Nowadays pharmacist become for patients a source of information about possibilities of quick detection or exclusion of diseases and interpretation of results obtained by more and more popular of quick diagnostic tests (so called “point of care tests”) available in pharmacies. An abnormal glucose concentration is frequently occurred in the populace and arouse need of further diagnosis to detection of diabetes mellitus. In this study clinical and diagnostic aspects of incorrect blood glucose concentration, symptoms and classification of diabetes mellitus are briefly presented. An acquaintance of symptoms and nomenclature of states related to anomalous glucose metabolism and methods of discovering them will be helpful and accelerate detection of a disease and prevent from development of latent vascular diabetic complications. Diagnosing and monitoring of diabetes require usage of diverse laboratory tests and may be done different types of blood samples and obtainment at the same patient of differing values of glucose concentration is not the measurement error. Diagnosis of diabetes by measurements of glucose concentration in plasma of venous blood in certified diagnostic laboratories by the biochemical analysers should be performed. Monitoring the course of disease and effectiveness of therapy by the measurement of glycemia in capillary blood with the use of portable small biochemical analysers (glucometers) is sufficient. A knowledge of rules of carrying out laboratory tests and methods of results interpretation will be helpful in cooperation with physician and laboratory diagnostican as well as in communication with patient.