The diagnosis of diabetes mellitus is essentially based on the determination of laboratory parameters. Determination of glucose and glycated hemoglobin A (HbA1c) are key parameters for the initial diagnosis, prognosis and treatment control. Diabetes-associated autoantibodies are used for the classification of diabetes and C‑peptide as well as insulin are markers for the endogenous secretion of insulin. Preanalytical and analytical limitations, such as influencing and confounding factors, have to be considered to ensure that laboratory measurements are reliably determined and correctly assessed. The specifications in the guidelines of the Federal Medical Association for the Quality Assurance of Laboratory Medical Investigations (Rili-BÄK) must be observed.