Risk factor assessment

A number of lab studies can aid in assessing a patient’s risk of stone formation.

Urinalysis: Routine urinalysis and culture should be performed in all stone formers and those at risk. Whereas urine pH >7.5 is characteristic of struvite stones, urine pH <5.5 points more to uric acid stones. Various crystals can also be observed on routine urine microscopy: calcium phosphate, calcium oxalate, uric acid, and cystine.

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A 24-hour urine collection is usually recommended in recurrent stone formers and in those with multiple risk factors predisposing to stone formation.

Stone analysis: Stones should be analyzed for composition whenever possible. Presence of struvite stones points to infection as the underlying cause. Calcium phosphate stones are seen in type 1 or distal renal tubular acidosis or even primary hyperparathyroidism. Calcium oxalate stones are common in those with inflammatory bowel diseases, albeit, not exclusively. Uric acid crystals are seen in uric acid nephrolithiasis, whereas cystine crystals are observed in patients with hereditary cystinuria.

Serum chemistries: The results of serum electrolyte studies and a chemistry panel can point to other predisposing diseases. Serum calcium elevations should be checked at least twice, so as not to miss underlying primary hyperparathyroidism. A low serum phosphorus and low plasma bicarbonate could be secondary to type 1 renal tubular acidosis or a chronic diarrheal state, both of which can lead to stone formation.

When prevention fails

Most patients with kidney stones complain of intermittent pain that can be correlated with stone migration. A stone in the area of the renal pelvis, for example, may cause severe, dull to excruciatingly sharp pain in the costovertebral angle, with radiation to the ipsilateral flank and upper abdominal quadrant. As the stone moves to the upper ureter, it may cause a mild to severe deep dull ache in the flank or back. Once the stone reaches the distal ureter, pain radiating to the ipsilateral testicle or labia is described.

Some stones are nonobstructive and don’t produce symptoms. They may be diagnosed incidentally, for example, during radiologic imaging for an unrelated indication.

Aside from actual passage of the stone, hematuria (gross or microscopic) is the single most discriminating predictor of a kidney stone in patients presenting with unilateral flank pain. Note that hematuria is not detected in approximately 10%-30% of patients with documented nephrolithiasis.6

Other symptoms include nausea, vomiting, dysuria, and urgency (Table 2). The last two usually correlate with stone passage through the bladder or urethra. Associated fever may indicate a need for prompt evaluation and management, especially if other signs of septicemia (hypotension, tachycardia, cutaneous vasodilation) are observed.

Imaging studies

Various imaging studies are available. They are used primarily to search for residual stones and assess for response to treatments and recurrence. Non-contrast enhanced spiral CT is now the radiologic imaging modality of choice in suspected nephrolithiasis. It has a specificity of 100%.7

For patients with contraindication to radiation (pregnant patients), renal ultrasonography (US) can detect urinary tract obstruction by showing hydronephrosis. US can also detect radiolucent stones, which are frequently missed on a plain abdominal x-ray. Occasionally, the proximal ureter can be visualized on routine renal US, while the distal ureter can be seen via transvaginal US. Although these studies can determine the size and number of stones, whether the stone is causing obstruction or not will dictate the urgency of diagnosis and treatment.

Abdominal x-rays and intravenous pyelograms have fallen out of favor with the advent of newer imaging modalities.The advantages and limitations of the various modalities are presented in Table 3.