Methods of vitamin deficiency testing: 1) Blood tests, where the level of vitamins in the blood is taken to determine whether there is a vitamin deficiency; 2) Urine tests, where the level of vitamins, especially water-soluble vitamins, are excreted in the urine, to determine whether there is a vitamin deficiency; 3) Isotope measurements, where the level of vitamins in the body is measured using stable isotopes. Stable isotope labeling can also be used to detect the level of vitamins in the body to determine whether there is a deficiency.