Abstract
The patient was a woman in her 60s. She was found to have proteinuria on a health checkup. She did not have any particular subjective symptoms, and no definitive diagnosis was made, despite serological findings indicative of immune abnormalities. A renal biopsy was performed. Light microscopy of renal tissue section revealed mesangial proliferative nephritis. Electron microscopic findings included electron-dense deposits and fibrillar/tubular structures with a diameter of 20-30 nm. These findings suggested the presence of cryoglobulin (CG), but CG was not detected in qualitative or quantitative hematologic tests. Thus, the serum samples were stored at 37°C for a long period of time and then cooled to 4°C. When the obtained precipitates were examined, CG was successfully detected. CG that precipitates only after a long period of time is referred to as slow cryoglobulin (sCG), and sCG is extremely rare. The present case is the first documented case, to our knowledge, of renal disorders caused by sCG. It should be noted that there are some cases in which it takes much time for CG to precipitate. Thus, when CG cannot be detected, it is necessary to spend much time to determine whether CG precipitates.