Cryptic activity of atypical hemolytic uremic syndrome and eculizumab treatment.

Atypical hemolytic uremic syndrome (aHUS) is a rare, life-threatening disease often related to uncontrolled complement activation. The use of eculizumab has changed the management and the outcome of aHUS, becoming the frontline treatment of the acute disease and for the prevention of relapses. We report the case of a male patient with aHUS due to complement factor H gene mutation who was shifted from plasmatherapy to eculizumab for preventing disease relapses. The shift to eculizumab was associated with a significant decrease in proteinuria, revealing disease activity otherwise unsuspected, being the classic criteria of disease activity (platelet, haptoglobin, LDH, schistocytes), all in the normal range.The condition of proteinuria as the only sign of thrombotic microangiopathy activity is here designated as "cryptic activity of aHUS."