Cornea. 2023 Nov 22. doi: 10.1097/ICO.0000000000003430. Online ahead of print.
PURPOSE: Schirmer test results are widely used for ocular surface disease assessment, but Schirmer strips are not standardized. We compare the characteristics and tear volume with millimeter moisture migration in different brands of Schirmer strips and introduce methods for volume-based, brand-independent calibration.
METHODS: Physical parameters of Haag-Streit, EagleVision, TearFlo, Contacare, and MIPL/A6 Schirmer strip brands were compared. Schirmer strip millimeter moisture migration distances were assessed 5 minutes after application of incremental microliter volumes of human tears. Linear regression analysis of data points from each Schirmer strip brand was performed, and the root-mean-square deviation of data points to the best-fit linear regression was calculated. Calibration correction was performed by converting migration distance to the corresponding tear volume. A reference table and calibration method formulas were created.
RESULTS: Schirmer strips differed in design, shape, and manufacturing precision. Strip width, weight, and length were different between the 5 brands (P < 0.05). A wide range of Schirmer strip moisture migration values for identical tear volumes was observed among brands. Statistical measurement resulted in a root-mean-square deviation of 2.9 mm for all data points from all brands. Millimeter to volume and weight to volume-based calibration correction methods resulted in a 2.2- and 3.1-fold measurement error reduction, respectively.
CONCLUSIONS: Our findings highlight the lack of standardization among different brands of Schirmer strips, raising concerns about potential sources of unintentional measurement errors. We propose volume-based Schirmer strip calibration methods and conversion of millimeter to microliter results to achieve brand-independent results and improve Schirmer test accuracy.