Impact of diurnal variation, sex, tear collection method, and disease state on tear protein levels in dogs

Objective To investigate the effects of various biological factors on total protein concentration (TPC) and serum albumin levels in canine tears. Animals studied 10 healthy beagles (5 female, 5 male) were used. Procedures Experiments were conducted on separate days, collecting tears with either capillary tubes or Schirmer strips, as follows: (i) Tear collection at 3 hours intervals (from 6 am to 12 am); and (ii) Tear collection before and 20 minutes following topical histamine application (1, 10, 375 mg/mL) to induce mild, moderate, and severe conjunctivitis, respectively. TPC and serum albumin were measured with infrared spectroscopy and ELISA, respectively. Results Tear film TPC and serum albumin ranged from 9.7-26.1 mg/mL and 6.4-1662.6 mu g/mL, respectively. Protein levels did not differ significantly among time points (P >= .080). Median coefficient of variation (CV%) was lower with Schirmer strips compared to capillary tubes for both TPC (12% vs 15%, P = .020) and serum albumin (57% vs 78%, P = .232). TPC (P < .001), but not serum albumin was greater in male vs. female dogs. Serum albumin, but not TPC (P >= .099), increased significantly with each grade of conjunctivitis severity (P < .001), with no differences between collection devices (P >= .322); median increase was 106%, 1389%, and 2871% in eyes with mild, moderate, and severe conjunctivitis, respectively. Conclusions There is no apparent diurnal variation in canine tear protein levels. Blood-tear barrier breakdown with conjunctivitis allows serum albumin to leak into the tear film at high concentrations. Schirmer strips compare well with capillary tubes for bioanalytical purposes in healthy and diseased eyes, and this collection method may offer improved reproducibility for protein quantification.