Alternative sites for self-monitoring of blood glucose (SMBG) (e.g., forearm, abdomen, calf, or thigh) are currently being introduced in clinical practice (1). However, blood glucose (BG) concentrations by these methods may differ from those by traditional fingertip pricking (2–4). In an elegant study, Jungheim and Koschinski (3) demonstrated that BG measurements, at the forearm by three commercially available devices, showed a less steep increase after an oral glucose load and a delayed decline after insulin administration. In addition, Ellison et al. (4) showed similar findings, which were less adequately followed at the forearm after a standardized meal.
Given the patients’ preference for alternative-site SMBG, an appreciation mainly based on the avoidance of painful fingertip pricking (1), clinical application will certainly ensue and, thereby, will introduce a new problem, i.e., what glucose value is actually measured and how does this relate to reference values?
We compared capillary BG taken at the fingertip (Glucotrend; Roche Diagnostics, Mannheim, Germany) and the abdominal wall (Freestyle; Disentronic, S’ulzbach, Switzerland) in 12 healthy nondiabetic males (age 25 ± 11 years [mean ± SD], BMI 23.7 ± 3.5 kg/m2). The participants arrived at the outpatient clinic after an overnight fast. They were clamped on their fasting BG by a varying glucose infusion and received an intravenous insulin infusion of 30 mU · kg−1 · h−1. After 60 min, the glucose infusion was stopped and BG was allowed to drop to near hypoglycemic levels. Then, after BG was increased by intravenously administered glucose (20% by weight, 0.15 × body weight × 12 [ml]), which was intended to increase BG 12 mmol/l above the actual BG level, the glucose infusion rate was increased. BG was measured every 5 min at the abdomen and fingertip. This was done every minute for 15 min after the sudden increase in BG. The agreement in fingertip and abdominal BG measurements during the gradual decrement and the sudden increment in BG levels was evaluated by repeated measurement ANOVA. Abdominal BG measurement adequately followed the decline in BG measured at the fingertip (see figure). In contrast, abdominal BG concentrations were 10–18% lower than fingertip BG the first 15 min after the rise in BG (P < 0.01, see figure).
Our finding that an increase in BG was less well followed by abdominal BG measurements supports the contention of Jungheim and Kochinsky (3) that alternative-site SMBG differs from classic finger pricking. It illustrates the existence of tissue-specific differences in glucose kinetics, and we previously noted that distribution effects may play a role in abdominal BG measurements (6,7). Obviously, compartment-dependent glucose characteristics should be taken into account with alternative-site SMBG. One solution is adjustment of glucose values generated from alternative sampling sites (forearm, abdomen, thigh, or calf) to arterial or nearby fingertip capillary values, known as the golden reference, and this was actually done in the report of Jungheim and Koschinski (3). Another approach could be that BG measurements are interpreted according to the site-specific characteristics. For instance, hypoglycemic episodes were more protracted in abdominal subcutaneous adipose tissue (8), suggesting that clinically relevant tissue glucopenia may be overlooked by conventional BG measurements. This illustrates that alternative sites may be preferable as they may better reflect tissue glucose homeostasis. Therefore, we challenge the view that fingertip capillary BG is the only reference in denoting BG excursions. We are entering a new era of BG monitoring with the first available glucose sensors that will provide us with a wealth of data on previously unavailable BG excursions, but at the same time confront us with compartment-specific differences in BG concentrations from traditional fingertip pricking.
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