Glucose is the major energy source for cells. Its blood levels are strictly controlled by hormones such as insulin and glucagon. Not surprisingly, the major cause for persistently elevated glucose concentration (hyperglycemia) is diabetes mellitus. However, also other conditions, such as stress and several internal diseases, can increase glucose concentrations. Furthermore, blood glucose is clinically measured only from fasting samples, as eating can increase its concentration. Conversely, low glucose concentration, hypoglycemia, may result from insulinoma, several internal diseases, paraneoplastic syndrome, or certain drugs. Also, delayed separation of red blood cells from the sample decreases glucose levels due to prolonged red blood cell glucose metabolism.