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.

 

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

  1. nicole

    Hello Dr. Coe,
    I am a 47 yr old female and I just had my first stone. It was 12 mm and therefore I had emergency surgery and the stone was lasered. Unfortunately, I wasn’t able to pass anything large enough to analyze after the surgery. I recovered and just did my 24 hr urine risk profile and got the results. i’ve been drinking 90 oz of water daily since i got the stone on April 15th and i only weigh 117 lbs. but i was hoping it would help me as i knew that hydration can cause stones). I fear that I caused this to occur because I went on a lectin free diet and was taking in very little calcium and was salting a lot of veggies, but mostly eating millet, spinach, dark chocolate, nuts and sweet potatoes on an almost daily basis as i had no idea anything about oxalates or kidney stone risk from diet until this happened to me. My hydration level was not this great in the past and i am wondering if my sodium level would have been much higher had i taken the test when i was only drinking about 40 oz of water daily? Can a person decrease their sodium levels by drinking more water daily? in looking at my numbers, do you think i need to cut salt or just increase calcium or should i also consider taking calcium citrate? Should i drink lemon juice for the citrate? Thank you for all the great articles and for responding to so many comments!
    Total Volume, Ur 3300 mL/24 hr
    Total Volume: 3300 mL
    Preserved Urine Volume 3300 mL/24 hr
    Ammonia, Urine 13630 ug/dL
    Ammonia, 24HR Ur 26 mEq/24 hr
    Calcium, Ur 3.4 mg/dL
    Calcium 24HR Ur 112.2 mg/24 hr
    Citrate, Ur 98 mg/L
    Citrate 24H UR 323 mg/24 hr
    Chloride, Ur 36 mmol/L
    Chloride 24HR Ur 119 mmol/24 hr
    Creatinine, Ur 21.6 mg/dL
    Creatinine,Ur 24hr 712.8 mg/24 hr
    Cystine, Quan. Urine 1.42 mg/L
    Cystine, Quant, Ur, 24hr 4.69 mg/24 hr
    Magnesium, Ur 1.6 mg/dL
    Magnesium, 24H Ur 53 mg/24 hr
    Oxalate, Ur 22 mg/L
    Oxalate, 24HR Ur 73 mg/24 hr
    Osmolality, Ur 180 mOsmol/kg
    pH, 24 Hr Urine 6.1
    Phosphorus, Ur 14.2 mg/dL
    Phosphorus, 24HR Ur 468.6 mg/24 hr
    Potassium, Ur 15.9 mmol/L Not Estab. mmol/L
    Potassium, 24HR Ur 52.5 mmol/24 hr
    Sodium, Ur 30 mmol/L
    Sodium, 24HR Ur 99 mmol/24 hr
    Sulfate, Ur, mEq/L 5 mEq/L
    Sulfate, 24H Ur 17 mEq/24 hr
    Uric Acid, Ur 8.6 mg/dL
    Uric Acid, 24HR Ur 284 mg/24 hr
    Brushite 0.19 ratio
    Calcium Oxalate 4.53 ratio
    Monosodium Urate 0.28 ratio
    Struvite 0.01 ratio
    Uric Acid 0.29 ratio

    Reply
    • Fredric L Coe, MD

      I believe this is a duplicate that I have already answered. Fred

      Reply
  2. Nicole

    Hello Dr. Coe,
    I am a 47 yr old female and I just had my first stone. It was 12 mm and therefore I had emergency surgery and the stone was lasered. Unfortunately, I wasn’t able to pass anything large enough to analyze after the surgery. I recovered and just did my 24 hr urine risk profile and got the results. i was told I am an unusual case with Excellent volume (I’ve been drinking 90 oz of water daily since i got the stone on April 15th and i only weigh 117 lbs. but i was hoping it would help me as i knew that hydration can cause stones), Borderline low citrate, Low to normal calcium but very high oxalate. I fear that I caused this to occur because I went on a lectin free diet and was taking in very little calcium and was salting a lot of veggies, but mostly eating millet, spinach, dark chocolate, nuts and sweet potatoes on an almost daily basis as i had no idea anything about oxalates or kidney stone risk from diet until this happened to me. My hydration level was not this great in the past and i am wondering if my sodium level would have been much higher had i taken the test when i was only drinking about 40 oz of water daily? Can a person decrease their sodium levels by drinking more water daily? in looking at my numbers, do you think i need to cut salt or just increase calcium or should i also consider taking calcium citrate? Should i drink lemon juice for the citrate? i am hesitant to take calcium citrate since i read through your site and it seems like trying to cut sodium and increase calcium as well as limit oxalate intake might help bring my levels back to normal, but i wanted your opinion on these results if you could please share it. Thank you for all the great articles and for responding to so many comments!
    Total Volume, Ur 3300 mL/24 hr
    Total Volume: 3300 mL
    Preserved Urine Volume 3300 mL/24 hr
    Ammonia, Urine 13630 ug/dL
    Ammonia, 24HR Ur 26 mEq/24 hr
    Calcium, Ur 3.4 mg/dL
    Calcium 24HR Ur 112.2 mg/24 hr
    Citrate, Ur 98 mg/L
    Citrate 24H UR 323 mg/24 hr
    Chloride, Ur 36 mmol/L
    Chloride 24HR Ur 119 mmol/24 hr
    Creatinine, Ur 21.6 mg/dL
    Creatinine,Ur 24hr 712.8 mg/24 hr
    Cystine, Quan. Urine 1.42 mg/L
    Cystine, Quant, Ur, 24hr 4.69 mg/24 hr
    Magnesium, Ur 1.6 mg/dL
    Magnesium, 24H Ur 53 mg/24 hr
    Oxalate, Ur 22 mg/L
    Oxalate, 24HR Ur 73 mg/24 hr
    Osmolality, Ur 180 mOsmol/kg
    pH, 24 Hr Urine 6.1
    Phosphorus, Ur 14.2 mg/dL
    Phosphorus, 24HR Ur 468.6 mg/24 hr
    Potassium, Ur 15.9 mmol/L Not Estab. mmol/L
    Potassium, 24HR Ur 52.5 mmol/24 hr
    Sodium, Ur 30 mmol/L
    Sodium, 24HR Ur 99 mmol/24 hr
    Sulfate, Ur, mEq/L 5 mEq/L
    Sulfate, 24H Ur 17 mEq/24 hr
    Uric Acid, Ur 8.6 mg/dL
    Uric Acid, 24HR Ur 284 mg/24 hr
    Brushite 0.19 ratio
    Calcium Oxalate 4.53 ratio
    Monosodium Urate 0.28 ratio
    Struvite 0.01 ratio
    Uric Acid 0.29 ratio

    Reply
    • Fredric L Coe, MD

      Hi Nicole, I believe you wrote about delayed testing. The urine seems under collected given the creatinine of 712 mg/d. This is common when volume is so high – the container fills up. Urine calcium is not low but actually a bit high: calcium is 3.4 and creatinine 21.6 (mg/dl for both) with a ratio of 157 mg calcium/gm creatinine – upper limit is about 140. Oxalate is simply immense at 73 mg/d and surely is from the low calcium high oxalate diet. But given a substantial urine calcium despite such a diet I suspect that with a normal calcium intake your urine calcium will rise significantly. Urine sodium is 99 mmol/d and the urine is not fully collected, so you have some room to lower sodium intake to control your urine calcium. Your prior note mentions this strategy. At 323 mg/d citrate is a bit low but with a full collection it may not be really low. As for the urine volume you have raised it, as you noted, and lowered stone risk. Urine sodium is diet sodium intake, on average, and independent of water intake. I presume you are not eating 1000 mg of calcium, and massively reduced your diet oxalate, and your next 24 hour urine will be very different. I think you have idiopathic hypercalciuria so urine calcium may be quite high – this is managable. By all means recheck soon! Regards, Fred Coe

      Reply

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