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.


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

  1. Roman Makarenko

    Would you mind assisting me please?
    My Stone Risk Factors /Cystine Screening:
    ½ days Vol 24 1.50/1.47, SS CaOx8.52/7.97, Ca 24 278/275, Ox 24 32/30, Cit 24 820/832, SS CaP 0.95/1.93, pH 5.617/6.078, SS UA 1.59/0.85, UA 24 0.615/0.727
    Dietary Factors
    Na 24 107/146, K 24 54/73, Mg 24 170/132, P 24 1.028/0.921, Nh4 24 32/33, Cl 24 114/155, Sul 24 36/37 , UUN 24 8.71/7.66, PCR 0.7/0.7,
    Normalized Values
    Cr 24 2451/2234, r 24/Kg 24.6/22.4, Ca 24/Kg 2.8/2.8 , Ca 24/Cr 24 113/123

    Thank you

    • Fredric Coe, MD

      Hi Roman, I do not understand 1/2 days. Do you mean you collected 12 hour urines? If so, no one can interpret them. If you mean days 1 and 2, your volume is too low, CaOx SS too high, calcium too high. Since I know nothing about your stones or your life, that is all I can say. Check out the values in this article so you can see why I said things were too low or high. Regards, Fred Coe

  2. Susan

    LithoLink Results
    2.72 Urine volume
    5.83 SS CaOx
    244 Urine Calcium
    34 Urine Oxalate
    1519 Urine Citrate
    1.48 SS CaP
    7.01 pH
    0.04 SS Uric Acic
    0.39 Urine Uric Acid
    44 Na
    60 K
    142 Mg
    0.409 P
    13 Nh4
    52 Cl
    10 Sul
    4.44 UUN
    0.6 PCR
    934 Cr 24
    14.2 Cr24/Kg
    3.7 Ca 24/Kg
    261 Ca 24/Cr24

    Taking 1000mg Calcium supplement; allergic to dairy
    Blood potassium 3.5 (at low end of range for months)
    Taking 1/2 of 12.5 mg Thiazide since December 2017
    Calcium oxalate stone confirmed by lab in 2015; no stones caught since then. Stone passed in January 2017. Seven stones on September 2017 CT. Ureteroscopy 2017 and shockwave 2018. Latest CT shows debris plus possible new stones.

    Possible next steps–what makes sense?
    1) Increase thiazide to half of 12.5/mg twice a day (this is double the current dose) to lower urine pH and urine calcium
    2) Increase calcium supplement to 1250mg/day to reduce oxalate and lower urine calcium
    3) Reduce oxalates in diet
    4) Eat more protein to increase phosphorus and PCR and lower urine pH
    5) Take potassium (NOT potassium citrate) to increase blood potassium; dosage? frequency?

    Confused about why pH and urine citrate are so high.

    • Fredric Coe, MD

      Hi Susan, Of course I cannot practice medicine on the web, so my suggestions are for your personal physician to consider along with yourself. I presume your calcium supplement is calcium carbonate, and therefore provided an alkali load – not all the calcium is absorbed, all of the excess alkali is absorbed – so it is like potassium citrate in a way. Your diet is low protein – pcr 0.6 – not good for health. Your urine sodium of 44 mEq/d is very low! so I presume you have very marked underlying hypercalciuria. What is wrong is at base how you get your calcium. Perhaps you do not take it with your high oxalate meals – it needs to be highly selective. The urine magnesium is very high, sulfate very low suggesting a plant based diet – high oxalate, low protein. Perhaps you could reorient your diet along better lines. The ideal kidney stone diet is the ideal US diet, so that is always a good change. Perhaps you might want to discuss this with your physician before doing anything as I have a very imperfect knowledge of your actual situation. Regards, Fred Coe

      • Susan

        I take cal/mag/zinc supplement (contains both calcium carbonate and calcium citrate) with meals. I am allergic to dairy. I also have severe osteoporosis. OK to boost calcium to 1250mg/day? Would increasing thiazide help? I will increase protein.

        • Fredric Coe, MD

          Hi Susan, You have a complex situation that I believe – I am far away!! – is genetic hypercalciuria with inadequate diet calcium and bone disease. I believe your urine sodium was very low – 44 mEq/d – and your diet is not fully clear to me. You need bone active medications if you have severe osteoporosis and also care from people specialized in mineral/ bone disease. At this stage we have to rely on your physicians and I had better keep quiet for fear of making things worse. Very likely mere change in diet calcium will not be enough to protect against fractures. Regards, Fred Coe

      • Clare

        I have been working on my diet for 8 months and have successfully lowered my protein from 2.0 to 1.1. All my levels have come down. My Urine calcium was 346 now down to 259 my urine ph is 6.8 was 7.0 but one of my 24 hour urine test was 6.1. I do not want to go on medication. Should I just keep going as I am? My urine volume was 3.45 My oxalate was 39 but I know it was my diet as I previously had it down to 25.

        • Fredric Coe, MD

          Hi Clare, Here is a good article on exactly this issue. In general we treat the individual values to lower supersaturation, and if we do lower it we stay with the diet and fluids unless more stones form. Take a look. You seem to be doing rather well, it would seem. Regards, Fred Coe

  3. Joyce

    My most recent report, while on Potassium Citrate 15 meq twice daily for a year showed— urine calcium 42, oxalate 49, Citrate 482, ph 7.3. Supersaturation calcium phos 0.15, volume 4.91 L.

    Because of the low numbers and extremely low supersaturations, a nephrologist advised me that I could stop Potassium Citrate completely. (My number was 334 before starting it), while my urologist advised increasing my dose of Potassium Citrate a bit.

    Who to believe?

  4. Aswin

    My question is
    How to recognise the type of kidneystone in ultra sound report without consulting the doctor???
    Please tell a proper and clear ans

    • Fredric Coe, MD

      Hi Aswin, I would not advise this. You need physicians for stone disease. The ultrasound report is not a good index of stone type; you need to have the stones analysed. Regards, Fred Coe

    • Fredric Coe, MD

      Hi Aswin, I would not advise this. You need physicians for stone disease. The ultrasound report is not a good index of stone type; you need to have the stones analysed. Regards, Fred Coe

  5. Judith Blades

    Hello Dr. Coe,
    After reading your directions I am still unable to decipher my lab readings. Would you mind assisting me please?
    Oxalate Urine
    Vol 1.99
    Creatinine Urine 3.9 mmol/L
    24 hr Urine Creatinine 7.8 mmol/d 4.0-17.0
    Oxalate 0.44 mmol/d H 0.04-0.32

    Urine Biochemistry Timed urine Sample
    Urine Volume 2.20
    Creatinine Urine 24h 7.3mmol/collection N 4.0-17.0
    Calcium Urine 24h 4.9mmol/collection N 2.5-7.5
    Calcium/Creatinine Urine 0.67mole ratio H 0.06-0.45

    My doctor told me oxalates are slightly raised but not enough to worry. I since raised my fluid intake to as close to 3L per day as possible. The urologist didn’t mention anything at all when I saw hime finally about 10 days ago. Then last week on a totally unrelated visit to my medical clinic another doctor said my oxalates are High! I am really confused now.
    None of them are talking diet at all and I have had 3 hospital admissions with passing kidney stones in the last 2.5 years.
    Is it possible to answer my query?

    Kindest thanks

    • Fredric Coe, MD

      Hi Judith, The problem is that the lab article uses mg for oxalate and calcium and your labs are in millimoles – presumably not in the US. So your urine oxalate is 44 mg/d in US units and that is indeed high – risk begins at 25 mg/day. The usual reason is a low calcium diet. Your urine calcium is just at the lower edge of stone risk. Your urine volumes are barely adequate. So indeed urine oxalate is your main problem. The best approach is increased diet calcium to 1000 mg/day putting the calcium where higher oxalate foods are found. Then you need to lower diet sodium to about 1500 mg/day; this keeps urine calcium from rising. Finally, you want to be careful to avoid very high oxalate foods – list in the oxalate article. All this is the kidney stone diet, ideal for general health and for stone prevention. Regards, Fred Coe

  6. Demetrius

    Dear Doctor,

    What is your opinion about Orthophosphate therapy (phosphorus) of Hypercalciuric stone formers due to IH?

    I came along this paper:

    albeit an old paper and low number of patients, it shows a significant decrease in Urine Calcium and serum 1.25OHD3. In this paper they cannot explain this phenomenon.

    Do you think is it because urinary Phosphate crystallizes with urinary Caclium in the urine and thus it is not detected as a free ion from the analyzer?

    They also show a small increase in iPTH. Could this signify a detrimental effect on bone mineralization ?

    Thank you.

    • Fredric Coe, MD

      Hi Doctor – I judge you are one; the work was very clever. phosphate loading lowered serum calcitriol, through a pathway well known even then, and also urine calcium. The authors reasoned the lower calcitriol lowered GI calcium absorption and also – perhaps by downregulating the renal CaSR – raised renal calcium reabsorption. PTH went up as expected. As for bone, no real data. Thanks for pointing it out. For clinical use, I would be wary. One can lower urine calcium and boost bone mineral balances with low sodium / high calcium diet, this measure has a very substantial trial, and indeed raising urine phosphate could possibly raise urine CaP SS and promote stones – though I doubt that happens. Regards, Fred

  7. Pat

    Hi, Dr. Coe

    I need help understanding the adjustments that LIthoLink has been making to the volumes of my 24-hr. urine samples. I understand that my reported volumes must be adjusted to fit a time period of exactly 24 hours, but that adjustment does not explain the differences I am seeing. I talked to my nephrologist about this, and he called LithoLink for an answer. The only explanation he got was for the time deviation from an exact 24-hr. collection period. I just noticed the differences recently when I had 2 more samples analyzed by LithoLink. When I went back to my notes on several prior tests and compared the volumes used by LithoLink for those samples vs. the volumes I actually reported, they also were different by an amount not consistent with the time deviation. In fact, in 2016 I reported a volume of 2.64 liters collected in 24 hrs. 0 minutes, and the volume used in my test report was 2.79 liters.

    Here are the volumes for my last 5 tests:

    Test Unex- Vol. used Vol. I reported, Vol. I Collection
    Date plained in LithoLink corrected actually period
    dif. report to 24 hrs. reported

    3/19/18 -0.38 2.39 2.77 2.75 23 hrs. 50 min.
    3/18/18 -0.08 2.40 2.48 2.65 25 hrs. 37 min.
    10/5/17 0.07 2.31 2.24 2.20 23 hrs. 35 min.
    10/4/17 0.09 2.65 2.56 2.62 24 hrs. 34 min.
    8/9/16 0.15 2.79 2.64 2.64 24 hrs. 0 min.

    The unexplained differences range from a negative 380 ml. to a positive 150 ml. They are in calendar order by size, but beyond that, they make no sense to me — will you please explain what’s going on?

    Many thanks for your help and your time,


    • Pat

      Sorry, the col. headings did not post as I expected. They are as follows: Test Date, Unexplained dif., Vol. used in LithoLink report, Vol. I reported – corrected to 24 hrs., Vol. I actually reported, and Collection period

    • Fredric Coe, MD

      Hi Pat, I can answer your question. Litholink puts a marker into the urine collection – a fixed amount of a harmless but detectable material and measures the concentration of that material in a sample of your urine. The concentration of the material in your urine is simply the total added (that is fixed by the company when you add preservative to the container)/volume of urine collected. One can calculate the volume from that amount (mg)/concentration (mg/l). Your reported volume is not the prime controller of the reported volume because trials by the company established that this dilution method gave more accurate results than you can get from reading your own volume using the scale on the container. Regards, Fred Coe

      • Pat

        Very interesting!

        So the graduations molded into the collection containers are not accurate enough to provide more than a rough estimate of the true volume, and the mysterious time trend suggested by my little analysis may have something to do with shrinking containers, but nothing to do with my kidney function . . . well, that’s a relief!

        And that also explains why the LithoLink instructions say to add not only the contents of the preservative vial to the collection container, but also the vial itself and the cap — to make sure that every mg. of the marker gets in there.

        Thank you so much, I will pass this info along to my doctor, and thanks again for this enormously helpful website.

  8. Kathleen Gomolka

    Hello Dr. Coe,
    I wondering if you could give me some direction… your opinion, is it possible to correct, in a type two diabetic, metformin only Results from two 24 hour urine collections); extreme high calcium (300-400), Oxalate (50-61), citrate of (2400-2600), sodium WNL(101-117) all other “dietary factors from litholink were WNL’s , output 1.8 L then 2.8L, ph 5.4 & 5.9 with a Whole Foods plant based vegan diet? Especially if I drop dairy & fish and keep my blood glucose and sodium intake in excellent control? History of two stones in five years, one passed without intervention, last one with a cysto/laser litho & a five day stent.
    Thanks for what you do!


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