Renal calculi, or kidney stones, may develop anywhere in the urinary tract. They may be excreted with the urine or become lodged in the urinary tract, causing hematuria, urine retention, renal colic and, possibly, hydronephrosis.

Ranging in size from microscopic to several centimeters, calculi form in the kidneys when mineral salts—principally calcium oxalate or calcium phosphate—collect around a nucleus of bacterial cells, blood clots, or other particles. Other substances involved in calculus formation include uric acid, xanthine, and ammonia.

Renal calculi result from many causes, including hypercalcemia, which may occur with hyperparathyroidism, excessive dietary intake of calcium, prolonged immobility, abnormal urine pH levels, dehydration, hyperuricemia associated with gout, and some hereditary disorders. Most commonly, calculi form as a result of urine stasis stemming from dehydration (which concentrates urine), benign prostatic hyperplasia, neurologic disorders, or urethral strictures.

Testing for the presence of calculi requires careful straining of all of the patient’s urine through a gauze pad or fine-mesh sieve and, at times, quantitative laboratory analysis of questionable specimens. Such testing typically continues until the patient passes the calculi or until surgery, as ordered.