Medical Articles

Lab Tests For Vitamin B12

Vitamin B12 is an important nutrient for both the production of red blood cells and for the nervous system.

Without enough B12 a condition where red blood cells (RBCs) don't properly mature can develop. The RBCs are larger than normal, but can't carry as much oxygen as they are supposed to. This is called macrocytic anemia.

Tests for macrocytic anemia include MCV (mean corpuscular volume) and MCH (mean corpuscular hemoglobin). If either of these is above normal, then there is likely a B12 or folic acid deficiency. Sometimes the MCV or MCH is at the high end of normal range. For example, MCV may be at 97 with a reference range of 80 - 100. Although technically still normal, this could indicate that B12 or folic acid levels are not optimal and supplementation may still be warranted.

Homocysteine is usually considered a marker for inflammation and cardiovascular disease. Elevated homocysteine can be lowered by taking folic acid or B12 supplements. Therefore homocysteine can also be used as a marker of B12 and folic acid status.

Neurological symptoms of vitamin B12 deficiency include: fatigue, parenthesis (burning, prickling, itching, or tingling skin sensation, with no apparent physical cause), difficulty with balance, dementia and psychiatric problems.

Sometimes B12 deficiency can cause neurological symptoms appear even if MCV, MCH and homocysteine are all normal. Therefore, if vitamin B12 deficiency is suspected other tests may be needed.

Blood can be tested for levels of vitamin B12. This can be useful, but potentially misleading. Some people have B12 levels within normal reference range on lab test, but still not enough to meet their body's unique situation. So while low blood levels of B12 can help diagnose a deficiency, it's not a good test to completely rule out need for supplementation.

This is where methylmalonic acid (MMA) is helpful. MMA is a normal chemical made in the body, which is metabolized by B12. If the body doesn't have enough B12 to meet its needs, less MMA will be metabolized. This raises levels of MMA in the blood and urine. Therefore high MMA can show a need for vitamin B12 even if MCV, MCH, homocysteine and blood serum B12 levels are all normal. This makes MMA is a valuable tool in uncovering hidden b12 deficiency as a cause of neurological symptoms.

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