Influence of Schirmer strip wetness on volume absorbed, volume recovered, and total protein content in canine tears

Objective Determine the protein content and volume of tears sampled by Schirmer strips wetness ranging from 20 to 35 mm. Animals Studied Ten healthy beagle dogs. Procedures Each dog underwent 20 tear collections per day (10 sessions in each eye, spaced by >= 1 h) for 4 separate days, providing 200 tear samples for each length of wetness evaluated: 20, 25, 30, and 35 mm. A Schirmer strip was placed in each eye until the selected mm-mark was reached, calculating the volume absorbed (VA) as the difference between the post- and pre-collection weight (assuming 1 mg(-1) mu L for tear fluid), and the volume recovered (VR) as the amount pipetted from the tube following centrifugation. Total protein content (TPC) was measured with infrared spectroscopy. Outcome measures were compared with the Kruskal-Wallis test. Results Median values for VA (mu L), VR (mu L) and TPC (mg/mL) were as follows: 20 mm (18, 10, 5.94), 25 mm (22, 12.5, 5.97), 30 mm (25.5, 16, 5.89), and 35 mm (31, 22.5, 7.13). Both VA and VR were significantly greater (p < .001) for Schirmer strips wetness of 35 >> 30 >> 25 >> 20 mm. TPC was significantly greater (p < .001) for 35 > 20-30 mm, but not among other groups (p = 1.000). Conclusions The study established normative data to consider when canine studies use Schirmer strips to collect tears for bioanalytical purposes (eg, proteomics, pharmacokinetics). Although 35 mm yielded higher VA and VR, the higher TPC could be explained by greater disruption of ocular surface homeostasis. Absorption to 20-30 mm is the suggested length of strip wetness for bioanalytical tear collection in dogs.