Comparability of venous and capillary glucose measurements in blood

Diabetes and glucose intolerance are diagnosed by measurement of glucose in blood. Glucose is usually measured as venous plasma or capillary whole blood and diagnostic criteria frequently provide equivalence estimates for these two methods. This study examined the relationship between glucose measured in capillary and venous samples collected at random, fasting and 2 h after oral glucose.

Simultaneous measurements of venous plasma and capillary blood glucose were performed on random samples in 609 people, fasting samples in 685 people, and 2 h after oral glucose samples in 463 people. Separate capillary and venous samples were collected each time. A variance component model was used to construct conversion algorithms between venous and capillary results.

The relationship between venous and capillary glucose values varied, with venous plasma being higher than capillary blood for random and fasting samples but lower for sampling 2 h after oral glucose. Discrepancies were observed between measured capillary blood values and the published WHO capillary blood equivalence values for venous plasma values for all except a fasting venous value of 7.0 mmol/l. For example, for a fasting venous plasma glucose of 6.1 mmol/l the WHO equivalent value is 5.6 mmol/l, while the measured value was 5.2 mmol/l, and for a 2-h venous plasma glucose of 11.1 mmol/l the WHO value is 11.1 mmol/l, while the measured result was 11.7 mmol/l.

These results highlight the difficulty in equating glucose levels from one sampling and measuring procedure to another, and raise uncertainties about current published equivalence values which could lead to misclassifications in glucose tolerance status.