You often hear docs on Grey's Anatomy and House ordering CBCs, DXAs, and other mystery tests (usually followed by "stat!") Here's the lowdown on three your M.D. may not have told you about:

1. CBC (Complete Blood Count)

This blood test screens for anemia, caused by lower-than-normal numbers of oxygen-carrying red blood cells. Unchecked, it can lead to heart failure.

You need it if you have heavy periods, feel extremely tired all the time, or eat a low-iron diet. These are the main causes of iron-deficiency anemia, which mostly affects young women, says Daniel Cosgrove, M.D., medical director of WellMax Center for Preventive Medicine in La Quinta, California.

2. BMD (Bone Mineral Density)

Often called a DXA scan, this low-radiation X-ray assesses your risk of developing osteoporosis and osteopenia. Caused by low levels of calcium and other minerals in your bones, these conditions weaken bones over time, making them vulnerable to fractures.

You need it if you smoke, have a family history of fractures, or have suffered from an eating disorder. Although women typically don't think about osteoporosis until after menopause, if you have low bone density, you can take preventive measures now, says Cosgrove.

3. Measles IgG Antibody (Measles Antibody Test)

This simple blood test can screen for immunity to measles, a contagious virus that can cause pneumonia and encephalitis (brain inflammation). Measles is especially dangerous for pregnant women and immuno-compromised adults. This year outbreaks have occurred in major cities, including Boston and London.

You need it if you were vaccinated before 1989 (you may have received one dose instead of the now-recommended two). Having an up-to-date vaccine makes you less susceptible during outbreaks, says Neal Halsey, M.D., director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health in Baltimore.