Vitamin B12 is a nutrient that helps keep your nerves and blood cells healthy and supports the production of DNA and red blood cells. It also plays a key role in energy metabolism and brain function. This test measures how much vitamin B12 is circulating in your blood. Low vitamin B12 levels can lead to anemia, fatigue, numbness or tingling, memory issues, and mood changes, sometimes even before levels fall below the standard “normal” range. Very high levels are less common and can sometimes be seen with supplementation or certain health conditions. Keeping B12 in an optimal range helps support long-term nerve health, energy, and cognitive function.