Evaluation of commercial glucometer test strips for potential measurement of glucose in tears.

Tear glucose measurements have been suggested as a potential alternative to blood glucose monitoring for diabetic patients. While previous work has reported that there is a correlation between blood and tear glucose levels in humans, this link has not been thoroughly established and additional clinical studies are needed. Herein, we evaluate the potential of using commercial blood glucose test strips to measure glucose in tears. Of several blood glucose strips evaluated, only one brand exhibits the low detection limit required for quantitating glucose in tears. Calibration of these strips in the range of 0-100 µM glucose with an applied potential of 150 mV to the working electrode yields a sensitivity of 0.127 nA/µM and a limit of quantitation (LOQ) of 9 µM. The strips also exhibit ≤13% error (n = 3) for 25, 50, and 75 µM glucose in the presence of 10 µM acetaminophen, 100 µM ascorbic acid, and 100 µM uric acid. Measurements of glucose in tears from nine normal (nondiabetic) fasting human subjects using strips yielded glucose values within the range of 5-148 µM (mean = 47 µM, median = 43 µM), similar to those for human tears reported by others with more complex LC-MS methods. The glucometer strip method could facilitate more clinical studies to determine whether tear glucose and blood glucose levels sufficiently correlate for application to routine measurements in tears to supplement blood glucose testing. This would be especially helpful for children, adolescents, other Type 1 diabetics, and also for Type 2 diabetics who require treatment with insulin and cannot tolerate multiple finger sticks per day.

Measurement of tear glucose levels with amperometric glucose biosensor/capillary tube configuration.

An amperometric needle-type electrochemical glucose sensor intended for tear glucose measurements is described and employed in conjunction with a 0.84 mm i.d. capillary tube to collect microliter volumes of tear fluid. The sensor is based on immobilizing glucose oxidase on a 0.25 mm o.d. platinum/iridium (Pt/Ir) wire and anodically detecting the liberated hydrogen peroxide from the enzymatic reaction. Inner layers of Nafion and an electropolymerized film of 1,3-diaminobenzene/resorcinol greatly enhance the selectivity for glucose over potential interferences in tear fluid, including ascorbic acid and uric acid. Further, the new sensor is optimized to achieve very low detection limits of 1.5 ± 0.4 µM of glucose (S/N = 3) that is required to monitor glucose levels in tear fluid with a glucose sensitivity of 0.032 ± 0.02 nA/µM (n = 6). Only 4-5 µL of tear fluid in the capillary tube is required when the needle sensor is inserted into the capillary. The glucose sensor was employed to measure tear glucose levels in anesthetized rabbits over an 8 h period while also measuring the blood glucose values. A strong correlation between tear and blood glucose levels was found, suggesting that measurement of tear glucose is a potential noninvasive substitute for blood glucose measurements, and the new sensor configuration could aid in conducting further research in this direction.

Optic disc imaging in conscious rats and mice.

PURPOSE: To determine whether useful images of the optic discs of conscious rats and mice can be obtained by using a photo slit lamp and a modified Goldmann-type fundus contact lens. METHODS: Testing was performed with a photo slit lamp equipped with two 2x teleconverters and a digital camera through a Goldmann-type fundus contact lens that was fabricated for the rodent eye. RESULTS: Images of the rat and mouse optic discs were obtained that are comparable to those used by ophthalmologists to assess optic neuropathy in glaucoma, a key part of the standard of care and of clinical investigation of this disease. The cup in the optic disc image of these rodents is darker than the neural rim of the disc, rather than lighter, as it is in humans. CONCLUSIONS: In addition to the application of this imaging method to studies of the effect on optic disc cupping of induced increased intraocular pressure in rats and mice, by detecting and documenting the onset and the course of optic neuropathy, it should be valuable in identifying animal models of glaucoma, in studying neuropathogenic mechanisms, and in assessing the effects of experimental therapies.

Evaluation of enzyme-based tear glucose electrochemical sensors over a wide range of blood glucose concentrations.

Miniature enzyme-based amperometric and coulometric glucose sensors were fabricated and applied to measure tear glucose concentrations in anesthetized rabbits. Without perturbing the eyeball, 3 µL of tear fluid was sampled from the marginal conjunctiva under the lower eyelid of anesthetized rabbits at various time points via a microliter glass capillary tube, and the miniature sensors were then inserted into the volume of collected tear fluids within the capillaries for detection. Intravenous bolus doses of insulin were administrated to the rabbits to lower the elevated blood glucose concentrations caused by anesthesia over the 7h test periods. A significant correlation was found between tear and blood glucose levels for multiple rabbits, suggesting that electrochemical sensor-based tear glucose measurements may be a potential supplementary method for point-of-care glucose monitoring.

Goldmann applanation tonometry free of the slitlamp.

A prototype device was developed for using the Goldmann applanation tonometer (GAT) without a slitlamp to make it potentially practical to measure intraocular pressure (IOP) away from the clinic. Inexpensive consumer video components and a small flashlight replaced the microscope and illuminator of the slitlamp in a device that was assembled to use a standard, unmodified GAT. The device was evaluated for IOP measurement by volunteer measurer-subjects, who had no experience using the GAT, on each other and also by volunteers on themselves (self-tonometry). In a single, short training session with the prototype, the naive volunteers learned to measure IOP with the GAT. Other naive volunteers made IOP measurements on themselves with the prototype that were comparable in reliability with those obtained with the prototype by a professional measurer.