This textbook helps nurses, physician assistants, medical students and residents to order appropriate tests and understand how to interpret them to improve their diagnostic reasoning. Children are not like adults, and interpreting of the results of their diagnostic laboratory tests requires knowledge of the biochemical and metabolic differences. Using a combination of information, questions and case studies, the book allows readers to gain an understanding of the key concepts of sensitivity, specificity, and positive and negative predictive values, as well as the indications for diagnostic lab tests. This textbook presents the state of art in testing across body systems and guidance on how to order and interpret diagnostic laboratory tests in pediatric patients. Each chapter includes learning objectives, tables and figures, as well as questions and references for further learning. This textbook provides an update for clinicians and is a valuable learning tool for students and new clinicians.