Effect of tear collection on lacrimal total protein content in dogs and cats: a comparison between Schirmer strips and ophthalmic sponges

Abstract

Background: Quantification of lacrimal total protein content (TPC) is an important tool for clinical scientists to understand disease pathogenesis, identify potential biomarkers and assess response to therapy, among other applications. However, TPC is not only affected by disease state but also by the method used for tear collection. Thus, the purpose of this study is to determine the impact on TPC of two methods of tear collection in dogs and cats: Schirmer strips and polyvinyl acetal (PVA) sponges.

Methods: (i) In vivo - Ten healthy dogs and 10 healthy cats were examined. Each animal underwent two sessions, separated by 10 min, in which a Schirmer strip was placed in one randomly selected eye until the 20-mm mark was reached, while a strip of PVA sponge was placed in the other eye for 1 min. (ii) In vitro - Schirmer strips and PVA sponges were spiked with various volumes of four bovine serum albumin solutions (0.5, 4, 10, and 20 mg/mL). In both experiments, the wetted absorbent materials were centrifuged for 1 min, and the TPC was quantified on the extracted fluid using Direct Detect™ infrared spectroscopy.

Results: Lacrimal TPC in dogs and cats ranged from 5.2 to 14.6 mg/mL and from 6.2 to 20.6 mg/mL, respectively. In cats, TPC was significantly lower with Schirmer strips vs. PVA sponges (P < 0.001). In dogs, the volume absorbed by PVA sponges was negatively correlated with TPC (r = - 0.48, P = 0.033). The inter-session coefficient of variation was significantly lower with Schirmer strips vs. PVA sponges in both species (P ≤ 0.010). In vitro, both absorbent materials resulted in a 'concentrating effect' of the TPC obtained post-centrifugation, which was most pronounced when the volume absorbed was low, especially for Schirmer strips.

Conclusion: Schirmer strips provide a repeatable method to quantify lacrimal TPC in dogs and cats, although care should be taken to absorb sufficient volumes of tears to minimize the concentrating effect from the absorbent material.

Keywords: Ophthalmic sponge; Schirmer tear test; Tear flow; Tear fluid; Total protein content.

Fig. 1

Box-and-whisker plots depicting the total protein content in tear samples collected in 10 healthy dogs (a) and 10 healthy cats (b) with either Schirmer strips or PVA sponges. Median values are shown by a horizontal line. First and third quartiles (25th and 75th percentiles) are represented by the lower and upper limits of the box, respectively. The 2.5th and the 97.5th percentiles are shown as the lower and upper whiskers, respectively

Fig. 2

Spearman’s rank correlation testing for TPC and VA of tear samples collecting with PVA sponges in 10 healthy dogs. A moderate negative correlation was found (Spearman’s rho = − 0.48, P = 0.033)

Fig. 3

Bar charts depicting the mean + standard deviation of TPC from in vitro experiments evaluating 12 combinations of volume absorbed/protein concentration in both Schirmer strips (white bars) and PVA sponges (gray bars): low, intermediate and high volumes of 0.5 mg/mL (a), 4 mg/mL (b), 10 mg/mL (c), and 20 mg/mL (d) BSA solutions. In each panel, statistical differences between the absorbent materials and the control BSA solution are shown by asterisks: * P < 0.05, ** P < 0.01, *** P < 0.001