Overview
Common blood tests are laboratory tests done on a sample of blood, usually taken from a vein in the arm. They can be used for routine health checks, to investigate symptoms, to monitor ongoing conditions, or to assess how treatments may be affecting the body. Different tests measure different parts of the blood, such as red and white blood cells, platelets, glucose, cholesterol, electrolytes, hormones, and markers of liver or kidney function.
Who Is Affected
People of all ages may have blood tests at some point, including children, adults, pregnant people, and older adults. They are commonly used in primary care, emergency care, hospital care, and public health screening programs. Which tests are appropriate depends on a person's age, symptoms, family history, medicines, and known health conditions.
Prevention
Blood tests themselves do not prevent disease, but they can support early detection and monitoring when used appropriately. Attending recommended health checks and screening programs may help identify some conditions earlier. It is important not to request or repeat tests without a clear reason, because unnecessary testing can lead to confusion or further procedures that may not be needed.
Diagnosis and Tests
Examples of common blood tests include a complete blood count, which looks at red cells, white cells, and platelets; blood glucose tests, which assess blood sugar; lipid tests, which measure cholesterol and triglycerides; and liver or kidney function tests, which look at chemicals linked to organ health. Some tests measure vitamins, iron, thyroid hormones, inflammation markers, or clotting. Your healthcare professional may ask you to fast, avoid certain medicines if advised, or come at a particular time of day, because preparation can affect some results. Laboratories use reference ranges, but these ranges can vary, and interpretation depends on the whole clinical picture.
Living With It
Many people feel worried when they see a result marked high or low, but one number on its own rarely tells the full story. Results may be influenced by temporary factors such as infection, stress, menstrual bleeding, hydration, or recent physical activity. If you receive your results through an online portal, discuss them with a healthcare professional rather than trying to interpret them alone.
Diet and Lifestyle
Before a blood test, follow the instructions you are given about eating, drinking, exercise, and medicines. Drinking water is often allowed and may make blood collection easier, but follow your local instructions if you have been told to fast. Over the longer term, habits such as balanced eating, physical activity, not smoking, and limiting alcohol can influence blood test results related to sugar, cholesterol, liver health, and nutrition.
When to Seek Medical Care
Seek medical advice if a healthcare professional recommends blood tests because of symptoms such as ongoing fatigue, unexplained weight change, fever, unusual bleeding, swelling, yellowing of the skin or eyes, or changes in urination. You should also contact a healthcare professional if you have questions about how to prepare for a test or if you do not understand your results. If a blood draw site becomes increasingly red, swollen, painful, or continues bleeding, arrange medical review.
When It May Be an Emergency
The blood test itself is usually not an emergency, but the symptoms that lead to testing sometimes are. Get urgent medical help for chest pain, severe shortness of breath, signs of stroke, heavy bleeding, fainting, severe allergic symptoms after a blood draw, or confusion. Follow local emergency guidance if a healthcare professional tells you a result needs immediate attention.
Questions to Ask a Healthcare Professional
You may want to ask: Why is this test being done, and what question is it trying to answer? Do I need to fast or change anything before the test? What do the results mean for me, and do they need to be repeated? Could my medicines, supplements, pregnancy, or recent illness affect the result?