G6PD and hyperinsulinemic hypoglycemia, familial, 4: The WHO classifies G6PDD-causing variants into five classes: class I is the most severe causing chronic non-spherocytic hemolytic anemia (G6PD activity < 1%), class II is also considered severe and associates with acute hemolytic anemia (G6PD activity < 10%), class III is considered as moderate deficiency and it is associated with occasional acute hemolytic anemia (G6PD activity of 10–60%), while classes IV and V are asymptomatic (G6PD activity of 60–150% and > 150%, respectively) [5].