Partial Pressure of Oxygen in Capillary Blood Samples from the Fingertip

Abstract

Many people with diabetes routinely measure their blood glucose (BG) on capillary blood samples from the fingertip. Beside other interfering factors, the blood samples’ partial pressure of oxygen (pO2) can affect BG measurements, particularly in systems based on glucose oxidase (GOx) enzyme reactions on test strips.1,2 Indeed, many of the available home-use systems for self-monitoring of blood glucose (SMBG) utilize the GOx enzyme reaction, which is prone to oxygen interference; however, in the literature, poor information is available concerning physiological pO2 values and possible variations in capillary blood from the fingertip in people with diabetes. In this investigation, the pO2 of capillary blood samples obtained from fingertips was determined in 110 subjects (55 female, 31 with type 1 diabetes mellitus, 69 with type 2 diabetes mellitus, 10 without diabetes; mean age 61 years, from 19 to 78 years); most of them were expected to perform SMBG regularly. The subjects had no acute serious diseases. They participate regularly in SMBG system evaluation studies at the Institute for Diabetes-Technology GmbH at Ulm University, Ulm, Germany. The study protocol was approved by the Ulm University Ethics Committee. Capillary blood samples were obtained by skin puncture, and the pO2 was analyzed on a blood gas analyzer (OPTI™ CCA-TS Analysator, OPTI Medical System Inc., Roswell, GA). Maintenance, handling, and quality control of the blood gas analyzer were performed according to the manufacturer’s labeling. Regular internal and external quality control measurements were performed, as required by German national guidelines. Sample collection and pO2 measurements were performed by trained clinical personnel. The 110 subjects showed a mean pO2 of 71.1 mmHg (standard deviation ± 6.9 mmHg), ranging from 49 to 86 mmHg. Female and male subjects showed similar mean pO2 values (72.5 and 69.8 mmHg, respectively). Ninety-four subjects (~85%) showed pO2 values between >60 and ≤80 mmHg, 6 subjects (~5%) showed pO2 values ≤60 mmHg, and 10 subjects (~9%) showed pO2 values >80 mmHg (Figure 1). Lowest pO2 values (53 and 49 mmHg) were found in two subjects with stable chronic respiratory disease. Figure 1. Relative number of subjects with pO2 values within the respective category. Our results indicate that a broad range of capillary pO2 values occur among a population of healthy people and people with diabetes without acute serious diseases. In a previous study using venous blood samples adjusted to different pO2 levels, we observed remarkable measurement deviations with some GOx systems. Particularly at pO2 ≤45 mmHg, we found considerably overestimated measurements.2 Decreased pO2 values can occur in patients with respiratory diseases, such as chronic obstructive pulmonary disease,3 which is described as being associated with type 2 diabetes.4 At high altitudes or also during long-distance flights, up to ~40% decreased pO2 is reported for arterial blood samples;5 a similar behavior can also be expected for capillary blood samples from the fingertip. In conditions with decreased pO2 values in capillary blood, measurements with oxygen-sensitive systems could be affected, and hypoglycemic events might not be detected adequately. Further investigations should be performed focusing on pO2 variations in capillary blood from fingertips in people with diabetes and the possible impact on glucose measurement results obtained with oxygen-sensitive systems.