The aims of stone prevention are to reduce the number of new stones formed, and to reduce the growth of stones in kidneys by lowering supersaturation. This means we gauge our success by counting and measuring stones.

Mostly, radiologists measure and we count.

That is why I wrote this article. Ultimately, physicians are responsible for counting, but patients can help a lot.

Moneylender and his Wife (Quentin Massys, 1465 – 1530)painted in 1514, hangs in the Musee du Louvre, Paris. Massys was ‘…the first important painter of the Antwerp school.’

How To Count New Stones

New vs. Pre-existing

A stone passed or removed or found in an image, and not present on a prior image, is a new stone. Therefore all such stones prior to the first available image are new. Subsequent ones must be counted against changes in the numbers of stones on subsequent images and counted as new or preexisting.

In patients with a massive stone burden, one may not be able to tell if a stone is new. I always assume it is, and work to lower supersaturations as much as possible.

What Does Active Mean?

Stone activity means new stones are forming or preexistent stones are growing. Obviously, any new stones means at some time or other the stone disease was active. But we use stone counts and growth to determine the need for and achievements of prevention efforts. So active or not is always placed in time, of which I can distinguish three periods: Remote, pre-treatment, and during treatment.

Remote Active Disease

Remote means we no longer believe prevention worthwhile. It is subtle. For example, decades ago seems reasonable, but I have no proof. A decade is perhaps reasonable. A few years ago is not remote. So this is pure judgment based on what the patient thinks, how severe the prior stones were, whether we have excellent reasons to believe we know why they occurred, and what has changed in the years between the last new stone.

Pre-treatment Active Disease

New stones have been forming in recent years, recent enough patients and their physicians believe prevention warranted. This level of activity is what we are treating against. We aim to reduce new stones below pre-treatment, ideally to 0.

In-treatment Active Disease

This is a reason to change things.

How to Quantify New Stone Activity

Stone Formation Rate

Stone formation rate is the number of new stones formed / years from last count or from the first stone(s) formed.

Both are of value.

The latter is like an overall average. The former tells about more recent changes, especially during treatment.

Because we cannot know when the first stone(s) formed, we cannot calculate from it a stone formation rate.

Inter-event Interval

This is the time between new stones, which means one requires successive images.

For example, if 1 new stone appears between successive CT scans, or if 10 appear, the measurable interval is the same. Likewise, if 1 or 10 new stones are passed or removed at a single ER visit or surgery and new images show one or more of these were new, it is still one event. So stone counts and inter-stone intervals are not necessarily the same.

If an event of stone passage or removal has no new stones in it, the event is not a new stone event and it does not count. The inter-event interval remains the time since the last new stone event.


Example 1. A women passed her first stone 10 years ago; her CT showed no stones in either kidney. She passed another stone 3 years ago, which had to be new, giving 1 new stone in 7 years. Her second CT 2 years ago showed 4 stones, 2 on each side. She has had 5 new stones in 8 years. A routine follow up CT one week ago showed 3 stones on the left and 2 on the right, meaning she had added one additional new stone, making a total of 6 new stone in 10 years, with a lifetime total of 7 stones. The bolding of ‘new’ is to emphasize that the first stone is of unknown duration, whereas the ones thereafter are new within a known period, of 10 years. She has obvious active and accelerating disease.

Example 2. A man passed his first stone 10 years ago, and his CT showed 4 stones, 2 on each side. He has formed 5 new stones, but when he formed them is unknowable. One year ago he passed two stones in a single event. His new CT shows 2 stones on the left, none on the right, and his pain was on the right. We presume he passed two old stones. New stone rate, 0/10 years. Since the event was null, without new stones, he has an inter-stone interval of >10 years. Because he has a stone burden, we would treat to reduce or prevent stone growth.

Example 3. A woman formed her first stone 1 year ago, and her CT showed no stones in either kidney. She has had no more stone events and a new CT shows no stones in either kidney. Her new stone rate is 0, her inter-stone interval is >1 year. Being a single stone former, we will choose prevention given reasonable information that untreated she has a significant risk of more stones.

Example 4. A woman with massive bilateral nephrocalcinosis had episodes of stone passage monthly, on average. One cannot detect if the stone burden is changing. Because we cannot be sure, we err on the side of ‘best for patient’ which means assume stones are  forming – on old stones, perhaps – and therefore that these are indeed new stone events. Reduction of supersaturation is the only reasonable approach, coupled, perhaps, with surgery to reduce stone burden and therefore morbidity and rates of surgery for stone passage. Her rate is 12 stones/year, inter-event interval 1 month.

Side of Passage and Counting

Example 2 illustrates the importance of records or clinical history. Suppose he had passed his newest stone from the left, not the right. Then, it would have been a new stone as the number of stones on the left remained at 2 before and after stone passage. Patients can help a lot here by keeping careful track of which side pain was on, and from which side stones were removed.

Stones During Treatment

In each of the four examples, stone counting has given us a baseline against which to evaluate the effectiveness of treatment. In example 1 we need to lower stone rate below 1/7 years – her baseline lowest rate -, and lengthen interstone interval above her baseline of 7 years. For example 2, we have little activity, and will treat to prevent new stone growth – not a matter we detail here. Example 3 should not form new stones, ideally, for many years, given only 12% relapse at 5 years of mere increase in fluid intake. Example 4 has very frequent stone events, perhaps new, and we would hope to lengthen the interval between events from the baseline of one month.

How To Count Stones in Kidneys

From everything I have just written, it is apparent that new stone counting depends utterly on the images we have and how well we read them.

Kinds of Images

Nothing Compares to CT Scans

I am devoted to CT images for stone work. Modern low dose instruments much reduce risk by reducing radiation exposure to below 3 mSv. Nothing comes close to a CT scan for counting stones. So whenever possible I get them and use them.

Flat Plates are Mediocre.

Simple flat plates are very hard to use. The bowel overlays the kidneys, stones show as faint white spots, or not. Ribs can be in the way. I squint at the image and try to figure out where, and if, stones are. If one is highly experienced and careful, they are of modest sensitivity. I say this after having read thousands of these images over decades of practice. A recent review gives a sensitivity of 57% vs. 95% for low dose CT, and a specificity of 76% vs. 98% for low dose CT. This means that nearly 1/2 of stones will be missed.

Renal Ultrasound

In the most expert hands ultrasonography has a sensitivity of 84% vs. 95% for low dose CT, and a specificity of 53% vs. 98% for low dose CT. This means that ultrasound can misclassify nearly 1/2 of suspicious objects as stones.

Errors From Changing Image Types

Imagine the first image for a patient is a flat plate showing 1 stone on the right. Several years later, a stone is passed from the right and a CT shows 2 stones on the right and 3 more on the left. What do we make of this? New stones or change of technique?

To me these are not new stones because a flat plate has poor sensitivity compared to a CT. But we cannot be sure, can we?

Imagine another patient with 3 stones in the right kidney seen on CT. After ureteroscopic removal of 3 stones, a flat plate shows no stones. One year later, a new CT shows 2 stones on the operated side. One presumes they are new, but the flat plate could be insensitive and the surgeon unable to see several stones during the surgery. Or, possibly, the ‘stones’ are tissue calcifications not seen during surgery. With ultra low dose instruments, perhaps a majority of such images will be CTs.

How I Record The Count

I list the number of stones in each kidney vs. date of the images and the image type. I draw the stones in on little kidney outlines so I have a record of what I could see at the time.

Value of Surgical Observations

During PERC or ureteroscopy surgeons can estimate numbers of ‘stones’ that are actually tissue calcifications. These will not pass, cannot be removed in most cases, and need to be subtracted from the stone count.

Renal Colic and Hematuria Without Passage or Removal

Quite apart from conventional new stone counting, we have patients who have attacks of pain with hematuria due to either stones too tiny or fleeting to visualize, or to passage of crystals. For them, activity is simply the numbers and spacing of attacks. But the larger problem is to identify these attacks as against the large complex of pain syndromes found among stone formers.

The key is renal colic, a peculiar pain produced by stone passage.

Colic is unique enough that we can use it as evidence for passage of a stone, or of crystals, even if we fail to find the stone or crystals by direct visualization. The details of the pain are discussed a bit later in this article, but right now I want to use it, so assume I will shortly tell you how to identify it.

Small Stones or Late or Insensitive Visualization

Not rarely, an attack of typical colic with hematuria is unaccompanied by a stone. Perhaps it passed unnoticed. Perhaps it was small and only ultrasound or flat plate available. Often, one finds on CT ‘stranding’ around the kidney on the same side as pain, indicating a recent obstruction, or even dilation of the renal collecting system. To me all of these are stone attacks until proven otherwise.

Crystal Passage Attacks

An attack of renal colic with hematuria and no obvious stone or obstruction or stranding is, to me, likely the passage of crystals, and I encourage physicians to look for them. Crystalluria/hematuria is very well known among children with idiopathic hypercalciuria and treated with stone prevention measures to lower supersaturation. I have seen it among adults but never published the cases.

Example 5. An 18 year old man had 3 attacks of renal colic with hematuria during the past 2 years. A single urological investigation shows no reason for bleeding, and a CT shows no stones. Calcium oxalate crystals have been found on one occasion. His second attack was 6 months after his first, his third was 8 months later.

Crystalluria/hematuria attacks should be investigated as to cause and then treated. Such attacks are disturbing and disruptive to the patient, even if no stones can be found. Example 5 has an inter-event interval averaging 7 months. Successful treatment would ideally bring these attacks to a halt or at least increase the interval markedly.

Special Problems

Crystal attacks not rarely reflect drugs, especially antivirals. Their prevention concerns drug management, not stone prevention, and is beyond this site at present. One diagnoses the drug crystal attacks by knowing about the drug and demonstrating the crystals.

Sometimes, bleeding from a tumor or other cause produces clots that pass and cause renal colic. For this reason we always need urological assessment to be sure bleeding has no obvious cause. Uncommon ‘matrix’ stones composed of only urine protein can form and pass causing colic. I always advise these patients to void through filters in hopes of tiny stones, matrix stones, even masses of crystals.

Specifics of Renal Colic

I promised to tell about the special properties of renal colic, and here they are. Mike Borofsky did a very fine article on this. I have only to elaborate from a more clinical and descriptive viewpoint.

I Assume Experienced Patients Can Tell

If my patient has had at least one certain episode of stone passage with pain, I accept their opinion about subsequent episodes of pain as being from a stone or not. The pain of stone passage is odd. It has a foreign, peculiar character. Patients recognize it, and I trust them to know.

When patients are not sure, from inexperience or otherwise, the traits below are very useful. I have found them so for all of the thousands of instances during which I struggled to sort out stones from everything else. 


The pain begins suddenly. Often it is not so severe as startling, and ominous.

I think the onset is sudden because pain arises from acute obstruction of a kidney. It is ominous because peculiar enough patients recognize it for what it is and experience immediate dread.

I should say, this pain is indifferent to its cause. A blood clot or sloughed renal papilla will feel the same as a stone. It is the acute obstruction that causes the pain.

Location, Course, Associations, Severity

Certainly at the beginning, the pain lies vaguely in one flank or the other most of the time. But I have examples of a midline upper abdominal beginning that only lateralized to one flank over an hour or so and was initially thought a myocardial infarction or gall stone. As it becomes more severe, the pain will eventually find its way to one side, unless stones are passing from both – a dreary and worrisome event.

Pain rises to a plateau of severity over some minutes to a few hours as the obstruction leads to increasing dilation of the urinary system with urine. It is indifferent to movement – bending, turning, walking, lying still. Being visceral, arising from an internal organ, it has a vague flank location and myriad internal reactions – nausea, vomiting, diarrhea, sweating, and changes in pulse rate or blood pressure.

Severity is famous, and terrible.


Among my patients have been poets, significant writers, passionate athletes well acquainted with their bodies, and to them as to most I have posed the question of character – what the pain feels like. All agree it is odd, unmistakable once you have known it, and the oddness has nothing to do with its severity as the pain seems odd even when mild.

Some call it burning, some boring – like a drill. Some combine the two – a hot piercing object. Many just stare, unable to find similes, but say it is like no other pain – strange, foreign, alien, and altogether evil.

Radiation and Movement with Stones

In men, stone pain can radiate to the testicle on the same side as the stone, in women to the labia.

If the obstructing stone moves down the ureter, pain usually follows it downward, along the path your fingers might make if spread out over the abdomen, the thumb hooked just below the lowest rib.

Magical Disappearance

Most important, if obstruction ceases – the stone passes, ceases to obstruct, for example – pain ends abruptly, magically. No pain of such consequence in all of clinical medicine has this property except if from a stone. I have observed it in retained biliary and common duct stones as perhaps my only other examples.

To detect this trait, be careful of what you ask for. Soreness and a general sense of ill dissipate over an hour or more, but the real pain – the one patients know as evil – that pain ceases with surprise, a sense of amazement. Even if pain meds have been used, this odd ending is evident.

Pain with Chronic Obstruction

If a stone fails to pass and obstructs for days, pain may get better. It does not so much magically disappear as fade. If under surveillance, physicians will tend to the problem. Sometimes, colic will gradually subside, no images made, and things simply go by with missed obstruction. This can destroy a kidney. So, if pain seems like a stone, one needs a physician to oversee matters. They will do images – perhaps ultrasound – and keep things safe.

Confused with Other Pains

I am rarely confused, nor are patients. Renal colic, gallstone passage, acute pancreatitis, and myocardial infarction can seem alike at the beginning. So physicians and patients need be careful. But in all my decades of work, colic had more or less always been stone passage.

If lodged at the junction of the bladder and ureter, a stone mimics infection – urinary frequency, urgency, burning. This is not typical stone pain, but may well lead to an image that discloses the stone.

Back Pains of Other Sorts

Back pain that is not renal colic is not a clue to a stone event per se but often to pathology caused by stone burden, stone events, surgery, and infection.

Stones in Kidneys

We know that many people who have significant stone burdens have back pain that is not typical of stone passage but severe and often disabling. Most of us believe it is from the stones but we lack trials to prove surgery helps. For this not uncommon situation we need trials to determine what characteristics of pain or stones justify surgery. Stone prevention efforts may be worthwhile to reduce new stones that could possibly worsen matters. 

If pain is on the same side as a kidney with even one stone, questions arise as to be benefits of surgery. Lacking trials, this becomes a difficult balancing of risk to benefit.

No Stones in Kidneys

If there are no stones in either kidney pain is not presently due to stones even if it is due to spasms and inflammation from prior stones.

But not rarely, pain altogether unrelated to stones, because in the flank, or back, or groin associates itself with stone pain in the minds of patients and physicians, and misleads. Here, physical medicine experts can help. Likewise, this maxim: If the pain responds to posture, movement, exercise, it may well represent long term effects of stone events on the back muscles but not pain from the immediate effects of stones themselves.

Events vs. Stones

One cannot help but see by now that events and stones are not the same. Events are when we have a chance to count and there is indeed at least one new stone. Counts are what we make at each event. So an event can have anywhere from one to many new stones.

Here are some of my own results. Joan Parks and I used all events and stones from 371 men to make this picture.

On the horizontal axis are the number of events for each man (shown as a single circle). On the vertical axis are the number of counted stones for all events that patient had.

To make each point visible, I jittered them randomly. That is why some lie just outside the graph borders.

One event, one or more stones

For example, many men clustered at the bottom left corner each had one event with one total stone. But many had one event and 10 or more stones. One man had 100 stones in his one event (upper left corner of the graph).

Two events, two or more stones

Another large group of men had 2 events each. They are centered above the second tick mark from the lower left corner. Many of these men had only 2 stones, one in each event. They form the ball of points. But a large number had many more than 2 stones. Once again several had 100 stones in their 2 events.

Three or four events

Given our limited number of cases, we could identify obvious clusters only for 3 and 4 events, easily seen on the graph. As in the other events, many men had equal event and new stone counts, many had more than one stone on average per event.

The Contour lines

The overlay of contour lines gives a sense of how many points are in an event space, as if they were showing the heights of hills viewed from directly above. The 1 and 2 event spaces have separate contours, meaning they are truly separate. The 3 and 4 are bounded by a single contour meaning that as the numbers become smaller these mounds are more like little hills filled in between.

What This Means

It means the stone counts can be astronomical but clustered in a few events. It can also mean that many events yield only few total stones. The two are not closely related. Of the two, the sturdier is event number. How many stones in an event was determined here because for decades we had the luxury of time to question our patients and pursue old records to get stone counts in an event.

Events vs. Time

The Longer You Wait, the More Events

From the same study, we were able to calculate the event rate (number/year) and the number of events in relation to the duration of

observation from first stone. On the graph, number of events is on the horizontal axis. Stars show the event rate, plotted on the vertical axis. Circles show the time of observation from the first stone, in years, also plotted on the vertical axis.

For example, one event had a mean observation time of 2.5 years from first stone, and a rate of 2.5 events/year.

There is no way you can miss the meaning of this graph. The more years that have elapsed from the first stone, the more events. This is what it means to wait once stones have begun. The common reward is more stone events, meaning more stones.

Event Rate Does Not Change

Because the number of events rises more or less with the time patients are observed, the event rate/year – plotted here – is more or less constant at a mean of about 1 – 1.5 event/year.

You might see an apparent error on this graph. For example, with 2 events and a mean observation time of about 7 years, one expects events/year to be 2/7 events per year, not about 1. But the mean of the ratio can depart greatly from the ratio of means, as is the case here because the means arise from a large distribution of observation times skewed left – downward. So many values would have been 2 events in a lot less than 7 years.

Events vs. Relapse

Everything in the figures arose from before treatment. But we then treated everyone as best we could and observed if relapse – new stones – occurred. We found something very important.

The vertical axis shows the fraction of patients free of relapse vs. duration of treatment (yrs) on the horizontal axis. Those with 1 event before treatment (circles) did best; at 10 years over 80% were free of new stones. Those with 2 events (triangles) did less well, and those with 3 or more events did worst of all (black squares) with less than 60% stone free.

Of course, though prospective, these are observations not a trial. Moreover, the patients were all men, as we lacked sufficient women, then, to make our distribution graphs.

But using only trial data, I have shown the very same phenomenon. During all of the randomized controlled trials we have had in stone disease, the percent of patients who relapsed during treatment with active drug rose with the number of stones formed prior to the trial. In other words, waiting treatment for more events means treatment gives a poorer outcome.

Avoid Confusion: Count

Confusion leads careless shopkeepers into bankruptcy, and distracted pilots into the sides of mountains. Counting tells us who to treat, and counting tells us how well we are doing.

All we do relies on counting well.

Good Prevention Can Look Bad

To count stones new when they are not makes treatment folly.

Like a mariner lost for want of proper charting, we despair of successful prevention which, present all the while, we cannot recognize because we counted wrong. And we, in consequent, regardless flailings, may undo what was a simple triumph.

Bad Prevention Can Look Good

To count stones old when they are new creates delusion.

Caught up in a fantasy of achievement, physician and patient await, unknowing, what nature will present them – in the act. What could have been and what will be are not a joyful pairing.

Pain Does Not Assure Active Stone Disease

Chronic pain is serious, important. It needs to be understood and treated. But pain need not mean active new stones are forming. Stone burden may cause pain that stone prevention may not abolish. As I have said, we lack trials to know if surgery benefits it.

Chronic pain without stones in either kidney may have nothing to do with stone disease. It requires astute evaluation on an individual basis. It requires treatment, and is a very important matter. But it need not denote active stone disease.

Renal Colic is a Good Indicator of Stones

On the other hand, renal colic is so distinctive it marks some kind of stone or crystal activity. While colic does not have the certainty of a passed or removed stone, it denotes a need for treatment – crystals, bleeding from some non stone related cause, tiny stones we are not imaging.

What Patients Can Do

Keep track.

When were stones passed or removed? How many stones did the radiologist report on each side and what kind of image was it? Did you fall into the trap of changing image sensitivity – flat plate after surgery shows no stones, CT one year later shows 5, and who knows what really happened?

This is not to suggest patients do without physicians. Physicians count new stones and read images more skillfully than patients can – they are trained to do it. But their time is limited, and most lack support people to help keep track of dates and events. They must do it all in the brief minutes allowed for a modern clinical visit.

Careful patients, who have their information at hand and good reasons to devote considerable time to the matter, can help immeasurably.

True, they must be very careful and accurate, but most should be so. We calculate our taxes, we make our budgets. We do a lot of dated quantitative counting work. What could be more important than to help clarify the state of our own disease?

I want to thank Dr John Asplin for reading this article and pointing out important confusions in the examples which I believe I have corrected. His help has been and is indispensable to me in running this site. 

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