Water-soluble antioxidants, in human tears: Effect of the collection method

PURPOSE. To resolve differences in published data on tear antioxidant level s by comparing the concentration of water-soluble antioxidants in human ref lex tears collected by capillary tube and by the Schirmer strip collection method and in basal and reflex tears collected using the Schirmer strip met hod. METHODS. Yawn-induced reflex tears (collected Simultaneously by capillary t ubes and by Schirmer strips) and basal tears (by Schirmer strips and using local anesthetic) were collected from 12 healthy subjects. Tear cysteine, a scorbate. glutathione, urate, and tyrosine were measured by high-performanc e liquid chromatography within a few minutes of collection. RESULTS. Cysteine, ascorbate, glutathione, and tyrosine were 5 to 10 times higher (P < 0.01) in both reflex and basal tears collected by Schirmer stri p compared with reflex tears collected by capillary tube from the same subj ect. Urate levels were slightly but nonsignificantly higher in Schirmer str ip samples (P > 0.05). CONCLUSIONS. The conflict in published data on tear antioxidants is caused by differences in collection methods. With the exception of urate, antioxid ants accumulate to very high levels in corneal cells. Spuriously high antio xidant levels in tears collected using Schirmer strips, therefore, are most probably caused by contamination with intracellular constituents. The capi llary tube collection method is proposed as the method of choice for reflex tear collection for biochemical studies. This less-invasive method facilit ates the evaluation of tear antioxidant levels as a biomonitoring tool for corneal health. Although moderately increased antioxidant levels may be ben eficial, the authors hypothesize that marked increases may indicate damage to the ocular surface.