Struvite (magnesium ammonium phosphate) stones form only in the presence of bacteria that produce urease. The common urease-producing bacteria that may populate the urinary tract are proteus, klebsiella, pseudomonas, and enterococci. Alkaline urine results from the urease-mediated splitting of urea and the generation of ammonium. Urine pH above 7.0 normally is associated with very low urine ammonium levels of less than 10 mM per 24 hr. However, urine ammonium levels above 30 mM/24 hr and urine pH above 7.0 virtually make the diagnosis. Other constituents of the stone may include calcium carbonate and brushite (calcium phosphate), which form crystals in the very alkaline urine. Patients who form struvite stones do not pass them spontaneously, but rather are at high risk for bleeding, obstruction, and decreased renal function. Some infection stones begin as calcium oxalate stones that become infected with a urease-producing bacterium. Spread of infection to the contralateral kidney may occur.

Because untreated staghorn calculi will require nephrectomy in 50% of patients, definitive treatment is indicated. Growth of infection stones and their progressive damage to kidney tissue may be limited by ESWL and percutaneous nephrolithotomy, however definitive treatment of struvite stones is surgical removal. Open surgical removal followed by vigorous lavage of the renal pelvis to remove all fragments of the infected stone has reduced recurrence rates from 40% to 2% during seven years of follow-up (54). Extended antibiotic therapy has proven ineffective in irradicating the infection and does not substitute for complete removal of even the smallest particulate of the stone. Acetohydroxamic acid inhibits urease produced by the bacteria and has been shown to be effective in eradicating chronic infection of struvite stones (55). Use of the drug has been limited, however, as it is associated with potentially serious side effects such as hemolytic anemia and venous thromboembolic disease.