This books explains in detail the science behind testing vitamins, organic acids, fatty acids, amino acids, hormones, nutrient and toxic elements, bacterial end products and much more to assist in assessing a patient's health.  The organic acids can be used to assess mitochondrial function, B vitamin status, and neurotransmitter function.  Hepatic Phase I and II activity measurement is explained as well as free radical damage.  If you are using the DFH profiles (comprehensive metabolic, basic metabolic with or without fatty acids or amino acids) available through Metametrix then you will find this book to be a great resource in explaining the results.  This type of testing helps determine where your patient/client may be depleted and allows you to tailor supplements to their needs leading to cost-effectiveness and optimal health.