A Subcutaneous Capillary Filtrate Collector for Measurement of Blood Chemistries

The capillary filtrate collector (CFC) contains 30,000 molecular weight cut-off, hollow fiber ultrafiltration membranes that are placed below the skin. A transcutaneous tube leads to an evacuated glass tube that provides a vacuum to pull ultrafiltrate at 40–60 μl/hr from blood, through the fibers, and past a sampling port to the glass tube. Long-term (1–6 months) animal and clinical studies have shown that the ultrafiltrate concentration of chemicals such as glucose and a variety of drugs is exactly the same as that of the blood plasma water when the ultrafiltrate is created. In this study, the device was placed in six home monitored diabetics and four in-center hemodialysis diabetic patients. Over the following month, blood glucose concentrations were compared to CFC glucose concentrations. In spite of difficulties in diluting and assaying small samples of filtrate, there was a good correlation between blood and CFC glucose levels. A flow-through enzymatic glucose sensor has been tested and shown to accurately measure glucose in CFC filtrate. When placed in the transcutaneous tubing near the skin, this should allow a small external device to continuously monitor glucose levels in brittle or out of control diabetics with high accuracy and little risk, discomfort, or cost.