CoeTie4There is no doubt about what I say to patients: “Bring me all the analyses that have been performed on your kidney stones. Bring in any stones you have tucked in a dresser drawer and we will get them analysed.” But what do I say to me, and what do I do as time goes on and more stones form or are passed? Do I analyse all of them? Do you?

The problem of keeping track

Whenever I get a new patient, the stone analyses are at the top of my mind. How can I do anything rational about prevention if I don’t know what the stone crystals are? If there are no analyses at the first visit, like you I do everything I can to track them down; and, I usually succeed. But as time goes on with a patient, and more stones come along – many of them old ones, I hope – there is a tendency to let them go. After all, there were two calcium oxalate stones documented 2 years ago, I say to myself; do I really need to send more of them off in order to prove the obvious, or to meet some standard of perfection?

The problem is, at least in the patients I have studied, things change, and not always for the better.

Evidence for stone conversion

You probably already know that calcium phosphate stones, brushite especially, are much more trouble than routine calcium oxalate stones. What I suspected, and have had an occasion to document Nephrology Dialysis Transplantation Volume 24, Issue 1Pp. 130-136. is that conversion from calcium oxalate to calcium phosphate stone formation is not so rare among our patients with sequential stone analyses during treatment. Out of 4767 patients, we found 445 who had two or more stone analyses, lacked any systemic disease as a cause of stones, had well preserved kidney function, and formed calcium stones without any admixed uric acid, struvite, or cystine: were, in short, idiopathic calcium stone formers with at least two stone analyses.

I would have thought, incidentally, this being a stone research center, we would have had many more stone analyses for this most common kind of patient, but we did not. If it were not for our research, and if we were not maintaining such complete research records as we do, I would never have known we did not have more.

The details of what we found

Of the 445, 62 had a first stone >50% calcium oxalate by analysis and a last stone at least 20% higher in calcium phosphate content. These were the patients who converted from calcium oxalate to calcium phosphate stone formers. As controls we selected from the 445 181 patients who met rather stringent criteria: First stone >90% calcium oxalate and increase of stone calcium phosphate was <20%. In actual fact, the median stone calcium phosphate percentage of those who converted were 12% at the start and 75% at the end, whereas those who did not convert began with a median calcium phosphate percentage of 2% and ended up at about the same. Given only the most rigorously selected patients, 62/(62+181) or 25.5% converted.

In an altogether unrelated study of VA hospital stone analyses, Mandel et al found that successive recurrences of stone had increasing calcium phosphate percentages. They specifically echo our idea, actually antedate our presentation of that idea in print, that stone analyses should be continuous because conversion is not at all uncommon.

Why is increase of stone calcium phosphate important?

I have already pointed out that calcium phosphate stones are more serious a problem than calcium oxalate stones: they are larger on average, often more numerous, and involve the kidney epithelial cells. Brushite stones are very hard and do not fragment well with shock wave therapy. So conversion is not a good clinical outcome.

An altogether different problem is that treatment may not be the same for idiopathic calcium phosphate and calcium oxalate patients. We treat idiopathic calcium stone formers like you do: fluids, reduced diet oxalate, reduced diet sodium, thiazide diuretic agents to lower urine calcium when it is high, and potassium citrate, and use these modalities in various combinations depending on the situation. All of these treatments are reasonable, and the two drugs each have some RCT support.

But potassium citrate has never been tried in calcium phosphate stone formers per se. Some calcium phosphate stone formers no doubt have been in the three RCT for potassium citrate, but we do not know which ones they were, and whether perhaps they did poorly with the drug – had more stones, or perhaps stone growth.

There are reasons to believe calcium stone formation might increase or decrease. Potassium citrate can raise urine citrate and thereby reduce calcium phosphate formation. This is true because citrate binds calcium in a soluble complex leaving less to combine with phosphate, and also because citrate can inhibit the formation and growth of calcium phosphate crystals. But citrate is an alkali and can raise urine pH, and therefore raise urine calcium phosphate supersaturation. So we do not know if it is a good or bad treatment for patients producing calcium phosphate stones. Lacking a trial the matter is moot and wisdom dictates caution.

There are other kinds of stone conversion

It is not just increasing stone phosphate content that has taken me by surprise. Although I have not written a paper about them, I have patients who started as idiopathic calcium oxalate stone formers and began making mixed calcium oxalate – uric acid stones over time. They needed treatment to raise their urine pH. Some had become diabetic; some became obese; some just got old and lost some kidney function. But how many patients have done this I do not know. Occasionally struvite begins forming in calcium stone formers. Eventually the infection becomes obvious: stones become large and gnarled; the urine is obviously infected. But perhaps a more timely analysis would have hastened diagnosis.

What I have begun to do

My message to myself, which I am sharing with you, is that stone analyses are really important over time, and being not too expensive (I have no financial relationships with this vendor, it just so happened to offer me a convenient web site for a reference) is probably worthwhile for most stones that are removed from patients or passed.

I am changing my ways. I send every stone for analysis. I suppose some money will be wasted, but maybe in the long run a lot more will be saved. Even a single extra stone attack can be very expensive.

Should we do research about this topic?

At first thought research would seem practical, and likely to help me decide if my new way is right – or wrong. But the matter is both mundane and not so easily transformed into an experiment. To do what Mandel did, analyse successive stones and determine phosphate content is to do what has already been done. What we did has also been done except for the specific search for new uric acid or struvite appearing over time.

This leads me, at least, to say no. We have a decent clue as to what happens to patients, and the belaboring of the matter may not be worthwhile. Others may say I am wrong in this, and even wrong to do too many stone analyses.

Fred Coe MD

Return to Stone Walking Tour


  1. Robin

    Hello. I’m 42yrs old (female) and passed a kidney stone for the first time 8 months ago. I passed another stone two weeks later, and then another 3 months later. A few weeks after the third one I had a bad reaction to valtrex (prescribed for cold sore) and it caused impaired kidney function for a few days. Imaging done at that time showed many small stones in both kidneys. I’m pretty sure I either passed another stone last night or its on its way. The only stone I was able to catch was that first one but no one wanted it. My primary care dr said to give it to the kidney specialist but I lost my medical insurance and wasn’t able to keep that appt. The stone has since been lost.
    I’m curious what may have changed to cause me to suddenly have kidney stones. Is this typical as we age?
    Thanks in advance.

  2. Emma Arnold


    Just come across your website whilst trying to look for new information or actually anything that will help me. I am 38 years old. For the past 3 years i have been producing Brushite stone at a rapid rate. In all i have 10 / 12 surgical operations to remove stones 2 of which were emergency. I am looking for anything or any advice to try and slow this process down. I have been diagnosed with Renal Tubular Acidosis which affects the kidney stone producing. I have been given Sodium Citrate to drink daily, tried cutting things out of diet, drink plenty but no one seems to know what to do with my case. I am from the Wales in the UK. Any advice or information will be given will be greatly appreciated. ANY!!

    • Fredric Coe, MD

      Hi Emma, Brushite stones are nasty and a special breed. Usually there is idiopathic hypercalciuria with increased urine pH and normal blood. Treatment is as for all calcium stone formers and we just put up our opus magnum. A good step program will help. Sodium citrate is not ideal because of the sodium; potassium citrate is better. Because they are nasty I would advise you follow the five steps with your physicians; Wales is said to be a lovely place and I am sure the physicians are wonderful. Regards, Fred Coe

    • Emma Arnold

      Thank you so much for your quick reply, this is the most information i have been given in 3 years so really appreciate it. I will definitely bring the information to my next appointment, whenever that will be. Unfortunately, at the moment i am only seeing a consultant who like is said is a surgeon, great for the removal of stones but so great for slowing the process down. Thank you again Fred, and if anyone over that side of the water wants a complex case to study (that’s what they said here, probably not that complex to you) they are more than welcome to have mine. P.S. Wales is a beautiful place you should come and see for yourself sometime. Many Thanks. Emma.

  3. yolanda rodriguez

    I am taking Norvasc and Flomax for suspected kidney stone. Today I noticed two round discs in my urine. They were very small and looked like white egg shell. I notice white is not listed as a kidney stone color. Could I be expelling something else beside particles of kidney stone? Thank you for your reply.

    • Fredric Coe, MD

      Hi Yolanda, whatever it is, turn it in for stone analysis. If it has any crystals in it they are a clue. Color is not a sure enough clue. Regards, Fred Coe

  4. Jim

    Thank you so much for sharing this information. I just collected a stone last month. My doctor said it’s consistent with calcium oxalate. I had 24hrs urine collection test after that. The report showed two of all are out of standard range:
    Oxalate Rate is 46mg/day (just higher than 45mg/day);
    BRUSHITE RELATIVE SUPERSATURATION is 2.96 (way higher than 2.00).
    Do you think I’m at high risk of calcium phosphate? What’s your advice on diet to prevent from forming calcium phosphate stone?
    Many thanks!

    • Fredric Coe, MD

      Hi Jim, I guess the first big question is just what that stone was indeed made of. The analysis would have been of the crystals, usually by infrared spectroscopy and your physician will know what they are. If they are calcium oxalate the high CaP SS is a problem because the initial nidus for formation is on calcium phosphate. Of course, if the stone has a lot of phosphate crystals, that SS is too high. So it needs to be lower. Such a SS is from low volume, high pH, high urine calcium, or some mixture – so take a longer look at the numbers. Treatment is usually a mixture – fluids, low sodium if calcium is high (over 200 is high), high diet calcium is oxalate is high, and possibly meds like potassium citrate or thiazide. Here is a pretty good outline of how things can be made to work for you. I think I am answering your question: The right answer is pretty specific to your results. Regards, Fred Coe

      • Jim

        Dr. Coe: Thank you so much for the quick response. I’m taking potassium citrate now, which brings up a higher PH I think. Here’s my recently 24hrs urine test result:
        Calcium rate, 24 hr urine: 212 mg/day
        Oxalate rate, 24 hr urine: 46 mg/day
        Uric acid rate, 24 hr urine: 647 mg/day
        Citrate, 24 hr urine : 443 mg/day
        PH, urine: 7.2
        Specimen volume, urine: 2.46 L/day
        Sodium, 24 hr urine: 151 mEq/day
        Phosphate, 24h ur,qn: 792 mg/day
        MAGNESIUM, 24 HR URINE: 90 mg/day
        POTASSIUM, 24 HR URINE: 66 mEq/day
        Creatinine, 24 hr urine: 1617 mg/day
        SULFATE, 24HR URINE: 16 mmol/day
        SUPERSATURATION INDEX WITH RESPECT TO: Brushite (Ca phosphate)
        SUSPECTED PROBLEM IS: Hyperoxaluric Nephrolithiasis
        Any advice will be greatly appreciated.

        • Fredric Coe, MD

          Hi Jim, with your present volume and supersaturation values, the high pH is not in your favor – but I still do not know the real stone analysis. Was it calcium oxalate? If so was there admixed calcium phosphate? How much? One cannot really say what is important in these data unless we know what crystals we want to stop. The high pH is from the potassium citrate, and perhaps your physician might want to balance the value of the increased urine citrate against the higher urine pH in the light of the stone crystals. Frankly, I am suspicious that you formed stones under conditions that differed from those of the present collection. Perhaps volume was lower? Regards, Fred Coe

  5. Megan Kalich

    I’m almost 21, and I have been plagued by brushite stones since I was in elementary school. I’ve had 4 surgeries to remove large stones. The past few days I have passed two small stones, which I believe are pieces of a larger stone. Doctors don’t seem to want to help me, I’ve been dealing with this for so long. I’ve tried every diet tip don’t eat this eat this try not eating this, etc. Nothing works, any advice?

    • Fredric Coe, MD

      Hi Megan, Your perceptions about brushite stones are accurate. This is a nasty kind of stone disease and physicians do not generally see enough cases to be expert about the problem as they are about the more common apatite stones or calcium oxalate stones. Brushite stones are very difficult to fracture with shock waves so most urologists avoid that modality – I gather yours did which is good. The stones can be large and grow rapidly. Plugging of renal ducts is common and the plugs can be large. Prevention is no different from that for other calcium stones – lowering supersaturation, but more urgent as the stones are a menace in your case. Follow the approach so supersaturation is lowered a lot day after day and during the course of every day and new stones will be reduced. The problem is that prior stones may seem like new ones if there is a lot of crystal in the kidneys. This is a matter for your urologist to reckon with, and I am sure he/she will already be aware of the problem. Regards, Fred Coe

  6. muhammad iqbal

    Dear sir,
    it was in the year 1996 when i felt severe pain in the flank area; i visited a surgeon and he advised me using antibiotics. since that time, i have been the victim of stones once in two-year time. Now the doctors say, collect the stones and get them analysed; but the problem is the stones are too minute and i am not able to collect that. some of my tests are as follows; (a) uric acid 5.7 (b) PTH 35 (c) calcium 10.00 (d) calcium and (e) the urine tests often show as normal. the ultrasound results says, concretions. Sir, what would you suggest in order to avoid this havoc.

    • Fredric Coe, MD

      Hi Muhammad, Even if minute the crystals can be caught and analysed. If you urinate through a coffee filter paper in a plastic cone the particles may be caught. No matter how small an analysis can be made. The blood values are normal. Until you know what the crystals are it is hard to use the urine values because the supersaturation of your urine with respect to those crystals you form is too high but right now you do not know the crystals. If you assume they are calcium based crystals try to lower the calcium oxalate and calcium phosphate supersaturations to one half of their present values. Ways to do that are in these articles on calcium and oxalate. Regards, Fred Coe

  7. Michelle Groleu

    A million thanks to you for sharing this information. For two years I told my primary that I thought something was wrong with my kidneys and I thought perhaps kidney stones might be contributing. No advice was given in any way so I assumed there was no harm. Now I realize that all this time I was passing multiple kidney stones. It is extremely difficult to find competent medical people. A urologist instructed me to go to a vegan diet which I did and several weeks later I called to find out lab resuts and I was told the stones had been “calcium”. No elaboration on what type of calcium stone and clearly the vegan diet had greatly increased any urine oxalate levels. I now pass many stones a day. I look forward to finally meeting with a nephrologist in a couple more weeks. Thanks to your articles I will be informed and understand what is being said.

    I can not thank you enough for posting these extremely informative essays. You are helping countless people and I know you will never receive adequate thanks.

  8. Chad R

    Dr. Coe, I would greatly appreciate it if you could contact me. I could really use some help. Thank you

    Chad R


    Sir,Thank for sharing valuable research work.
    I had stones since 2002 now I am 34.Never they hurt me too much even though come out through urine.I had picked up the stones for analysis,but no doctor interested in.recently a stone size 10x6mm blocked in my upper part of the ureter below 6 cm to the pelve…and had successfully come out with out much pain.Sir,I humbly request you to kindly analysis the reason for continuos formation of stones and suggest changes in my life style. Give me the address where shall I send the reports and stones

    • Fredric Coe, MD

      Hi Rakesh, Right now I am unable to provide personal health care for patients who do not come to the university of chicago – my employer. We are trying to begin telemedicine which would enable such but for the moment it is not available. All I can offer is the kind of general advice that is already on the site. It is very important to analyse these stones, and I am sure a laboratory is available to do this – it is the first step to prevention. Regards, Fred Coe

  10. April Suzuki

    Hi…I have had kisney stones ans my analysis report back to me. My brushite number is 9.04 and I have other areas that are concerning also. If I send you my report and can you please help me to prevent as much as I can…I am pregnant ans not wanting to ezperoence these again let alone while Im pregnant.

    • Fredric Coe, MD

      Hi April, I guess you mean the percent brushite in your stone is 9% but I cannot be sure. If you wish, I can surely help you interpret your reports. Please call my secretary at 773 702 1475 to arrange sending. Fred Coe

  11. sandhya

    I need a suggestion for my disease from you how I get that from you. Shall I send my report


Leave a Reply