G6PD deficiency is an X-linked disorder. The presence of anemia will vary depending on the severity of deficiency in the affected cells. WHO classified it according to the magnitude of the enzyme deficiency and the severity of hemolysis.
- Class I – Class I variants have severe enzyme deficiency (eg, <10 percent of normal) associated with chronic hemolytic anemia.
- Class II – Class II variants also have severe enzyme deficiency (<10 percent of normal), but there is usually only intermittent hemolysis, typically on exposure to oxidant stress or drugs.
- Class III – Class III variants have moderate enzyme deficiency (10 to 60 percent of normal) with intermittent hemolysis, typically associated with significant oxidant stress.
- Class IV – Class IV variants have no enzyme deficiency or hemolysis.
- Class V – variants have increased enzyme activity (more than twice normal). They are of no clinical significance.