Some are useful; others may just lead to more tests and treatments that, at best, waste time and money -- and, at worst, can even be harmful.
In fact, in April, nine major medical groups released a report called Choosing Wisely that urged doctors and patients to think twice before scheduling unnecessary tests.
"Most often a skilled physician, by taking a careful history and physical exam, has a good idea of what is going on," says Dr. Glen Stream, president of the American Academy of Family Physicians. But, in certain situations, he adds, testing can help confirm a tricky diagnosis or determine the course of treatment.
Read on for the symptoms that should trigger a test from your doctor -- plus, a guide to cancer screening, routine checkups, and other tests you truly need.
If your symptoms include.. pain or burning during urination, funky-smelling or cloudy urine, or pelvic pain
You should get... a test for a urinary tract infection (UTI)
A simple urine-dip test in the doctor's office can confirm infection, but a more extensive urine culture is often required to determine exactly which bacteria are causing the problem.
"Different bacteria are resistant to certain antibiotics, so make sure your doctor does the extra test to help guide him to the correct treatment," Stream says.
If your symptoms include... light-headedness, pale-looking skin, and fatigue, especially if you also have heavy bleeding during your period
You should get... a blood test for anemia (low red blood cells)
Women of reproductive age are especially prone to iron-deficiency anemia, often due to blood loss from menstruation, Stream says.
By checking your blood count, your doctor can assess if the condition is serious enough to warrant treatment, which could include anything from taking supplements to managing your period with hormonal birth control, or, if you have an extremely heavy menstrual flow and aren't planning on becoming pregnant, endometrial ablation (scraping of the uterine lining).
If your symptoms include... pain or tenderness in your abdomen
You should get... a pelvic ultrasound
While doctors are generally cautious about referring patients for expensive imaging tests, an ultrasound is the best way to check for problems such as uterine fibroids, ectopic pregnancy, or ovarian cysts.
"An ultrasound is noninvasive, not too costly, and has few side effects," says Dr. David W. Lee, a family physician in Lorton, Virginia.
If your symptoms include... rash, fatigue, chills, or unexplained muscle and joint pain
You should get... a blood test for Lyme disease
Yes, you can have Lyme even if you don't get the characteristic bull's-eye rash (20 to 30% of those infected don't). The disease -- spread by tick bites -- has symptoms that can overlap with other conditions, such as rheumatoid arthritis, so testing is important to confirm the diagnosis, explains Dr. Deborah Horwitz, a doctor of internal medicine in Bethesda, Maryland.
However, the earlier Lyme is treated, the better (and leaving it untreated can lead to serious complications), Horwitz adds. So if you've been bitten by a tick and come in with classic symptoms, your doctor should start treating you right away (typically with a 14- to 21-day course of antibiotics), even before the test result comes back.
If your symptoms include... fatigue, constipation, unexplained weight gain or loss, or unusually dry or unusually oily skin
You should get... a thyroid function blood test
"Women are more prone to diseases of the thyroid, which controls metabolism," Stream says. If you're in your 40s or 50s and show up at the doctor's office complaining of fatigue and weight gain or loss, thyroid problems should be at the top of his or her list of possible diagnoses, Lee says.
A blood test to measure your levels of thyroid-stimulating hormone (TSH, which is produced by the pituitary gland) and the thyroid hormones T3 and T4 can confirm whether you have an underactive thyroid (hypothyroidism), which typically leads to sluggishness and other side effects, or an overactive thyroid (hyperthyroidism), which can lead to weight loss, palpitations, and other effects of overstimulation.
If your symptoms include... fever, sore throat, difficulty swallowing
You should get... a strep test
"Even the most experienced doctors often have trouble distinguishing strep throat, which is a bacterial infection, from many viral infections that look identical," Stream says.
A rapid test with a throat swab can be done in your doctor's office in minutes, but the test misses about 5% of strep infections, so you should request that she also send a throat culture to a lab just to be sure.