The usefulness of diagnostic and clinical outcome of a single laboratory test is determined by due quality of its performance in the interest of patients. A laboratory diagnostician is obliged to perform an assessment of the analytical quality of the test result according to current knowledge based on available scientific evidence. A helpful element in laboratory work, both analytical and microbiological, is participation in external and internal quality control of tests carried out, and correct interpretation of control results obtained in the laboratory. Creating quality management systems in medical diagnostic laboratories and developing, in the near future, the rules for a national system of work quality assessment in such institutions, is a very important factor that will determine the quality of diagnostic services.