Why your doctor orders blood tests and what they show

Blood tests are one of the most common ways doctors catch health problems early, before you feel sick. A single vial of blood can reveal whether your heart is working well, if your kidneys are filtering waste, whether you have diabetes, and dozens of other things. For people over 65, regular blood work often catches problems when they are easiest to treat.

Your doctor orders blood tests for two main reasons: to check on a health problem you already know about, or to screen for problems you may not know you have. A screening test looks for disease in people who have no symptoms. A diagnostic test confirms or rules out a specific condition your doctor suspects based on how you feel.

The results come back as numbers and measurements. Your doctor compares them to a normal range. If your result falls outside that range, it does not always mean something is wrong — ranges vary by age, sex, and lab — but it signals that your doctor should look closer.

Key Takeaways

  • Blood tests can detect diabetes, heart disease, kidney problems, anemia, and infections before you have symptoms.
  • Medicare covers certain screening blood tests at no cost to you, including cholesterol panels and diabetes screening for people at risk.
  • You do not need to fast before most blood tests, but your doctor will tell you if fasting is required for yours.
  • Results usually come back within a few days, and your doctor's office will contact you to discuss what they mean.
  • Bring a list of all medications and supplements you take, because some can affect blood test results.

Common blood tests for older adults

A complete blood count (CBC) measures red blood cells, white blood cells, and platelets. It shows whether you have anemia (too few red blood cells), infection, or blood clotting problems. Doctors often order this as a baseline or when you report fatigue or frequent infections.

A comprehensive metabolic panel (CMP) checks kidney function, liver function, blood sugar, and electrolytes like sodium and potassium. This test is especially important for people taking blood pressure medications or diabetes drugs, because those medications can affect kidney and liver function over time.

A lipid panel measures cholesterol and triglycerides — fats in your blood that contribute to heart disease. It breaks down total cholesterol, LDL (the kind that clogs arteries), HDL (the protective kind), and triglycerides. Your doctor uses this to decide whether you need medication or lifestyle changes to lower your heart disease risk.

A hemoglobin A1C test shows your average blood sugar over the past three months. It screens for diabetes and prediabetes, and monitors control if you already have diabetes. This test does not require fasting.

A thyroid-stimulating hormone (TSH) test checks whether your thyroid is working properly. An underactive thyroid is common in older adults and can cause fatigue, weight gain, and depression. An overactive thyroid can cause heart problems.

What Medicare covers for blood screening

Medicare Part B covers several preventive blood tests at no cost to you — meaning no copay, coinsurance, or deductible — if you see a doctor who accepts Medicare assignment. The tests must be ordered as screening, not diagnosis, and your doctor must order them as part of a preventive visit.

Cardiovascular screening includes a lipid panel once every five years for all adults 65 and older. If you have risk factors like diabetes or high blood pressure, your doctor may order it more often.

Diabetes screening is covered once per year if you have high blood pressure, or once every three years if you do not. The test is usually a fasting glucose or A1C.

Colorectal cancer screening sometimes includes a blood test called a fecal immunochemical test (FIT), though colonoscopy is more common. Medicare covers this screening through age 85.

Other tests — like a CBC, thyroid panel, or kidney function test — are covered when medically necessary, meaning your doctor has a reason to order them based on your symptoms or medical history. These are not free preventive screenings, so you may owe a copay or coinsurance depending on your plan.

If you have a Medigap or Medicare Advantage plan, your coverage may differ. Call your plan to ask what blood tests are covered before your appointment.

How to prepare for a blood test

Most blood tests require no special preparation. You can eat and drink normally, take your medications, and go about your day. Your doctor's office will tell you if fasting is required — usually for a lipid panel or fasting glucose test, which means no food or drink except water for 8 to 12 hours before the test.

Wear loose, short-sleeved clothing so the person drawing blood can access your arm easily. Bring your insurance card and photo ID. If you take blood thinners like warfarin or apixaban, tell the person drawing blood before they start, because some tests require special handling.

Drink extra water the day before and the morning of your test. Staying hydrated makes your veins easier to find and reduces the chance the technician will need to stick you more than once.

Bring a written list of all medications and supplements you take. Some can affect test results — for example, vitamin K supplements can interfere with blood clotting tests, and some antibiotics can affect kidney function readings. Your doctor needs to know what you are taking to interpret your results correctly.

Understanding your results

Blood test results come back as numbers with a "normal range" or "reference range" printed next to them. That range is based on what is typical for the general population, but it is not a hard line between healthy and sick. Your result may fall outside the range and still be normal for you, especially if you have a chronic condition.

Your doctor will call you or have your results available in your patient portal. Do not assume no news is good news — ask your doctor to explain what each number means and whether any results need follow-up. If a result is abnormal, your doctor will tell you whether it needs treatment, another test, or just monitoring.

Keep copies of your results. Over time, your doctor looks for trends — a cholesterol level that is creeping up, or kidney function that is slowly declining. One abnormal result may mean nothing; a pattern means something.

Blood tests and medication side effects

If you take medications regularly, your doctor may order blood tests to make sure the medication is working and not harming your organs. People on blood pressure medications need periodic kidney and electrolyte checks. People on cholesterol drugs need liver function tests. People on diabetes medications need glucose and kidney function monitoring.

These tests are not optional — they are how your doctor catches problems early. If your doctor recommends regular blood work and you skip it, you may not know that a medication is damaging your kidneys or liver until serious harm has occurred.

If you cannot afford the copay for blood work, tell your doctor. Some labs offer reduced-cost testing, and some community health centers provide free or sliding-scale blood tests. Your doctor can refer you or help you find a resource.

When to ask for a blood test

You do not have to wait for your doctor to order a blood test. If you are experiencing unexplained fatigue, weight loss, frequent infections, or other symptoms that concern you, ask your doctor whether blood work would help. Bring a list of your symptoms and when they started.

If you have a family history of heart disease, diabetes, or cancer, mention it at your annual visit. Your doctor may recommend screening blood tests earlier or more often than standard guidelines suggest.

If you are starting a new medication, ask whether you will need blood tests to monitor it. If you have not had blood work in over a year, ask whether it is time for a checkup.

Frequently Asked Questions

Do I need to fast before my blood test?

Most blood tests do not require fasting. Your doctor will tell you if fasting is needed — usually for lipid panels or fasting glucose tests, which means no food or drink except water for 8 to 12 hours before the test. If you are not sure, call your doctor's office the day before.

How long does it take to get blood test results?

Most results come back within two to three business days. Some tests, like cultures that check for infection, take longer — up to a week. Your doctor's office will contact you when results are ready, or you can check your patient portal if your doctor uses one.

Can medications affect my blood test results?

Yes. Blood thinners, antibiotics, vitamins, and supplements can all affect results. Tell the person drawing blood and your doctor about everything you take, including over-the-counter drugs and herbal products. Your doctor may ask you to stop taking something before the test or will interpret results knowing what you take.

What if my blood test result is abnormal?

One abnormal result does not always mean you have a disease. Your doctor will explain what the result means, whether it needs follow-up testing, and what to do next. Sometimes a result is abnormal because of dehydration, stress, or a temporary infection. Your doctor will help you understand whether treatment is needed.

Does Medicare cover all blood tests?

Medicare covers certain preventive blood tests at no cost, like cholesterol screening and diabetes screening. Other tests are covered when medically necessary, meaning your doctor has a reason to order them. You may owe a copay or coinsurance for those. Call your plan to ask about specific tests before your appointment.