Influence of sample type on the interpretation of the oral glucose tolerance test for gestational diabetes mellitus.

The World Health Organization (1985) criteria allow evaluation of the oral glucose tolerance test using venous or capillary whole blood or plasma glucose measurements. However, the empirical factors used for interconversion may not reflect observed differences, especially during pregnancy, causing inconsistent classification. To investigate how choice of sample would influence the interpretation of results, venous and capillary blood was taken during oral glucose tolerance tests in 36 pregnant women at risk of gestational diabetes and in 21 non-pregnant control subjects. Glucose was measured on whole blood and plasma by a glucose oxidase method. No cases of gestational diabetes were identified. Eight subjects had gestational Impaired Glucose Tolerance using either venous plasma or venous whole blood results, but only five were similarly classified with capillary whole blood and only four using capillary plasma. Plasma-whole blood differences (venous 0.6 +/- 0.2 (+/- SD) mmol l-1, capillary 0.7 +/- 0.3 mmol l-1) and capillary-venous differences (plasma 0.5 +/- 0.4, whole blood 0.4 +/- 0.5 mmol l-1) at 2 h were lower (all p less than 0.05) than in the WHO criteria (1.1 mmol l-1). When compared with venous plasma, capillary measurements may give a lower incidence and venous whole blood measurements a higher incidence of Impaired Glucose Tolerance in pregnancy.