How the test is performed: An angiocatheter will be placed in a vein, typically from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet 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 to allow placement of the angiocatheter. The needle is then removed while the angiocatheter is left in the vein. The tourniquet is then removed to restore circulation.
Methylene blue is then injected into the vein.
How to prepare for the test: No special preparation is required for this test.
How the test will feel: When the needle is inserted, you may feel moderate pain or a stinging sensation. Afterward, there may be some throbbing.
Why the test is performed: 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.
Normal Values: Normally the IV administration of methylene blue rapidly lowers the levels of methemoglobin in the blood.
What abnormal results mean: If the administration of IV methylene blue does not significantly lower blood levels of methemoglobin, then a rare form of hereditary methemoglobinemia is suspected.
What the risks are: Risks associated with an angiocatheter are minor:
- 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
Special considerations: Veins and arteries vary in size from one patient to another and from one side of the body to the other. Inserting an IV may be more difficult for you or your child than for others.
