Your blood plays a big role in your overall health and contains a lot of information about what may be going on in your body. That’s one reason why blood tests are a common medical test. A healthcare provider may do a blood test because:

It’s time for your regular physical. During a checkup, your provider may order blood tests to check on your overall health. They may order a blood test that evaluates many parts of your blood, such as a complete blood count (CBC), basic metabolic panel (BMP) or a comprehensive metabolic panel.
Your provider recommends screening tests. Screening tests are done before you have any symptoms. They may recommend screening tests if you’re at risk of developing certain conditions, such as cancer. For example, if you’re at risk for developing coronary artery disease, your healthcare provider may order several blood tests to evaluate that risk.
You don’t feel well. If you have specific symptoms, your provider may order blood tests to determine what’s causing them. For example, if you have symptoms that may be signs you’re pregnant, your provider will do a pregnancy test. The blood test looks for a specific hormone your body only ever makes when you’re pregnant.
You have a medical condition that happens when certain genes change (mutate). Depending on your situation, your blood cells and platelets may show information about the specific changes. Understanding which genes changed may help your provider plan your treatment.
You’re receiving treatment for a medical condition. Your provider may use regular blood tests to see if treatment is working.
You may have inherited certain genetic mutations that cause medical conditions. Your provider may take blood samples for genetic analysis so you know if you’re at risk of developing a specific condition.
What do blood tests show?
In a broad sense, a blood test shows changes in your body. Blood test results don’t show a complete picture. Instead, they’re a kind of snapshot. After seeing that snapshot, your provider may do other blood tests to get a closer view. Here’s a glimpse of what your healthcare provider may see with blood tests:

The tests show if your blood is working as it should. For example, your red blood cells carry oxygen throughout your body. A blood test may show you have low red blood cell levels (anemia). If healthcare providers look at your cells under a microscope, they may see your red blood cells are larger than normal or shaped differently than normal red blood cells. These differences may be signs of blood disorders or blood cancers.
They show if you have normal levels of enzymes and electrolytes. Enzymes are proteins that help speed up the chemical reactions that build up and break down substances in your body. Electrolytes do several things, such as helping your body regulate chemical reactions and maintaining the balance between fluids inside and outside your cells.
What are the most common blood tests?
There are many different blood tests. Some tests — such as complete blood count tests, basic metabolic panels, complete metabolic panels and electrolyte panels — check on several different elements in your blood at the same time. Other blood tests look for very specific elements in your blood.

What does a complete blood count (CBC) test show?
This is the most common blood test that includes several specialized tests. CBC tests:

Count and measure your red blood cells, white blood cells and your platelets. A CBC test includes a hematocrit test that measures the percentage of red blood cells in your blood.
Measure your hemoglobin levels.
Measure variations in your red blood cells’ size and volume with an RDW blood test (RDW stands for “red blood cell distribution width”).
Measure the average size of your red blood cells with a mean corpuscular volume (MCV) test.