Reference ranges for blood tests, sorted by mass concentration. A reference range is a set of values used by a health care provider to interpret a set of medical test results. The range is usually defined as the set of values 95 percent of the normal population falls within, or two standard deviations from the mean. All values (with some exceptions) denote blood plasma concentration, which is approximately 60–100% larger than the actual blood concentration if the amount inside red blood cells is negligible.Image credit: Mikael Häggström