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National and international recommendations on diagnostics of diabetes are based on laboratory venous plasma examinations. In outpatients, however, most blood sugar determinations in Europe are performed with capillary whole blood and portable blood glucose meters. Our own investigations confirmed the literature findings that capillary blood sugar results cannot be converted to venous plasma values on an individual basis, whereas transformations of the recommended cutoff-values from venous plasma to capillary whole blood are feasible. On the basis of 203 oral glucose tolerance tests (75g) in non-diabetic pregnant women in the 24th-28th week of gestation, the transformation in fasting individuals amounts from 95 mg/dl in venous plasma to 82 mg/dl in capillary whole blood, whereas the 2-hour values of 155 mg/dl are identical. Capillary blood sugar determinations are the only qualified way to determine blood sugar in general practice at the moment, as up to this day no venous blood devices with complete antiglycolytic preservatives are commercially available. Blood sugar determination is one of the most frequently performed and simultaneously least reliable laboratory tests.