To obtain valid medical laboratory results, it is imperative that the in vivo state of the quantity in the body fluid under investigation be presented unchanged to the analytical process. In this book an international team of authors summarizes the information on biological influences, analytical interferences and on the variables affecting the collection, transport and storage as well as preparation of samples. They cover age, gender, race, pregnancy, diet, exercise and altitude, plus the effects of stimulants and drugs. National and international standards are described for sampling procedures, transport, sample identification and all safety aspects, while quality assurance procedures are shown for total laboratory management.