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.


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

  1. Dawn

    I passed a kidney stone in February after which an ultrasound showed that I had several largish sized stones in my left kidney. The urologist ordered a 24-hour urine test. Every single result was well within normal parameters except that I was just shy of 2L total urine volume (1.99L). Before the kidney stone I had been taking a multi-vitamin with 500 mg of calcium, but I had stopped that before the urine test because the doctor’s office said it could contribute to stone forming. I was also drinking a lot more water at the time of my 24-hr test than I had been before. If they had done a 24-hr test before I made those changes, I’m sure it would have been much different. Because of the number and size of the stones in my left kidney, the doctor ordered a ureteroscopy and laser lithotripsy which I had done last week. I never, ever want to have to go through that again, but the feeling I get from the doctor’s office is that they view it as inevitable. “Some people are just stone formers”, they say, and it does run strongly in my mother’s family. But is it possible that with a completely normal 24-hr urine sample that I am destined to make more stones? I wish the doctor’s office had told me not to make any changes until after they had got a 24-hr test because then I would be able to see the difference that resulted from the changes I made. In your opinion could the combination of the calcium in the supplement and the much lower volume of water that I was drinking have caused my problems considering the fact that my 24-hr test came back so normal after I fixed those 2 issues? The doctor’s office is pretty much telling me to reconcile myself to the fact that I am destined to have more stones, which really bothers me. There is so much information on your site that I am literally overwhelmed at times…in a good way! I’m trying to take it all in, but after reading this article about the 24-hr urine test and seeing that all of my results were normal I’m at a loss of what to think about my situation.

    • Fredric Coe, MD

      Hi Dawn, Your comments make remarkable sense. Your doctors were neither right or wrong. They wanted to see you as you are, but in fact with a lot less fluid and more calcium you may well have looked more abnormal. I notice you used my article – so your calcium would have been below 200 mg, oxalate below 25 mg, citrate above 400 mg – you gave the volume. As for inevitability, that is not true. If you have really brought everything to normal, and can keep things the way they were then you did your test, you may make no more stones. I would recheck in a month or two to be sure things remain normal. Even one more stones and you can just push things more below normal. Here is a good source for you. Also, be sure you have been fully evaluated. Regards, Fred Coe

      • Dawn

        The only number that wasn’t below your recommendations was the oxalate. That was 30. The lab considered anything under 45 normal, but I see now my number needs to 25 or below. My citrate was 691, which may have been partially due to the lemon water I had been drinking after passing that first stone. I am seriously applying what I’ve read about diet and fluid–drinking close to a gallon a day and being careful to consume enough calcium from food as well as being sure to eat calcium foods with oxalate foods at meals. Hopefully this will help bring that oxalate number down. I cannot tell you how thankful I am for the information you have on here. I’m thankful for my doctor, but as you mentioned in another article, he is a busy man and I have had so many questions that it would have taken far longer for him to answer them all than could be done in the office. The information on your site has given me hope and direction. THANK YOU!

  2. Annette

    Calcium Ur 616mg/day, Oxalate Ur 115mg/day, Uric Acid 1584mg/day, sodium Ur 513mEq/day, sulfate Ur 39mmol/day, Phosphorous Ur 2607mg/day, Ammonium Ur 85mEq/day, Creatinine Ur 4247mg/day, Calcium Oxalate 3.08, Brushite 4.20, Sodium Urate 5.67…..results of a 24 hr urine study 9 days after passing kidney stone. Doctor not very concerned, but not sure if I should be because these are all very high….Opinion?? I do have a history of Calcium kidney stones and off the charts low bone density….

    • Fredric Coe, MD

      Hi Annette, These data include a urine creatinine of 4247 mg/day, an impossible amount. Given your female name and the general size of women’s muscle mass, I would judge it is off by about 3 – 4 fold. So the calcium of 616 mg is really more like 250 or 200 mg. My immediate suggestion is to call your physician and or the lab and let them know about this problem. Perhaps they can identify the reason for the error – for error it is – and correct matters. Feel free to use this note. No medical decisions should be made using this kind of mistaken information. Regards, Fred Coe

      • Annette

        yes, that is what the results said, also the total Urine volume was 3.30 L/day and PH Urine was 6.5 , and I questioned the doctor’s office twice since they were so high, and they were not concerned and said see you in May for your appointment to go over the results… not sure I shouldn’t worry.

        • Fredric Coe, MD

          Hi Annette, Even though I know nothing more than what you sent, that creatinine is high by threefold or more so I do not trust the lab. I would repeat the study with a reliable national kidney stone laboratory. The data are unusable in their present form and of that I am certain without reservation. This is not a medical opinion about you but merely a technical appraisal of the lab data. Regards, Fred Coe

  3. Jean Chow

    Hi Dr. Coe,
    My calcium ur is 399 mg/24hrs.
    Oxalate ur is 38mg.,Utica acid ur is 785mg/24 hrs.,citrate ur is 453mg/24hrs., ph ur is 6.9 and brush its is 3.83.
    Can you tell me how shall I deal with that. Thank you so much.

  4. Ken

    Hi, if the normal range of supersaturation for CaOx is for example 6-10, does it mean that almost all of us are constantly forming CaOx crystals ? And if that is the case, what makes the difference between who actually form stones and who just form crystals but never forms stones?

    • Fredric Coe, MD

      Hi Ken, A great question and thank you. Metastable supersaturation means that a solution can be supersaturated and yet form no solid phase. Urine is metastably supersaturated. The upper limit of metastability is higher than the supersaturation. When the upper limit is exceeded, by definition, crystals form. For CaOx the ULM actually rises with the SS, and is often above it by a large margin. For Calcium phosphate, it is very close to the SS so the SS itself is rarely above 2 – 4. For CaP citrate and perhaps inorganic pyrophosphate seem to create the inhibition of crystal formation needed to maintain the ULM – that is an open research question. For CaOx I have no idea – no one does – what the inhibitory molecules are. Stone formers are unlucky – they make crystals and stones at values of SS normal people easily tolerate. Regards, Fred Coe

  5. natasha

    Hello! Can you help me understand the ratio you talk about for the supersaturations of calcium oxalate, calcium phosphate and uric acid? You mention what happens to crystals <1, at 1, and above 1 . . . however, the ratios I see listed on the metabolic report have ranges of 6-10 for CaOx and 0.5-2.0 for Ca Phosphate. Is there something further that needs to be done for this number to get the ratio? Thank you!

    • Fredric Coe, MD

      Hi Natasha, the numbers you quote are the supersaturation ratios. For example supersaturation for calcium oxalate (CaOx) commonly ranges from 6 to 19 fold, meaning 6 – 10 fold above saturation (at 1). So they are ratios and represent the final stone risk measure. The normal ranges are not really important here. WHat matters is this; If you are actively forming stones, your supersaturation is too high in relation to the crystals in those stones. For example if you are forming CaOx stones, and your supersaturation ratio is 7, that is too high for you and needs to be lowered. But, it is up to you to be sure the collections from which that SS is derived represent your life as lived – your real average life, not simply weekends or other potentially special times. Forming new stones means passage or appearance of stones by CT or ultrasound not already present in the past on prior CT or ultrasound studies. Regards, Fred Coe


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