title: Water-soluble antioxidants in human tears: effect of the collection method.
abstract: OBJECTIVE
  To resolve differences in published data on tear antioxidant levels by comparing the concentration of water-soluble antioxidants in human reflex tears collected by capillary tube and by the Schirmer strip collection method and in basal and reflex tears collected using the Schirmer strip method.
  
  
  METHODS
  Yawn-induced reflex tears (collected simultaneously by capillary tubes and by Schirmer strips) and basal tears (by Schirmer strips and using local anesthetic) were collected from 12 healthy subjects. Tear cysteine, ascorbate, glutathione, urate, and tyrosine were measured by high-performance 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 strip compared with reflex tears collected by capillary tube from the same subject. Urate levels were slightly but nonsignificantly higher in Schirmer strip 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, antioxidants accumulate to very high levels in corneal cells. Spuriously high antioxidant levels in tears collected using Schirmer strips, therefore, are most probably caused by contamination with intracellular constituents. The capillary tube collection method is proposed as the method of choice for reflex tear collection for biochemical studies. This less-invasive method facilitates the evaluation of tear antioxidant levels as a biomonitoring tool for corneal health. Although moderately increased antioxidant levels may be beneficial, the authors hypothesize that marked increases may indicate damage to the ocular surface.
authors.affiliation: Department of Optometry and Radiography, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China.
authors.forename: C K
authors.initials: CK
authors.lastname: Choy
authors.forename: P
authors.initials: P
authors.lastname: Cho
authors.forename: W Y
authors.initials: WY
authors.lastname: Chung
authors.forename: I F
authors.initials: IF
authors.lastname: Benzie
mesh_terms: D000328:Adult; D000975:Antioxidants / Q000032:analysis* / Q000737:chemistry*; D005260:Female; D006801:Humans; D008297:Male; D012995:Solubility; D013048:Specimen Handling / Q000379:methods; D013666:Tears / Q000737:chemistry* / Q000378:metabolism; D014867:Water; D015000:Yawning / Q000502:physiology
chemical_list: D000975:Antioxidants; D014867:Water