MDNG: Which patients should get a kidney stone metabolic workup? Why is this significant? GP: We try to base the need for a comprehensive metabolic evaluation on a patient's risk for kidney stone...
Glenn M. Preminger, MD, Professor of Urologic Surgery, Director of the Comprehensive Kidney Stone Center, Duke University Medical Center
MDNG: Which patients should get a kidney stone metabolic workup? Why is this significant?
GP: We try to base the need for a comprehensive metabolic evaluation on a patient’s risk for kidney stone formation. As a typical stone former is a middle-age, white, male, someone who falls outside of this definition might benefit from a comprehensive metabolic evaluation. In addition, patients who already have formed previous stones, patients with a strong family history for stones, or patients with associated medical illnesses that might increase their risk for stones such as patients with bone disease, GI diseases, or other endocrine problems might also be good candidates to undergo a comprehensive metabolic evaluation.
MDNG: What are the differences between simplified metabolic evaluations and extensive metabolic evaluations?
GP: Basically, we have patients collect two 24-hour urine samples on their regular diet and then, in some cases, a third 24-hour urine sample on a diet that has been limited in sodium and calcium. In some of our patients, we also perform a “fasting calcium test,” in which we have a patient collect a four-hour fasting urinary specimen and then calculate the calcium-to-creatinine ratio. Based on this information, we’re able to design a selective treatment program that directs the medical therapy toward the patient’s underlying metabolic problem.
A simplified evaluation would be one in which only a single 24-hour urine specimen is collected; in some cases, we might not even collect one at all. This is the case in patients with uric acid stones, because we already know that their underlying metabolic diagnosis is going to be gouty diathesis; we could empirically start them on alkaline therapy, usually in the form of potassium citrate, in order to prevent a recurrent uric acid stone.
MDNG: What importance does stone analysis play in the diagnosis and prevention of stone formation?
GP: Stone analysis is oftentimes very helpful in defining the underlying cause of stones. Certain stone compositions would be specific tip-offs to the metabolic etiology. For example, patients with uric acid stones by definition have gouty diathesis as at least one of their underlying abnormalities. If a patient has a calcium phosphate stone, in most cases, we would expect that the patient is likely to have either renal tubular acidosis or hyperparathyroidism, because these are the two most common causes of calcium phosphate stones. However, if a patient has a “run-of-the-mill” calcium oxalate monohydrate or calcium oxalate dihydrate stone, the problem is that there are a myriad of potential underlying causes for stone formation. Since 80% of the patients are going to form calcium oxalate stones, it’s much more difficult to use that stone analysis to define the metabolic etiology; so, in most patients, we definitely would collect 24-hour urine samples.
MDNG: What role does diet play in stone formation, specifically levels of dietary calcium, sodium, and fluid intake, and how can dietary modifications affect stone recurrence?
GP: Diet is a factor that really has not received enough emphasis, but over the past few years there have been a number of studies to suggest that diet should be addressed when the physician sits down with the patient to discuss options for stone prevention. In the past, it was thought that a dietary restriction of calcium would be beneficial for patients with calcium stones; however, over the past 10 years, we’ve seen growing evidence to suggest that a normal calcium intake is the best dietary recommendation that one can provide the patient.
More important than restricting dietary calcium in someone who forms calcium stones is the restriction of salt and red meat protein. In fact, there was a very important prospective, randomized trial performed about two years ago in Italy that suggested a normal-calcium, low-salt, low-protein diet was significantly more effective in preventing recurrent stone disease than was a low-calcium, normal-salt, normal-protein diet. So, in general, our dietary recommendation to our patients is to maintain a normal calcium intake but restrict dietary sodium, restrict red meat protein, and avoid “oxalate gluttony;” that is, stay away from high oxalate-containing foods like spinach, tea, nuts, and chocolate.
Fluid intake is essentially important in preventing recurrent stone formation, and it goes hand-in-hand with medical therapy. Besides taking a certain medication or following dietary recommendations, it’s imperative that a patient maintain a urine output of more than 2,000cc per day. We actually try to have our patients produce a urine output of more than 2,500cc per day if possible. Our general recommendation to patients is that they should take in a minimum of 3,000mL (100 ounces) of fluid. If we can get all our patients to drink 100 ounces of fluid per day, then most of them will make 2,000cc of urine.
MDNG: While dietary calcium has been found to be beneficial for patients dealing with stones, there is also some research pointing out that calcium supplements are actually detrimental for some menopausal patients at risk for developing kidney stones. Can you clarify this concern about menopausal women taking calcium supplements?
GP: Calcium supplements present a potential problem in some women. It is thought that in the majority of postmenopausal women, calcium supplements are probably not a risk factor, because most postmenopausal women already have impaired vitamin D production and, therefore, impaired absorption of calcium from the intestinal tract. It’s the premenopausal women who have a normal intestinal absorption of calcium who might be at a slightly increased risk of stone formation with calcium supplements. What we would recommend is that after three or four months on calcium supplementation, if there’s a concern in that particular woman, a 24-hour urine specimen be performed to assess the urinary calcium excretion. If the patient is hypercalciuric, we would recommend treatment with diuretics. However, if the urinary calcium is normal while on calcium supplementation, we would only recommend a high fluid intake to maintain a urine output of at least 2,500ml per day.
MDNG: On a scale of 1-10, what importance would you place on hydration, diet modification, and medication inthe treatment and prevention of stone formation?
GP: I would say that hydration is probably a 10, medication is probably an 8, and diet modification is probably a 7.
MDNG: Is the resource you developed, “The ABCs of Medical Management of Stones,” a good guide for routine use by urologists? What value does it have for practitioners in other specialties?
GP: I think that “ABCs” is an excellent guide for anyone treating patients with kidney stones. It provides a very straightforward and easy-to-understand definition of the various metabolic causes for stones, offers easy-to-follow methods for evaluating a patient, and reviews very specific instructions on managing the specific metabolic abnormalities that might be uncovered during the evaluation process.
Visit www.abcsofstonedisease.com to download a .PDF copy of “ABC’s of Medical Management of Stones.”