TEST Vitamin B₁₂
If you never seem to have enough energy, this simple screen may be in order. It measures the amount of vitamin B₁₂ in the blood, which helps maintain healthy nerve cells and red blood cells in the body. "In addition to fatigue, low levels of this nutrient can cause numbness or tingling in the arms and legs, weakness, loss of balance, and anemia," says Lloyd Van Winkle, M.D., a clinical associate professor at the University of Texas Health Science Center in San Antonio.

Over the long run, a vitamin B₁₂ deficiency can raise your risk for depression and dementia. If you're diagnosed with the condition, your doctor can prescribe high-dose supplements in pill, shot, or nasal spray form. She may also test you for pernicious anemia, a disease in which the body is unable to absorb vitamin B₁₂ properly.

Who Needs It
Consider this test if you're a vegetarian, since the only dietary sources of vitamin B₁₂ come from animals. One German study found that 26 percent of vegetarians and 52 percent of vegans had low B₁₂ levels. You should also ask your doctor about the test, which costs $5 to $30 and is covered by insurance plans, if you have any of the symptoms mentioned above.