The Impact of Post-Tear Collection Storage on Tear Film Substance P Concentration

Substance P is a sensory neuropeptide increasingly used as a biomarker for ocular and systemic neuropathic conditions. Due to the limited studies on tear storage conditions compared to other bodily fluids including blood and urine, the aim of this study was to investigate whether different storage durations at 4 °C can impact on substance P concentrations prior to storage at -80 °C. This is important to assess potential practical limitations in the handling and storage of tear fluid essential.

Tears were collected and pooled from both eyes of 31 healthy participants using the flush tears method. The samples were centrifuged and aliquoted into three sets of microcentrifuge tubes with each stored at 4 °C for <2 h, 4 h or 6 h (Timepoints 1, 2 or 3). After each respective storage duration, the aliquoted samples were than stored at -80 °C before analysis, within 6 months. Tears were analyzed for the concentration of substance P and the total protein content (TPC).

Substance P concentrations across the three timepoints were not significantly different (p > 0.05), including Timepoint 1 (Median [interquartile range]: 10.7 ng/ml [1.6-37.9]), Timepoint 2 (10.9 ng/ml [1.6-32.6]) and Timepoint 3 (5.2 ng/ml [1.3-25.2]). There were also no significant differences in TPC concentrations measured at the three timepoints, including Timepoint 1 (3.1 mg/ml [1.7-3.8]), Timepoint 2 (2.9 mg/ml [1.9-4.1]) and Timepoint 3 (2.7 mg/ml [1.6-3.7]).

While the levels of substance P were stable while stored at 4 °C prior to proper -80 °C storage and analysis, future research should investigate the impact of other storage conditions such as ambient room temperature to optimize the feasibility of using tears for biomarker purposes in clinical settings.