The RBC G6PD test measures the amount (activity) of the enzyme G6PD in red blood cells.

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleaned with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any continued bleeding.

No special preparation is usually necessary.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is performed when the diagnosis of G6PD deficiency is suspected.

Red blood cells are almost solely dependent on blood glucose as a source of energy. This biochemical pathway is called glycolysis. Red blood cells can also divert glucose into a secondary pathway which results in the production of NADPH from NADP (nicotinamide adenine dinucleotide phosphate).

NADPH plays an important role in maintaining the proper 3-dimensional structure of proteins in the cell membranes and also in maintaining hemoglobin in the reduced state. Oxidized hemoglobin (i.e., met-hemoglobin) is unable to bind oxygen. The biochemical pathway needed for the generation of NADPH requires the enzyme G6PD.

A deficiency of G6PD results in hemolysis (destruction) of red blood cells. There are many kinds of G6PD deficiency, and in most cases an additional environmental factor is necessary to bring about a hemolytic episode.

Hemolytic episodes can be triggered by drugs (sulfonamides, nitrofurantoin, phenacetin, antipyretics, primaquine, quinidine, thiazide diuretics, and tolbutamide), infections, severe stress, or certain foods such as fava beans.

It is important that this test not be performed in the setting of an acute hemolytic episode. Following hemolysis, the cells most likely to have low G6PD levels (older cells) have been destroyed, and those remaining may show normal G6PD levels.

Following recovery from the episode, aged cells will show the decreased levels of G6PD, resulting in a positive test.

8 to 8.6 units/gram of hemoglobin

G6PD deficiency

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.