Blood tests can be used to evaluate heart disease. Your blood sample may be tested for one or more of the following:
- Cholesterol. A full test includes measuring total cholesterol, high-density lipoprotein (HDL) cholesterol (the good kind), low-density lipoprotein (LDL) cholesterol (the bad one) and triglyceride levels. These results allow your doctor to calculate the ratio of LDL to HDL. Blood levels of triglycerides vary according to food intake. To get a true reading, do not eat or drink (other than water) for at least 8 hours before the blood is drawn.
- Cardiac enzymes. After a heart attack, enzymes from damaged heart cells may leak into blood. If you experience chest pain, your doctor may test your blood for these enzymes. Specific enzymes include CK (creatine kinase), CK-MB (the form of creatine kinase found in heart muscle), and cardiac troponin I or T.
- C-reactive protein. This test measures inflammation throughout the body. Because atherosclerosis causes inflammation in your body’s arteries, C reactive protein (CRP) tends to go up in people with more atherosclerosis. Other diseases that cause inflammation, like arthritis, can also make it rise.
- B-type natriuretic peptide. This is a newer test that detects a chemical released by the heart when it is under strain. BNP can help separate shortness of breath due to heart failure from shortness of breath caused by lung disease. Higher BNP levels mean a higher risk of heart complications.
- Oxygen content. Measurement of blood oxygen helps to determine whether your overall circulation is sufficient, whether the lungs are providing enough oxygen to the bloodstream and whether there is evidence of poor blood flow from the heart to the lungs. Oxygen is measured using blood drawn from an artery, whereas standard blood tests use blood from a vein, usually drawn at the bend of the elbow.
- Prothrombin time. Medications that slow blood clotting include anticoagulants, such as warfarin (Coumadin), and antiplatelet drugs, such as aspirin. Although they are referred to as blood thinners, they don't actually "thin" the blood; instead anticoagulants alter proteins in the blood that are responsible for clotting and antiplatelet agents prevent platelets from clumping and forming clots. Because people respond differently to anticoagulants, blood tests that measure prothrombin time are used to determine whether the drug dose is correct — effective, yet safe (too high a dose can cause bleeding). Prothrombin time is not measured in people taking antiplatelet agents.
- Thyroid disease. Doctors often use blood tests to check for thyroid disease because an overactive thyroid gland can lead to a racing heartbeat.