It is known that glucose levels in capillary blood in the fingertip after a liquid glucose load are constantly higher when compared with venous blood measurements (1). Recently alternative sites for capillary blood drawing (e.g., forearm) have been proposed (2) that are less painful compared with fingertip. Data have shown that there was no significant difference between the capillary blood drawn from forearm and fingertip in diabetic patients with glucose values in a wide range (3). Nevertheless, some data have shown that glucose results from alternative sites and fingertip were not identical. This difference was more pronounced when there was a rapid increase or decrease of blood glucose values (4). It seemed that significant differences appeared when glucose values declined at a mean rate >2 mg · dl−1 · min−1 (5), but not at a lower rate (6). All the above-mentioned reports compared capillary blood drawn from either the forearm or fingertip, but so far, it appears that no direct comparison has been made between venous plasma blood and capillary forearm blood.
Thus, the purpose of this investigation is to study the pattern of capillary forearm blood and that of capillary fingertip blood glucose using the same glucometer (FreeStyle; Therasense) and to compare both with venous blood laboratory measurements during a 100-g oral glucose tolerance test (OGTT) in pregnant women. A total of 47 pregnant women (age 31 ± 3 years, BMI 24 ± 3 kg/m2, and gestational age 24–28 weeks) underwent a 100-g OGTT. Half of these women (n = 23) had simultaneous glucose samples drawn from the forearm after rubbing (7) using FreeStyle in 0′, 60′, 120′, and 180′, whereas the other half (n = 24) underwent the same procedure with blood drawn from the fingertip. The two groups were matched for age, BMI, and gestational age. Glucose difference in percentage (GDP) was calculated for both groups separately. Mean GDP between finger glucose and venous glucose samples was significantly higher at 60′ (14.6 ± 20.4%), 120′ (25.2 ± 34.7%), and 180′ (26.4 ± 26.7%) than at 0′ (−3.1 ± 14.1%) (P < 0.01). Mean GDP between forearm glucose and venous glucose samples was significantly higher at 120′ (16.3 ± 21.5%) and 180′ (16.3 ± 21.5%) than at 0′ (−2.5 ± 16.3%) (P < 0.01). On the contrary, mean GDP at 60′ (6.7 ± 20.9%) was not found significantly different.
These findings confirmed the already reported observation that up to 3 h after a liquid glucose load, capillary finger glucose levels are constantly higher (15–26%) than venous glucose levels. On the contrary, forearm glucose levels were closer to venous plasma glucose levels: There was no significant difference between them after 1 h, whereas a significant increase of 16% appeared at 2 and 3 h. These findings are in accordance with the concept of slower glucose kinetics at the forearm than the fingertip due to lesser arteriovenous anastomoses (4). To be sure, this physiological difference needs to be taken into consideration in the detection of hypoglycemia in diabetic patients. However, it is precisely this physiological difference that supports the suggestion that capillary forearm glucose measurements using a portable glucose meter may be useful for the 50-g challenge test for gestational diabetes screening in an outpatient environment.