges is suspect. In people who have a high body fat percentage the ratio can be as low as 12 mg/kg/day, and people who have little body fat and a high muscle mass can be as high as 30 mg/kg/day.

Conditions of Collection

Did you eat and drink as usual, or show off? Had you ‘improved things’ since a recent stone so your collections reflect new habits and not those that led to the stones? Every collection is a one day frame out of a movie that is running 365 frames a year, so if these few days do not reliably represent your average life, their results will inevitably mislead you and your physicians and falsify what you came to do.

Reading the Results

Urine Volume

Curhan and colleagues have linked new onset of stones to urine volume in three large cohorts reviewed in our other articles. Take a look now. The graph is on page 2 of the article. Risk is on the vertical axis, and urine volume is on the horizontal axis. When red shows above the line at 1 it is above normal.

Risk is low above 2.25 liters/24 hours. But the day is long and the 24 hour urine is an average, so for insurance it is best to overshoot so the low times are still not too low. I would say 2.5 – 3 liters liters/24 hours is ideal.

Urine Calcium

Stone risk increases with urine calcium excretion above 200 mg/day in men and women – take a look. The calcium risk is at the upper left corner of the graph.

If you have above 200 mg/24 hours you have ‘hypercalciuria’, a high enough calcium to pose risk of stones – and also bone disease. There are many causes of hypercalciuriaThe commonest is simply a genetic tendency, called idiopathic hypercalciuria. ‘Idiopathic’ means your physician has ruled out any of the other causes of hypercalciuria.

You lower urine calcium in idiopathic hypercalciuria with reduced diet sodium, avoidance of sugar loads, and medications if needed. You compare treated to untreated excretion to see how well the treatment worked.

Urine Sodium

Urine sodium is essentially the diet sodium intake. Because urine losses can lag intake when intake varies sharply, urine sodium is a good estimate of the average over 3 – 4 days. If you tried to lower your diet sodium to 100 mmol/24 hours, 2,300 mg intake, and you find 200 mmol/24 hours, you can be sure that your average intake is on average twice what you desired.

Suppose your urine calcium is 250 mg/24 hours, urine sodium 200 mmol/24 hours – twice the upper limit of diet sodium in the US and above the optimal value of 65 mmol (about 1,500 mg/24 hours). You would want to lower your diet sodium. Suppose your urine sodium was already 65 mmol/24 hours and urine calcium was 250 mg/24 hours; you would want to take other steps like thiazide diuretics or potassium citrate.

What if your urine calcium is 450 mg and your urine sodium is 200 mmol? Sounds like a bigger problem, but it is not. When urine calcium is higher the slope dependency of urine calcium on urine sodium is steeper, so the same reduction of sodium could bring urine calcium quite a way down.

If you lowered diet sodium and there is no change, do not assume the test is wrong. Check the creatinine values – do they match? If so, you have not lowered your average sodium intake. Sodium is an atom and you are not a cyclotron; you cannot make sodium or destroy it.

Sugar Loads

Given to people with idiopathic hypercalciuria they cause a rapid rise in urine calcium and supersaturation. The 24 hour urine will tend to underestimate this because of averaging throughout the day.

Thiazide Diuretics and Potassium Citrate

These drugs come after diet changes have not proven sufficient and are added to the diet changes.

Urine Oxalate

Stone risk increases with increasing urine oxalate and if the risk ratio is not as high as for calcium risk appears at very low levels of excretion – above 25 mg/day. In general, high urine oxalate comes from high oxalate diets, low calcium diets, or the combination of high oxalate and low calcium together. Therefore treatment is dietary. Excellent food lists are in this site – linked from the oxalate diet article. Rarely, urine oxalate is raised from a hereditary overproduction state – primary hyperoxaluria. Likewise, bowel diseases can raise urine oxalate. These are complex conditions and diet alone is rarely enough.

If urine oxalate is high and you have corrected both your diet oxalate intake and added significant diet calcium, you may have some form of genetic or acquired oxalate overproduction or an otherwise inapparent intestinal absorption problem.

Urine Citrate

Citrate is a powerful force against calcium stones. It binds calcium in a soluble complex. It interferes with calcium crystal formation and growth. Low urine citrate is a risk factor for new stone onset – bottom left panel of the graph. Above 400 mg daily there is no extra risk of stones in men or women, so ‘hypocitraturia’ means a urine citrate below 400 mg daily.

If you begin taking, as an example, four 10 mEq potassium citrate pills a day – 40 mEq, that extra 40 mEq of potassium should appear in the urine. If it is not there, the pills may not be delivering the medication into the blood from the bowel. The urine pH should rise because citrate imposes an alkali load. If it does not and the urine potassium does rise, you may need more of the medication.

Urine ammonia – I will cover this in the second half of this two part article – is a major way the body removes acid, so when you take citrate ammonia excretion should fall. If it does not fall, and urine potassium rises perhaps your pills are potassium chloride – the pharmacist switched from what was prescribed.

Strangely, the urine citrate itself does not rise in everyone even when potassium and pH rise and ammonia falls.

Urine pH

A low value – below 5.5 poses a risk of uric acid stones. A high value, above 6.2 raises calcium phosphate supersaturation and risk of calcium phosphate stones. Potassium citrate can raise urine pH; there are no medications to lower urine pH except methionine and ammonium chloride and these are not usable for stone prevention because they will increase urine calcium and pose a risk of bone mineral loss.

Urine Supersaturations

Get Them

If there are no supersaturations, ask that your samples be processed by a vendor that provides them. Supersaturation is the driving force for crystal formation and growth, and invaluable for monitoring stone prevention.

Relate Them To Your Stone Crystals

There will be three supersaturations: Calcium oxalate, calcium phosphate, and uric acid. Inspect the ones related to your stone crystals. For calcium oxalate both calcium oxalate and calcium phosphate supersaturations matter. If calcium phosphate, that one matters most. If pure uric acid, it is uric acid supersaturation.

Reduce Them

One should not compare values in a stone former to values in normal people, who often have urine supersaturations as high or higher than those of stone formers. Active stone formation means supersaturation is too high for you whatever the value may be and needs to be lower. You reduce your supersaturations by increasing urine volume and reducing excretions of calcium and oxalate, or raising excretion of citrate without undue increase of urine pH. That is why we make these measurements.

Compare Then to Now

Compare you now to you before your most recent fluid and diet intakes or medications: If they have fallen, things are going well; if they have risen things are not going well. If stones are still forming, supersaturations need to be lower. If there have been no more stones and supersaturations are stable, stand pat. If supersaturations are below 1 and stones are still forming, the urine samples do not properly represent your real life.

A Good Schedule

When? I like two 24 hour urines before treatment – it gives a sense of averages. After treatment has begun – diet changes, fluid goals, lifestyle, it is important to get another. The timing is up to patients: You know when something has changed, or ought to have changed, and need to be sure it has indeed changed in the right direction. It goes on like that until treatment is reaching its goals – lowering supersaturation by at least half – after which once a year is a good idea.


274 Responses to “HOW TO READ YOUR KIDNEY STONE LAB REPORT – Calcium stones”


    Good day-I am 69yo and a Family doc, I have been plagued with kidney stones for years. I have passed 7 stones the last almost 2 years ago requiring hospitalization. Recent CT scan still revealed multiple stones with largest 11mm. Previous stone and recent stones have been CaOxalate stones. Being a vegetarian, I have always had low protein in my diet and plenty of fruits and vegetables. Almost every meal includes cheese to keep up with protein and bind with any oxalate in my meals. I have stopped almost all high oxalate foods(many of my favorites) and only have something sweet for special occasions-a few times a year. The fluids have become an issue as I have increasing urgency and drink 2-3 liters a day unless I have to go out. I restrict my intake of fluids to 5 hours before I go to sleep so I do not have to get up more than 2-3 times.
    Recent 24 hr. Urine analysis:
    Ca 175mg
    Oxalate 31g
    UA 519
    Citrate 882mg
    Na 132 meq/d
    Sulfate 19mmol/d
    Phos 901mg/d
    Mg 127mg/d
    Ammonia 34 meq/d
    K 53meq
    Cr 1,635
    CaOx 1.02
    Brushite 0.3
    NaUrate 0.79
    Struvite ).04
    UA 2.30 (H)

    I’m not sure what else I can do to reduce ‘my fertility’ for producing kidney stones. Appreciate your thoughts.
    All the best-Fred

    • Fredric L Coe, MD

      Hello Dr Arredondo, I think the clue lurks in your history: “The fluids have become an issue as I have increasing urgency and drink 2-3 liters a day unless I have to go out. I restrict my intake of fluids to 5 hours before I go to sleep so I do not have to get up more than 2-3 times.” Crystals form when supersaturation rises – repeated periods of reduced urine volume are a setup for stones. I also notice SS uric acid above 2, a very high number, suggesting your urine pH must be below 6 – also you do not show the 24 hour urine volume. Potassium citrate might be a good idea even though your stones are calcium oxalate if urine pH is indeed low enough to cause UA SS of 2.3, and of course the low volume needs tending to – I need not elaborate. Best, Fred

  2. Kimberly

    Greetings Dr. Coe – and thank you in advance for all your help on this forum 🙂 Like everyone else here, I’m confused about my 24 hour results so if you have any tips, I’d be so grateful!
    I am female, 54, 5’10 and 185# and train 6 days a week. I had 9mm stone surgery in April – renal sono says “multiple stones present” and hydronephrosis.

    Urine Volume 3.07
    SS CaOx 5.64
    Urine Calcium 159
    Urine Oxalate 70
    Urine Citrate 539
    SS CaP 0.60
    pH 6.291
    SS Uric Acid 0.32
    Urine Uric Acid 0.851

  3. jen

    Hello Dr. Coe,
    Thank you for your generosity! You are very kind to share your expertise.
    Vol 24 4.71
    SS CaOx 2.93
    Ca24 420
    Ox24 31
    Cit24 1100
    SS CaP 1.74
    pH 6.881
    SSUA 0.07
    UA24 1.012
    P24 1.172
    Ca24/kg 4.6
    Ca24/Cr24 273

  4. Kate Arata

    Dr. Coe – I have just received my 24 hr test.

    Vol 1.96
    SS CaOx 8.80
    Ca 24. 315
    Ox 24 30
    Cit 24. 746
    SSCaP. .62
    pH. 5.64
    SS UA. .98
    UA 24. .502

    I was told my risks are the 8.80 SS CaOx, the 315 Urine Calcium and pH of 5.64 was borderline low for Uriel acid stones.
    I have had CaOx stones.

    I do not take supplemental Calcium. I have read the Harvard list for oxalate content and I’m ok with that.

    What are your recommendations, please?

    Do you need other values?
    Thank you.

    • Fredric L Coe, MD

      Hi Kate, Your urine calcium is high and if your serum calcium is normal I presume you have genetic (idiopathic) hypercalciuria. Usual treatments are low sodium diet – to lower urine calcium – and thiazide type diuretics to lower urine calcium more. This is a common problem. Bone mineral needs measuring as it can be lost in the urine and bone can fracture in some cases. Regards, Fred Coe

  5. Suzanne Steenblik Schiess

    I just received my 24-hour results and meet with my doctor soon. I’m wondering what all this means in the meantime. In February I went to the ER and found that I had a 7 mm stone in my left urethra. When I saw the urologist he said I also had many stones in my kidneys. I had Ureteroscopy done in March where they were able to get all of the stones out of my kidneys as well as the 7mm one. They sent 3 stones (from 1 mm to 9mm) to be analyzed and found that they were 70% calcium oxalate monohydrate, 10% calcium oxalate dihydrate, and 20% calcium phosphate. Can you help me understand all of these numbers. (I’ve included my Litholink numbers below)
    Urine Volume 2.48
    SS CaOx 4.80
    Urine Calcium (mg/day) 261
    Urine Oxalate (mg/day) 23
    Urine Citrate (mg/day) 351
    24 Hour Urine pH 6.190
    SS CaP 0.82
    SS Uric Acid 0.27
    Urine Uric Acid (g/day) 0.461

    • Fredric L Coe, MD

      Hi Suzanne, You have high enough urine calcium to raise risk of stones, and low enough urine citrate to do the same. We have found this is very typical of women calcium oxalate stone formers (I have not written our research into an article for the site, it is still a research publication. You may be an ideal candidate for potassium citrate to raise urine citrate and low diet sodium – 1500 mg/d – to lower urine calcium. See if your physicians might not agree. Regards, Fred Coe

  6. Meghan

    1700 mL/24 hr
    Higher Than Normal

    312.8 mg/24 Hr
    Higher Than Normal

    160 mmol/24 hr

    1314.1 mg/24 Hr
    Higher Than Normal

    772 mg/24 Hr
    Higher Than Normal

    60.4 mmol/24 hr

    109 mmol/24 hr
    Lower Than Normal

    493 mg/24 Hr

    32 mg/24 Hr
    Higher Than Normal

    95 mg/24 Hr

    27 mEq/24 hr

    17.70 mg/24 Hr

    493 mOsm/kg

    Creatinine Calculated
    1655.8 mg/24 Hr


    25 mEq/24 hr

    Calcium Oxalate Saturation
    8.37 ratio
    Higher Than Normal
    Calcium Hydrogen Phosphate Dihydrate (Brushite)
    3.09 ratio
    Higher Than Normal
    Sodium Urate Saturation
    4.38 ratio
    Higher Than Normal
    Urate Saturation
    1.85 ratio
    Higher Than Normal
    Magnesium Ammonium Phosphate Hexahydrate (Struvite)
    0.02 ratio

    Any thoughts?

  7. William D Cassabaum

    My Stone Risk Factors are:
    Urine Volume: 2.11
    SS CaOx: 7.53
    Urine Calcium: 127
    Urine Oxalate: 70
    Urine Citrate: 705
    SS CaP: 0.99
    24 Hr Urine pH: 6.468
    SS Uric Acid: 0.26
    Urine Uric Acid: 0.671
    Should I increase my dietary calcium, lower my dietary sodium, and increase my fluid intake? What else can I do?

    • Fredric L Coe, MD

      Hi William, I believe I answered you in my other response. Fred

  8. William D Cassabaum

    Thank you for the information. I have had two 24hr collections done. I’m wondering if going to a plant based diet (milk elimination) has increased my risk for oxalate stones because the lower calcium does not supply enough calcium to bind with the oxalate. Is this a valid concern? Or do I need to just continue lowering my oxalate intake, e.g. spinach, kale, etc. Here are my collection results.
    Urine volume: 2.11
    SS CaOx: 7.53
    Urine Calcium: 127
    Urine Oxalate: 70
    Urine Citrate: 705
    SS CaP: 0.99
    24hr Urine pH: 6.468
    SS Uric Acid: 0.26
    Urine Uric Acid: 0.671

    Thank you

    • Fredric L Coe, MD

      Hi William, I believe your low calcium diet has raised your urine oxalate – a very common occurrence. I would add back a normal amount of diet calcium right away and repeat the 24 hour collection – urine calcium will rise, oxalate will fall, perhaps low enough it is no longer an issue – and you can focus on whatever is really wrong. Regards, Fred Coe

  9. Andrew

    Hi Dr Coe,
    Thanks for providing such valuable resources when it comes to kidneys and kidney stones. I had a Calcium Oxalate stone removed in February, my first ever at 45 yrs old. After two months of increasing my water intake to nearly a gallon a day, I had a 24 hour urine test. The results are confusing to me, largely because my Calcium Oxalate Saturation levels seem to be low, while my Calcium level seems to be quite high. Seems contradictory? Here are my results, any interpretation or advice would be greatly appreciated. Thanks!

    Urine Volume (Preserved) 4440 High mL/24 hr
    Calcium Oxalate Saturation 2.63 Low 6.00-10.00
    Calcium, Urine 329 High mg/24 hr 450
    Calcium Phosphate Saturation 0.47 Low 0.50-2.00
    pH, 24 hr, Urine 6.109 5.800-6.200
    Uric Acid Saturation 0.22 <1.00
    Uric Acid, Urine 563 mg/24 hr <800
    Sodium, Urine 137 mmol/24 hr 50-150
    Potassium, Urine 63 mmol/24 hr 20-100
    Magnesium, Urine 118 mg/24 hr 30-120
    Phosphorus, Urine 875 mg/24 hr 600-1200
    Ammonium, Urine 51 mmol/24 hr 15-60
    Chloride, Urine 141 mmol/24 hr 70-250
    Sulfate, Urine 57 meq/24 hr 20-80
    Urea Nitrogen, Urine 10.62 g/24 hr 6.00-14.00
    Protein Catabolic Rate 0.9 g/kg/24 hr 0.8-1.4
    Creatinine, Urine 1603 mg/24 hr Not Applic.
    Creatinine/Kg Body Weight 17.2 mg/24 hr/kg 11.9-24.4
    Calcium/Kg Body Weight 3.5

    • Fredric L Coe, MD

      Hi Andrew, You have a high urine calcium loss (329 mg/d), possibly genetic, and have reduced your SS values by so high a urine volume. No doubt you formed your stones when urine volumes were considerably lower. You might lower urine volume to 3000 ml/d and still have safe SS values. All the rest of your study looks rather reasonable, especially the lowish diet sodium of 137 mEq/d. Regards, Fred

      • Andrew

        Thanks so much for your response Dr Coe. Yes, I believe my urine output was around 1L per day previous to 2021. Again, thank you for the analysis, it is much appreciated – Andrew

  10. lisa

    help……what does all this mean
    dr. just said he was going to put me on a diuretic 55mg twice a day and to majorly increase my fluid

    urine vol. = 1.77, ss ca ox 6.98, urine cal=301, oxalate=24, citrate 828, sscap 2.62, urine ph6.682, ss uric acid 0.11, urine uric acid 0.398
    I have had several calcium oxalate stones
    what can I do to stop this . 60 yrs old , 200 lbs,

    • Fredric L Coe, MD

      Hi Lisa, you have a high urine calcium at 301 mg/d, and a low urine volume below 2.4 liters (the risk level for volume). Your SS CaP of 2.62 puts you at risk for calcium phosphate stones – are you really sure about the stone analysis. Given stones began later in life, the most worrisome possibility is that you have primary hyperparathyroidism, and that can be cured. Possibly you are losing bone mineral at a high rate, and that is raising urine calcium – you need a bone DEXA scan to find this out. Regards, Fred Coe


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