Urine, serum, plasma, whole blood, red blood cells, feces, hair, fingernails … the list goes on. How do you decide what biological sample(s) to use for element analysis? Can results be compared to scientific literature or do they have clinical significance? Is it possible for values to be elevated or low in one sample type and normal in another? Do test results indicate recent intake, body burden, acute toxicity, chronic toxicity, deficiency, or homeostatic regulation? These are just some of the questions facing a testing laboratory when they want to develop and validate essential and toxic element profiles that will provide clinically meaningful results.