A metabolic evaluation is a series of tests used to identify the underlying causes for the formation of kidney stones. Once this is done, a specific treatment can be selected. The goals of a metabolic evaluation are:
identify treatable metabolic abnormalities
identify underlying medical diseases that predispose to stone formation
outline a treatment plan
Who needs a metabolic evaluation? The type of evaluation and the extent of the evaluation depend on the severity of stone disease, whether the stone episode represents a single occurrence or recurrent stone formation, and whether there is evidence of involvement of more than one body system. The decision to investigate a stone former should be shared by the urologist and the patient. We can gauge the extent of evaluation according to the patient's risk for recurrent stone formation.
As with all urologic diseases, the evaluation starts with talking to the patient. The history obtained should elicit:
the frequency of stone disease
the past medical and surgical history
a list of medications
a family history of stones
a dietary history
whether any previous stones were analyzed to see what minerals and crystals made up the stone
Some patients are at high risk for recurrent stone disease and would benefit greatly from a metabolic evaluation. Strong indications for a metabolic evaluation are:
stones in both kidneys
recurrent kidney stones
family history of kidney stones
children who have kidney stones
black patients with stones
stones in a patient with only one kidney
medical conditions which predispose to kidney stones, such as gout and sarcoidosis
bowel disease (chronic diarrhea, inflammatory bowel disease) or previous bowel surgery
skeletal diseases, such as osteoporosis and pathologic fractures (broken bones in locations weakened by certain diseases)
Other patients are at a lesser risk of stone formation than the above group, but still have an increased risk of forming stones in the future or require a work-up. These patients include:
a first time stone former who is a pilot
a first time stone former who is a white male less than 20 years old or greater than 50 years old
a patient with stones difficult to treat
a first time stone former who is a female
a patient with struvite (infection) stones
A single stone former without risk would fall into the uncomplicated stone disease category. An example of this would be a man who formed a calcium-containing stone with no underlying medical conditions. In this group, a limited metabolic evaluation may be all that is necessary. This would include:
a thorough medical history (see above)
several blood tests to identify certain systemic problems, such as:
primary hyperparathyroidism--elevated serum calcium, decreased serum phosphorus
renal phosphate leak--decreased serum phosphorus
gout--elevated serum uric acid
renal tubular acidosis--low serum carbon dioxide
voided urine specimens for urinalysis and culture
Urine for urinalysis is analyzed for
pH determination--How acidic or alkaline the urinary environment is in the patient. A pH less than 5.5 represents a very acidic urine, and predisposes to uric acid stone formation. A pH greater than 7.5 is very alkaline, and may indicate struvite (infection) stones.
crystals--Certain stones are made up of minerals whose crystals are readily identifiable in the urine. These would include calcium oxalate crystals, uric acid crystals, triple phosphate crystals, and cystine crystals.
Urine is sent for culture to identify any possible infection. Urine infected with certain bacteria (Proteus, Pseudomonas, and Klebsiella bacteria) may suggest struvite (infection stones).
radiologic studies, such as an abdominal x-ray and an intravenous pyelogram
An abdominal x-ray can identify:
residual stones within the urinary tract
nephrocalcinosis (calcifications within the solid portion of the kidney), which is suggestive of renal tubular acidosis
staghorn calculi (stones filling up the inside portion of the kidney where urine collects before it goes down to the bladder), which is suggestive of infection stones
An intravenous pyelogram (IVP)--This is an x-ray of the urinary tract after iodine solution is given intravenously. This test visualizes the kidneys, ureters, and bladder. The IVP can:
identify the exact location of stones within the urinary tract
assess the degree of blockage caused by the stones
outline radiolucent stones (stones not made of calcium that do not show up on a plain abdominal x-ray)
identify anatomic abnormalities that may be responsible for stone formation
stone analysis--All stones should be analyzed to determine the crystals that make up the stone. The crystal make-up of a stone can give clues as to the cause of the stone disease.
uric acid crystals--suggest gout
cystine crystals--suggest cystinuria
carbonate apatite crystals--suggest infection stones
triple phosphate crystals--suggest infection stones
hydroxyapatite crystals--suggest renal tubular acidosis or primary hyperparathyroidism
In patients with recurrent stones as well as patients at increased risk for further stone formation, a more extensive evaluation is indicated. It is possible to determine the cause of stone formation in 97% of patients.
A thorough history may provide clues to the cause of the stone disease.
A history of skeletal fractures and peptic ulcers suggests primary hyperparathyroidism.
Intestinal disease such as chronic diarrhea, Crohn's disease, colitis, or surgical removal of a portion of the ileum may lead to too much oxalate in the urine or too little of the stone inhibitor citrate in the urine resulting in calcium oxalate stones.
Patients with gout can form uric acid stones or calcium oxalate stones.
A history of recurrent urinary tract infections may suggest struvite (infection) stones.
A history of dietary habits, fluid intake, and over-the-counter drug usage is obtained.
Eating too many foods high in salt, calcium, oxalate, and purines could aggravate stone disease.
Inadequate fluid intake leads to a more concentrated urine and makes it easier for stones to form.
Frequent use of calcium-rich antacid tablets can aggravate calcium stones. High doses of Vitamin C can lead to calcium oxalate stones as Vitamin C is converted into oxalate by the body.
Radiologic exams should be performed, such as an abdominal x-ray or intravenous pyelogram, to evaluate the anatomy of the urinary tract and look for residual stones.
Blood samples are drawn for mineral levels and for evaluation of kidney function.
Urine is collected for 24 hours and analyzed for volume, mineral content of the urine, and levels of inhibitor substances that occur naturally in the urine and prevent stone growth.
Depending on the results of this 24 hour urine collection, the patient may need further tests to determine the exact etiology of the stone disease.
The patient is placed on a low calcium, low oxalate, low sodium diet for 7 days. A 24 hour urine collection is then repeated to assess the impact of environmental factors in the stone formation.
If the patient is found to be excreting an abnormally large amount of calcium in the urine (hypercalciuria), a "calcium fast" and "calcium load" test can be performed. These are additional urine tests performed after withholding calcium and then loading up on calcium to determine the exact reason for the large amount of calcium found in the urine.