Evidence of tosufloxacin deposition in the kidneys of a patient presenting with crystal nephropathy.

Tosufloxacin tosilate is classified as a new quinolone antibacterial agent, which has been reported to cause crystal nephropathy. However, the origin of these crystal deposits has not yet been elucidated. We encountered a case of renal failure that progressed slowly owing to crystal-forming interstitial nephritis after long-term exposure to tosufloxacin. Mass spectrometry of the renal specimens revealed that tosufloxacin was deposited in the kidneys. The patient's renal function improved slowly with the withdrawal of tosufloxacin and steroid therapy. This is the first case to demonstrate the presence of crystal deposits consisting of tosufloxacin.