Paroxysmal nocturnal hemoglobinuria (PNH) is a descriptive term for the clinical manifestation of red blood cell (RBC) breakdown with release of hemoglobin into the urine that is manifested most prominently by dark-colored urine in the morning (see image below. The term “nocturnal” refers to the belief that hemolysis is triggered by acidosis during sleep and activates complement to hemolyze an unprotected and abnormal RBC membrane. However, this observation was later disproved. Hemolysis has been shown to occur throughout the day and is not actually paroxysmal, but the urine concentrated overnight produces the dramatic change in color.
