The guide is organized the way clinicians think; by clinical presentation and by organ system. So, if you have a patient with a skin problem, go to the section on dermatology. If a patient presents with fatigue, go to the section on fatigue. In addition to information on lab investigations, we provide, where useful, differential diagnoses, etiologies, and summaries of signs and symptoms. We also share some ‘pearls’—particular knowledge about lab investigations we have gathered as experts and clinicians in our fields.