HOW TO READ YOUR KIDNEY STONE LAB REPORT – Calcium stones

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

Time

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.

 

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

  1. Gerald Arcuri

    I am wondering how a person could possibly produce more than 2 liters of urine a day. Since being diagnosed and treated for kidney stones, I have done two 24hr urine samples analyzed for stone risk factors, two months apart. My first sample had a 24 hr volume of 800 ml. My second sample had a volume of 1,400 ml.

    Between samples, I altered my diet to force myself to drink more liquids, reduce dietary oxalate, and increase dietary calcium. I began taking TheraLith ( contains 180 mg of magnesium citrate and magnesium oxide 50/50 plus B6 and 99 mg potassium per two pill dose ) twice a day as a supplement. I did not alter my meat/protein intake, which includes one of the following, daily: beef, pork, chicken and fish. I use “light salt” ( NaCl and KCl blend ) in moderation.

    I was pleased, but confused with the results. My 24 hr urine pH went up from 5.5 to 6.8. My 24 hr urine calcium went from 150 to 252 mg. 24 hr urine oxalate went down from 40 to 27 mg. 24/hr uric acid went up from 448 to 504 mg. Uric Acid went from 450 to 850 mg(!) 24 hr urine sodium went from 75 to 162 mg. 24 hr urine sulfate went from 22 to 17 mg. 24 hr urine phosphorus went from 882 to 875 mg. 24 hr urine magnesium went from 100 to 159 mg. 24 hr potassium went from 48 to 54 mEq. 24 hr urine creatinine went from 1325 to 1222 mg. Calcium oxalate went from 4.41 to 1.82. Brushite went from 1.31 to 5.22 (!!!!) Sodium urate went from 2.55 to 3.29 Uric acid went from 5.30 to 0.28.

    Supersaturation indices for the first sample were:
    Calcium oxalate
    Monosodium urate
    Uric acid

    With the “Suspected Problem” being stated as “Uric Acid Lithiasis”

    Supersaturation indices for the second sample were:
    Brushite ( Calcium phosphate )
    Monosodium urate

    With the “Suspected Problem” being stated as “Hypercalcuric Nephrolitiasis”

    I am a male, currently 69 years of age. At approximately age 30, I had two bouts of kidney stones, which I passed, and which were sent out for analysis. They were small stones ( 2 mm ) described as being composed of calcium oxalate.

    The stone that formed in my renal pelvis this year was 7mm and was successfully treated with ECSL. Unfortunately, no stone material was recovered for analysis.

    So, I am left wondering: Do I potentially have a propensity to form all three types of stones? And if so, how do I control dietary factors to limit my risk, and still try to eat a healthy diet? (Oxalate restrictions* alone are draconian, excluding from the diet an awful lot of things considered “healthy”, for example, in a diet designed to lower CAD risk. ) Finally, how can one practically produce >2 liters a day of urine? Pushing myself to drink, I can not even achieve 1.5 liters, and I am a fairly active 69 year-old.

    Thanks for your advice with renal stone prevention.

    *By the way, I have carefully researched the data on oxalate levels in foods, and I have found WIDE variations from studies performed at several well-regarded universities and the American Dietetic Association. I have even found outright contradictions, with one study reporting some foods as “extremely high” in oxalates, with another equally-plausible study reporting those same foods as “low” in oxalates.

    Reply
    • Fredric L Coe

      Hi Gerald, Here are your changes: My 24 hr urine pH went up from 5.5 to 6.8. My 24 hr urine calcium went from 150 to 252 mg. 24 hr urine oxalate went down from 40 to 27 mg. 24/hr uric acid went up from 448 to 504 mg. Uric Acid went from 450 to 850 mg(!) 24 hr urine sodium went from 75 to 162 mg. 24 hr urine sulfate went from 22 to 17 mg. 24 hr urine phosphorus went from 882 to 875 mg. 24 hr urine magnesium went from 100 to 159 mg. 24 hr potassium went from 48 to 54 mEq. 24 hr urine creatinine went from 1325 to 1222 mg. Calcium oxalate went from 4.41 to 1.82. Brushite went from 1.31 to 5.22 (!!!!) Sodium urate went from 2.55 to 3.29 Uric acid went from 5.30 to 0.28.
      You took alkali so urine pH went up. You added diet calcium but did not lower urine sodium – raised it! – so urine calcium went up. You tok magnesium so magnesium went up. You added calcium, so urine oxalate went down. The CaOx SS fell because oxalate fell, the BR SS went up because urine pH and calcium went up. The big problem is that you did part of but not all of the kidney stone diet. It requires increased diet calcium AND lower diet sodium both together. As for diet oxalate, you no longer have a problem as diet calcium cured it – as it almost always will. Most lists are corrupt, ours is as best we can get it given an excellent source (Harvard) and a wonderful curator (Ross Holmes). Regards, Fred Coe

      Reply
      • Gerald Arcuri

        Thanks, Dr. Coe,

        I will work on my sodium intake, but I can tell you, I ingest very little to begin with. I am skeptical that lowering it any further will be of much value. What is the mechanism by which lowered urine sodium affects urine stone-forming chemistry?

        Also, I did not increase my dietary calcium intake at all between my two 24 hour urine collections.. I take no calcium supplement, and what calcium I ingest comes solely from dairy products, which I eat only in moderation. Increased calcium intake cannot possibly account for the reduction in my oxalate values. Not only that, but my latest results indicate that I am now at risk of “hypercalciuric nephrolithaisis”. Out of frying pan and into the fire? I think that the reductions in urine oxalate and calcium oxalate are a reflection of my rigorously following a low-oxalate diet… which is not easy.

        1. Having “solved” the oxalate problem that I would expect to have given rise to calcium oxalate stones, I appear to have somehow traded this off with radically increased brushite ( calcium phosphate ) levels. I assume that this is due to a shift in urine pH. You did not comment on this. Am I now at risk of forming calcium phosphate stones? If so, will reducing my intake of sodium impact brushite concentrations? If not, what will impact ( lower ) them?

        2. I continue to struggle with the recommendation to produce > 2,000 ml of urine per day. I managed with persistent effort to increase my diurnal output from 800 ml to 1,400 ml. I have read the example fluid intake suggestions in the diet section of this website and find them extremely unrealistic. If I can normalize my supersaturations at an output of 1,500 ml per 24 hours, is there any reason to produce more urine ( other than the fact that “more is always better” )?

        I need to find a way to consistently eliminate all possible supersaturations. I seem to be only halfway there, at best.

        Thanks again for your advice!

        Reply
        • Arcuri Gerald

          Dr. Coe,

          I want to apologize for making two seemingly contradictory remarks in my sequential comments in this blog regarding my calcium intake. In my first message, I indicated that, along with making other dietary changes between two 24 hour urine collections ( which were separated by a 2 month timespan ), I had increased my dietary calcium. I actually tried to do so, but I don’t think I was very successful, as measured by any significant increase in dairy products or other calcium-rich foods. So, when I noted in my last comment here that I had, in fact, not increased my calcium intake, I was using that frame of reference.

          Reply
          • Fredric L Coe

            Hi Gerald, I saw this after my note. Diet oxalate may have been the main factor lowering your urine oxalate. The sodium raised your urine calcium. Best, Fred

            Reply
        • Fredric L Coe

          Hi Gerald, I copied your prior results below for reference. You raised your urine sodium from 75 to 162 mEq/d, and urine calcium varies quite a lot with urine sodium in that range. Renal sodium excretion must match diet sodium because there is little storage space for sodium, so very powerful and evolutionarily conserved mechanisms link them. Calcium handling by the kidney parallels sodium in the early parts of the nephron, and although they are separated more distally the sodium effect is remarkable and very well established. You have been labeled ‘hypercalciuric’ because your urine calcium rose. You say you did not increase your diet calcium but you said you did: “Between samples, I altered my diet to force myself to drink more liquids, reduce dietary oxalate, and increase dietary calcium. I began taking TheraLith ( contains 180 mg of magnesium citrate and magnesium oxide 50/50 plus B6 and 99 mg potassium per two pill dose ) twice a day as a supplement.”. You also took alkali, as I noted in my first response and that raised the urine pH, so the calcium phosphate supersaturation rose, as you noted. Of course lowering diet sodium back down to around 75 will lower urine calcium, but the alkali raise urine pH. The purpose of the alkali is to raise urine citrate, but you do not say if it went up; I suspect the pH went up more than citrate as the SS for CaP rose and it is strongly lowered by citrate that binds calcium. As for urine volume, it is hard to control supersaturations with less than 2 liters of urine, and in the only prospective observations stone risk begins to increase below 2 liters/d. I said before you need the whole kidney stone diet to get anywhere, and your situation illustrates the problems people have implementing it. Sometimes is it just to hard, and physicians use medications. I hope this helps clear up what appear to have been less than ideal answers from me the first time. Regards, Fred Coe

          My 24 hr urine pH went up from 5.5 to 6.8. My 24 hr urine calcium went from 150 to 252 mg. 24 hr urine oxalate went down from 40 to 27 mg. 24/hr uric acid went up from 448 to 504 mg. Uric Acid went from 450 to 850 mg(!) 24 hr urine sodium went from 75 to 162 mg. 24 hr urine sulfate went from 22 to 17 mg. 24 hr urine phosphorus went from 882 to 875 mg. 24 hr urine magnesium went from 100 to 159 mg. 24 hr potassium went from 48 to 54 mEq. 24 hr urine creatinine went from 1325 to 1222 mg. Calcium oxalate went from 4.41 to 1.82. Brushite went from 1.31 to 5.22 (!!!!) Sodium urate went from 2.55 to 3.29 Uric acid went from 5.30 to 0.28.

          Between samples, I altered my diet to force myself to drink more liquids, reduce dietary oxalate, and increase dietary calcium. I began taking TheraLith ( contains 180 mg of magnesium citrate and magnesium oxide 50/50 plus B6 and 99 mg potassium per two pill dose ) twice a day as a supplement. I did not alter my meat/protein intake, which includes one of the following, daily: beef, pork, chicken and fish. I use “light salt” ( NaCl and KCl blend ) in moderation.

          Reply
  2. Colleen Murray

    Hello, Trying to figure out where I should go next. Had Kidney Stone in March, Removed by Urologist in April, Stent removed in May. Parathyroid levels were 75 in March, 57 in June and 67 in Aug. Urologist called and told me my 24 Hr Urine came back high for Calcium. Referral to ENT for possible Parathyroid removal. He said all levels were within normal range. Sent for Thyroid Ultrasound that showed probable Chronic Lymphocytic Thyroiditis and no definite Parathyroid lesion.
    Volume 1.66 Na 24 141 Cr 24 1510
    SS CaOx 10.94 K 24 59 Cr24/kg 13.2
    Urine Calcium 726 Mg 24 200 Ca24/kg 6.3
    Urine Oxalate 31 P 24 1.493 Ca24/Cr24 481
    Urine Citrate 681 Nh4 24 51
    SS CaP 1.99 Cl 24 144
    24 Hr Urine PH 5.511 Sul 24 66
    SS Uric Acid 1.65 UUN 24 13.73
    Urine Uric Acid 0.613 PCR 0.9

    Reply
    • Fredric L Coe

      Hi Colleen, Your urine calcium is indeed remarkably high and PHPT is not unlikely. Here is a review of that disease. Serum calcium, not scans, are the crucial thing. Get them fasting, in the morning, and ask if multiple readings are above the upper limit of normal – usually about 10.1-10.3 mg/dl. Given a PTH level that is not suppressed and so very high a urine calcium I would suspect this disease. Regards, Fred Coe

      Reply
  3. pankaj

    Sir just wana to weather these investigations in spot urine

    Reply
    • Fredric L Coe

      Hi pankaj, spot urines are not worthwhile. Urine chemistry changes with meals too much, and all we know about risk vs urine chemistry comes from 24 hour urines. The cost of running the tests is the same for a spot as for a 24 hour urine. Regards, Fred Coe

      Reply
  4. Morris Brum

    Can you explain what the Normalized Values ca24/cr 24 mean in the 24 hour results. Someone on the Facebook page said that their doctor looks at that rather than looking at the urine calcium value at the top of the report. My urine calcium was high at 414, but my ca 24/cr 24 was 165.
    Any significance there?
    Thanks,
    Morris

    Reply
  5. Susan

    Hi I have had 4 calcium oxalate stones. I recently did my 24 hour test. I would love for any feedback you may have as the only advice i have been given is to drink more water.
    Volume was 2.10
    Creatinine 4.6 mmol
    Creatinine (24hr Urine) 9.7 mmol
    Oxalate (urine) 106 umol/L
    Oxalate (24hr) 223 umol/L
    Urate 0.9
    Urate (24h) 1.9
    Calcium 3.61 mmol
    Calcium (24hr) – HI – 7.58 mmol
    Sodium 42 mmol
    Sodium (24hr) 88 mmol
    Parathyroid Hormone Intact – 6.2 pmol
    Citric (urine) .84 mmol
    Citrate (24hr) 1.8 mmol

    Reply
    • Fredric L Coe

      Hi Susan, Your urine volume is just adequate, calcium is quite high, sodium is reasonable if ample and citrate low. I would think your physicians might want to raise your urine volume and ask you to lower your already reasonable diet sodium further to lower your urine calcium. If that is not enough, potassium citrate might be of value as it will lower urine calcium and also provide citrate. Of course your physician is in charge here, and these are just suggestions to that person – and you. Regards, Fred Coe

      Reply

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