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.


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Fredric Coe, MDVishnumoxRoman MakarenkoClareSusan Recent comment authors

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My urine diagnosed report showed I have calcium oxalate crystalls present.I don’t know how much but my Dr advise me to drink more water … And he doesn’t told me to take any scanning…why??.. My urine pH level is 5.
So Dr what is your opinion…..

Roman Makarenko
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


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… Read more »


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?


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

Judith Blades
Judith Blades

Hello Dr. Coe, After reading your directions I am still unable to decipher my lab readings. Would you mind assisting me please? 11/12/17 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 17/12/17-18/12/17 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… Read more »


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.


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… Read more »


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

Kathleen Gomolka
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… Read more »