G6PD

The range depends heavily on the laboratory method (some use 6.4 to 15.6 U/g Hb); a high reticulocyte count can push the result into the range even when the enzyme is deficient; use the range your lab printed.

G6PD (glucose-6-phosphate dehydrogenase) is an enzyme inside red blood cells that protects them from chemical damage. Some people inherit a version that works less well, which is common in people whose families come from Africa, the Mediterranean, the Middle East and South Asia. The test measures how active the enzyme is, adjusted for the amount of haemoglobin in the sample.

Doctors request it before starting certain medicines that can break down red cells in people with low G6PD, when looking into episodes of red cell breakdown or jaundice, and when a family member is known to carry the condition.

• A high reticulocyte count, because young red cells have more of the enzyme
• Recent blood loss or red cell breakdown, for the same reason
• A recent blood transfusion, because the donor cells carry normal enzyme

• Inherited G6PD deficiency
• Older red cells, since the enzyme declines as the cell ages

• A test taken during or shortly after an episode of red cell breakdown can read falsely normal, because the deficient cells have already gone
• A recent transfusion masks a deficiency
• Different laboratory methods give different numbers, so results are not comparable between laboratories
• Women who carry one copy of the gene can have a result anywhere in or below the range

No fasting needed. Best measured when well and at least three months after any transfusion or episode of red cell breakdown.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.