A test to determine the type of methemoglobinemia, based on the responsiveness of blood methemoglobin to the administration of methylene blue (a dark green powder that can function to turn methemoglobin back into normal hemoglobin).

An angiocatheter (IV) is usually placed in a vein (venipuncture), typically from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and then removed while the angiocatheter is left in the vein. The tourniquet is then removed to restore circulation.

A quantity of methylene blue, based on weight (1-2 mg/kg), is then injected into the vein.

Note: mg/kg = milligrams per kilogram

If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. This test requires temporary placement of an angiocatheter, an IV, and this should be explained to your child. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety they will feel.

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

Methemoglobinemia has several causes, many of which are genetic. This test assists in distinguishing between methemoglobinemia caused by a deficiency of NADH cytochrome b5 reductase, and certain types of hereditary methemoglobinemia caused by hemoglobin M disease.

Most methemoglobinemia quickly responds to IV administration of methylene blue, and can be controlled by daily doses of oral methylene blue and ascorbic acid. However, some types of hereditary methemoglobinemia do not respond to methylene blue. This test allows distinction between the two, and helps your health care provider develop a plan of care.

Normally the IV administration of methylene blue rapidly lowers the levels of methemoglobin in the blood.

If the administration of IV methylene blue does not significantly lower blood levels of methemoglobin, then a rare form of hereditary methemoglobinemia is suspected.

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken, but the chances of infection increase the longer the IV remains in the vein)
• 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 or inserting an IV may be more difficult in you or your child than in others.