The incidence of paroxysmal nocturnal hemoglobinuria cell clones in the Nordic countries.

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired stem cell disorder associated with hemolysis, thrombosis, organ dysfunction and immune mediated bone marrow failure. Studies estimate the incidence of PNH between 1.3-3.1/million/year. However, there are no studies on the incidence of PNH in the Nordic countries. Here we report 126 patients diagnosed with a PNH clone between 2011 and 2016 with an annual incidence of 1.71 cases/million in Denmark, Finland, Norway and Sweden. The number of new cases varied from 15 to 28 yearly with a mean clone size of 23.8% at diagnosis. For the smaller clone sizes < 10% the test indications were predominantly aplastic anemia and unexplained cytopenias, while test indication for larger clone sizes > 50% typically was direct antiglobuline (DAT) negative hemolytic anemia. 23 patients (18.2%) had a history of venous or arterial thrombosis and 30 patients (22.2%) experienced kidney failure. This is the first study to report on the incidence of PNH in the Nordic countries. As expected, the PNH test indications varied according to clone size, while the high number of thromboses and kidney failures underlines the need for knowledge about this rare disease. Cross-country collaboration gives us the opportunity to gather larger patient populations and encourage further studies on PNH in the Nordic countries.