366 CHAPTER 8 Stone disease
Evaluation of the stone former
Determination of stone type and a metabolic evaluation allows identification of the factors that led to stone formation, so advice can be given to prevent future stone formation.
Metabolic evaluation depends, to an extent, on the stone type (see Table 8.2). In many cases a stone is retrieved. Stone type is analyzed by polarizing microscopy, X-ray diffraction, and infrared spectroscopy, rather than by chemical analysis. Where no stone is retrieved, its nature must be inferred from its radiological appearance (e.g., a completely radiolucent stone is likely to be composed of uric acid) or from more detailed metabolic evaluation.
In most patients, multiple factors are involved in the genesis of kidney stones and, as a general guide, the following evaluation is appropriate in most patients.
Risk factors for stone disease
-Diet: Enquire about volume of fluid intake, meat consumption (causes hypercalciuria, high uric acid levels, low urine pH, low urinary citrate), multivitamins (vitamin D increases intestinal calcium absorption), high doses of vitamin C (ascorbic acid causes hyperoxaluria).
-Drugs: Corticosteroids (increase enteric absorption of calcium, leading to hypercalciuria); chemotherapeutic agents (breakdown products of malignant cells leads to hyperuricemia).
-Urinary tract infection: Urease-producing bacteria (Proteus, Klebsiella, Serratia, Enterobacter) predispose to struvite stones.
-Mobility: Low activity levels predispose to bone demineralization and hypercalciuria.
-Systemic disease: gout, primary hyperparathyroidism, sarcoidosis
-Family history: cystinuria, RTA
-Renal anatomy: UPJ, horseshoe kidney, medullary sponge kidney (up to 2% of patients with calcium-containing stones have MSK)
-Previous bowel resection or inflammatory bowel disease causes intestinal hyperoxaluria.
Metabolic evaluation of the stone former
Patients can be categorized as low risk or high risk for subsequent stone formation. High risk includes previous history of a stone, family history of stones, gastrointestinal (GI) disease, gout, chronic UTI, nephrocalcinosis.
Low-risk patient evaluation
Assess urea and electrolytes, CBC (to detect undiagnosed hematological malignancy), serum calcium (corrected for serum albumin), and uric acid, as well as urine culture, and urine dipstick for pH (see below).
High-risk patient evaluation
Evaluation is the same as for low-risk patients plus 24-hour urine for calcium, oxalate, uric acid, and cystine, and evaluation for RTA.
EVALUATION OF THE STONE FORMER 367
Table 8.2 Characteristics of stone types
Stone type |
Urine acidity |
Mean urine pH (SEM) |
Calcium oxalate |
Variable |
6 (± 0.4) |
Calcium phosphate |
Tendency toward alkaline urine |
>5.5 |
Uric acid |
Acid |
5.5 (± 0.4) |
Struvite |
Alkaline* |
— |
Cystine |
Normal (5–7) |
— |
|
|
|
* Urine pH must be above 7.2 for deposition of struvite crystals.
Urine pH
Urine pH in normal individuals shows variation, pH 5–7. After a meal, pH is initially acid because of acid production from metabolism of purines (nucleic acids in, for example, meat). This is followed by an alkaline tide, with pH rising to >6.5.
Urine pH can help establish what type of stone the patient may have (if a stone is not available for analysis) and can help the urologist and patient in determining whether preventative measures are likely to be effective or not.
•pH <6 in a patient with radiolucent stones suggests the presence of uric acid stones.
•pH consistently >5.5 suggests type 1 (distal) RTA (~70% of such patients will form calcium phosphate stones).
Evaluation for RTA
Evaluate for RTA if there are calcium phosphate stones, bilateral stones, nephrocalcinosis, MSK, or hypocitraturia.
-If fasting morning urine pH (i.e., first urine of the day) is >5.5, the patient has complete distal RTA.
-First and second morning urine pH is a useful screening test for detection of incomplete distal RTA; over 90% of cases of RTA have a pH >6 on both specimens. The ammonium chloride loading test involves an oral dose of ammonium chloride (0.1 g/kg; an acid load). If serum pH falls <7.3 or serum bicarbonate falls <16 mmol/L, but urine pH remains >5.5, the patient has incomplete distal RTA.
Diagnostic tests for suspected cystinuria
These include the cyanide-nitroprusside colorimetric test (cystine spot test). If positive, a 24-hour urine collection is done. A 24-hour cystine >250 mg is diagnostic of cystinuria.1
1 Millman S, Strauss AL, Parks JH, Coe FL (1982). Pathogenesis and clinical course of mixed calcium oxalate and uric acid nephrolithiasis. Kidney Int 22:366–370.
368 CHAPTER 8 Stone disease
Kidney stones: presentation and diagnosis
Kidney stones may present with symptoms or be found incidentally during investigation of other problems. Presenting symptoms include pain or hematuria (microscopic or occasionally macroscopic). Struvite staghorn calculi classically present with recurrent UTIs. Malaise, weakness, and loss of appetite can also occur.
Less commonly, struvite stones present with infective complications (pyonephrosis, perinephric abscess, septicemia, xanthogranulomatous pyelonephritis).
Diagnostic tests
-Plain abdominal radiography: Calculi that contain calcium are radiodense. Sulfur-containing stones (cystine) are relatively radiolucent on plain radiography.
-Radiodensity of stones in decreasing order is calcium phosphate > calcium oxalate > struvite (magnesium ammonium phosphate) >> cystine.
-Completely radiolucent stones (e.g., uric acid, triamterene, indinavir) are usually suspected on the basis of the patient’s history and/or urine pH (pH <6: gout; drug history: triamterene, indinavir), and the diagnosis may be confirmed by ultrasound, computed tomographic urography (CTU), or magnetic resonance urography (MRU).
-Renal ultrasound: Its sensitivity for detecting renal calculi is ~95%.1
A combination of plain abdominal radiography and renal ultrasonography is a useful screening test for renal calculi.
-IVP is increasingly being replaced by CTU. IVP is useful for patients with suspected indinavir stones (which are not visible on CT).
-CTU is a very accurate method of diagnosing all but indinavir stones. It allows accurate determination of stone size and location and good definition of pelvicalyceal anatomy.
-MRU cannot visualize stones but is able to demonstrate the presence of hydronephrosis.
1 Haddad MC, Sharif HS, Abomelha ME, et al. (1992) Management of renal colic: redefining the role of the urogram. Radiology 184:35–36.
This page intentionally left blank