Aim: To provide conversion formulae between measurements based on different specimens in use in epidemiological studies and clinical practice, and to evaluate the relative precision for the different methods. Background: The current guidelines emphasize the use of venous plasma for determining glucose concentration. Nevertheless, the World Health Organization (WHO) guidelines provide cut-off points for different specimens for the determination of the glucose concentration in circulating blood (venous plasma, whole blood, serum and capillary blood). There is a lack of data about the comparability between the values obtained by using different specimens. Methods: Eleven different combinations of specimens and methods of measurement of blood glucose were used in 294 blood samples from 74 subjects. The methods were grouped by the specimen used for analysis (venous plasma, whole blood, serum and capillary blood). Results: The result of the analysis is a set of linear equations allowing conversion of the result from one specimen or method to another. Furthermore, it was estimated how much of the variation for each method can be attributed to laboratory variance. Conclusions: Measurements based on capillary blood had a very large variability compared with other methods. Measurements based on venous whole blood tended to give results 0.5 mmol/L lower than other methods. Our data indicate that the current diagnostic cut-off points, as recommended by WHO for non-plasma specimens, are not fully compatible and may differ as much as 0.5 mmol/L between specimens.