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Clayton J Radke, Young Hyun Kim, Wing Li, Meng C Lin; Schirmer Strips Provide Reliable Tear-Production Rates. Invest. Ophthalmol. Vis. Sci. 2017;58(8):478. doi: https://doi.org/.
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Reliable measurement of tear production is critical to distinguish between aqueous deficient and evaporative dry eye. We show that Schirmer strip wetting lengths provide quantitative tear-production rates.
In a standard Schirmer tear test (STT), tear production after 5 min of eye closure is classified as “normal” when the strip wetted length is more than 10 mm, “deficient” or “dry eye” if less than 5 mm, and “equivocal” when lying between (Cassen et al., 1997). With this procedure, it is not possible to predict quantitative tear production. For quantitative analysis, measurement of wetting length is required at several time points.When SST wetted length is measured as a function of time, a linear increase occurs after an initial nonlinear period (Holly et al., 1997) (see Figure 1 from Clinch et al, 1983). Upon careful analysis of STT wetting kinetics, we identify the initial nonlinear wetting period as due to local depletion of tear in the lid margin followed by a slower linear time regime where exhaustion of the lid-margin tear slows tear supply. Our analysis yields the simple result that volumetric tear-production rate (in μL/min) is calculated from the slope of linear portion of wetting kinetics multiplied by the cross-sectional area of the Schirmer strip (strip width times thickness). By measuring the time course of the wetting front, quantitative tear-production rate is assessed.
From the linear slope in Figure 1 and the described calculation, we obtain an average tear-production rate of 1.7 μL/min for 50 subjects with no apparent ocular surface disease, a realistic value. For best clinical results, we suggest that subject eyes be anesthetized and the Schirmer strip be sheathed to minimize evaporation. We also recommend utilizing dyed Schirmer strips with mm markings and recording at least three wetting lengths at, for example, 3, 4, and 5 min.
We rigorously establish tear-wetting kinetics in a Schirmer strip and present a simple methodology to obtain quantitative tear-production rate.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Figure 1. STT average wetted length for 50 human subjects under local anesthetic as a function of time (Clinch et al., 1983). Solid line connects datum points. Standard deviation error bars are shown.
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