Blood test
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Let's schedule some blood work." Most people hear that from their doctor every so often, but what exactly does "blood work" entail? Labs can test a blood sample for a variety of conditions, such as thyroid problems, risk for heart disease, and diabetes. But depending on your age and your health, some blood screenings make a lot of sense, while others can be put off until you're older. We spoke to a number of experts to come up with the right protocol for your 20s, 30s, 40s, and beyond.

In Your 20s:

Cholesterol:
Get your first test at age 20, and repeat it every five years if it's normal. (Retest every year or on your doctor's recommendation if your levels are unhealthy.) Ask for a fasting check of your LDLs (bad cholesterol), HDLs (good cholesterol), and triglycerides (blood fats). "Knowing your total cholesterol isn't enough," says Ann Zerr, MD, director of primary care at Indiana University's National Center of Excellence in Women's Health. "You need to know if your HDL is high enough and your LDL and triglycerides are low enough to keep your risk for heart attack low." 
The healthy target: LDL below 130 milligrams per deciliter (lower if you have diabetes or high risk for heart problems), HDL over 50 mg/dl, triglycerides under 150 mg/dl. [See: How to lower bad cholesterol]

Blood sugar: How well your body metabolizes sugar is a window on your insulin function. Even if you don't have the insulin dysfunction that characterizes full-blown diabetes, merely having insulin resistance can leave you vulnerable to heart disease. You should get your first test at age 20 and repeat annually if you're even a little overweight and have any additional diabetes risk factors, such as high blood pressure or a family history of diabetes. "Although the American Diabetes Association recommends that testing begin at age 45 if you have no diabetes risk, fewer and fewer women are risk-free," notes Janet Pregler, MD, director of the Iris Cantor–UCLA Women's Health Center. That's why she and other experts recommend testing at a younger age. 
The healthy target: Under 100 mg/dl (100 to 125 mg/dl indicates prediabetes, and 126 mg/dl and higher indicates diabetes).

HIV: Get tested at least once if you're sexually active. It's also smart for you and your partner to get checked before starting a sexual relationship, and anytime you suspect you might have been infected. 

In Your 30s:
Along with blood tests for cholesterol, triglycerides, blood sugar, and HIV, consider the following screenings.

Thyroid function: An overactive or underactive thyroid can trigger a laundry list of symptoms, such as weight gain or loss, depression, anxiety, and skin problems such as eczema. Many experts now recommend a blood test for TSH—thyroid-stimulating hormone—for all women at age 35, to be repeated every five years. Doctors also test women who are having difficulty becoming pregnant, have a family history of thyroid problems, or are suspected of having subclinical hypothyroidism (a mildly underactive thyroid). "Signs could include new and unexplained depression, irritability, and anxiety," Pregler says. Set aside a day to relax before you get tested, and let your doctor know if you're taking medications such as aspirin, corticosteroids, or heparin. Being under stress or on one of these drugs can skew your numbers; so can a recent X-ray that used iodine dye. The healthy target: A TSH level of 0.4 to 4.5.