Blood samples. Close-up of a rack of sample tubes containing blood for analysis. Blood samples are tested to check the health of a patient. The analysis may be to determine blood cell composition or the levels of chemicals, such as drugs and hormones, in the blood. Blood tests may indicate disorders of the metabolism and immune system, and they can detect infections. The tube at centre right has been separated into blood cells (red/black) and serum (orange). The watery blood serum contains salts, glucose and proteins, such as antibodies. The tubes have bar codes for identification.