Laboratory diagnostics is an analytical branch of medicine that enables us to obtain valuable information about the health and disease of particular organs (heart, kidney, liver, etc.) or the whole organism, by analysing readily available samples (blood, urine, stool). In addition to being a diagnostic discipline, it also makes possible to predict the course of the disease and to follow-up the effects of treatment. Most medical decisions are made based on laboratory findings, so the patient preparation before specimen sampling is imperative for accurate and reliable laboratory test results. Therefore, laboratory diagnostics is closely related to other branches of medicine since complex analysis of laboratory results is required in majority of cases.