In the Netherlands, guidelines for the diagnosis of diabetes mellitus are confusing and differ from the international guidelines. Capillary blood-glucose testing using a blood-glucose device is allowed used as a diagnostic tool, although this test is imprecise. The Dutch laboratories measure blood-glucose concentrations by a more precise accurate method, but sometimes measure glucose levels in capillary whole blood and sometimes in venous plasma. These results are not comparable, because the results of capillary measurements are lower than the plasma measurements. In daily practice, health-care professionals are using different methods and are often not aware of the differences in glucose values that may result. They do not realise that glucose devices and laboratory glucose measurements may differ and that capillary- and plasma-glucose values are not interchangeable. Uniformity within the Dutch laboratories with regard to the glucose measurements is urgently needed, as is revision of the Dutch guidelines concerning the diagnosis of diabetes mellitus. This should be based solely on venous plasma-glucose values determined in a laboratory. Portable blood-glucose devices should not be used as a diagnostic tool for diabetes mellitus. These should only be used for blood-glucose control monitoring during treatment or as a screening tool.