Blood tests are usually performed by taking a small sample of blood, often from a vein in the arm using a sterile needle (venipuncture), or sometimes via a finger prick (for certain tests like blood sugar level). The blood is collected in special test tubes containing different anticoagulants or preservatives depending on the specific test required. The samples are then sent to a laboratory for analysis using specialized equipment and techniques. Blood tests can measure the number and types of blood cells (such as a Complete Blood Count - CBC), evaluate organ function (such as kidney and liver function tests), detect signs of inflammation or infection (such as C-Reactive Protein - CRP), measure hormone levels, assess blood clotting factors, and screen for specific diseases like diabetes, heart disease, or certain types of cancer. Patients may be asked to fast for several hours before some blood tests to ensure accurate results, especially those measuring sugar or lipid levels.