Rupture of some of the red blood cells (hemolysis) in the blood sample may affect test results.

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

The health care provider will usually take blood or red blood-cell folate levels when testing for megaloblastic anemias of any kind.

The blood test for levels of vitamin B12 has become much more accurate within the past few years. Now, there are fewer false-normal results, because the test only measures biologically active B12.

A Schilling test can find the cause of a vitamin B12 deficiency.