For the purposes of Nutrition Counseling, I prefer the use of hair tests to blood test because I feel they are more accurate indicators of the overall metabolic trends in the body.
Blood is a transportation medium.
The components of your blood must remain fairly stable at all times, or you would die. Acidity, alkalinity, levels of certain nutrients, minerals, etc., all must remain within fairly tight limits. This equilibrium, or balance, is incredibly important to your health. It could be that your body is robbing calcium from your bones and teeth to support a major organ. A hair test would show the actual mineral status of your tissues.