In contemporary healthcare provision, there are many different types of tests, scans and investigations that are available to help make a diagnosis, exclude a diagnosis or to help with screening. This series of articles concerns tests, scans and investigations, providing the healthcare assistant (HCA) and assistant practitioner (AP) with information that will assist them in understanding the various procedures, as well as helping them to ensure positive patient outcomes. This first article focuses on one of the most commonly used medical tests: blood tests.