To compare the quantity of fluorescein delivered to the eye via fluorescein-impregnated paper strips of various sizes and surface areas and via various microliter volumes of fluorescein sodium using an in vitro assay.
A commercially available fluorescein-impregnated strip (75 mm2) and three modified strips of reduced fluorescein-impregnated surface areas (10, 7.5, and 5.0 mm2) were used. The amount of fluorescein delivered to the eye for each of the four strips was approximated by applying each strip to a Whatman No. 1 filter paper under conditions simulating application of the strip to the eye, extracting the fluorescein from the filter paper in an aqueous solution, and performing spectrophotometric analysis at 484 nm. Similarly, this filter paper analytical system was calibrated using 1, 2, and 3 μl volumes of 2% w/v fluorescein delivered to the filter paper.
Using calibration curves, linearity was observed between absorbance and concentration of fluorescein sodium with an R2 value ≥0.99. Using these calibration curves, the amount of fluorescein delivered to the eye for the four strips and the three fluorescein solution samples was determined. Fluorescein-impregnated strips with surface areas of 75, 10, and 5 mm2 delivered approximately the same quantity of fluorescein to the ocular surface as 3 μl, 1 μl, and 0.5 μl of fluorescein 2% solution, respectively.
The surface area of the fluorescein-impregnated portion of the strip can be designed to control the amount of fluorescein delivered to the eye.