There are actually several types of blood tests that your doctor may perform, depending on your symptoms and what they are looking for. These include tests to look at your blood chemistry, the enzymes present in your blood, tests of how well your blood is able to form a clot, tests to assess risk for heart disease, or a complete blood count. A complete blood count can detect the presence of many diseases and immune system disorders by measuring the numbers of red and white blood cells, platelets (blood cell fragments that promote clotting), hemoglobin (the protein that carries oxygen), hematocrit (the amount of space taken up by red blood cells), and mean corpuscular volume (a measure of the size of red blood cells).

Blood chemistry tests provide information about the health of your muscles, bones, and organs. This test reports blood glucose, calcium, and electrolyte levels. It will also test the function of your kidneys. This is often a test that requires you to not eat any food for some length of time before the test so that the doctor can get an accurate measure of your blood chemistry that is not influenced too heavily by what you have eaten recently.

Blood tests to assess heart disease focus on measuring your cholesterol levels. This includes low-density lipoproteins (LDLs, often called “bad” cholesterol), high-density lipoproteins (HDLs, often called “good” cholesterol), and triglycerides (a type of fat). This test also typically requires you to not eat for about half a day prior to having your blood taken, as your cholesterol levels can be easily influenced by what you have recently had to eat.

While blood tests typically cannot diagnose diseases themselves, they can provide a strong indication of what disease(s) you may have and direct your physician to provide other tests that can confirm a diagnosis.