I like Doctor's Data for their HAIR ELEMENTS/essential and other elements test. Andy Cutler has used this DDI test to create some 'counting rules' to help determine if you are mercury poisoned. These counting rules are available in his Hair Test Interpretation book or on-line. I also use DDI for urine toxic metals testing while chelating my Autistic son. Great Smokies and Great Plains are both reputable labs, too, for tests such as Elemental Analysis (packed erythrocytes) and Detox Profile. Mercury rapidly leaves the blood stream to do it's dirty work in the cells. A simple blood test will only measure very recent exposure. Pam (mercury toxic, and mom to a vaccine injured child)