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(Investigative Ophthalmology and Visual Science. 2000;41:3293-3298.)
© 2000 by The Association for Research in Vision and Ophthalmology, Inc.

Ascorbic Acid Concentration and Total Antioxidant Activity of Human Tear Fluid Measured Using the FRASC Assay

Camus Kar Man Choy1, Iris Frances Forster Benzie2 and Pauline Cho1

1 From the Departments of Optometry and Radiography and 2 Nursing and Health Sciences, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China.

PURPOSE. To evaluate a novel method (FRASC) for total ferric reducing (antioxidant) activity and ascorbic acid concentration applied to human tears, to investigate the stability of ascorbic acid, and to determine the antioxidant status of human reflex tears.

METHODS. Linearity, sensitivity, and precision of FRASC and ascorbic acid loss during 7 days’ storage were assessed; total antioxidant activity and ascorbic acid and uric acid concentrations of reflex tears from 47 healthy subjects were measured.

RESULTS. FRASC has good precision, linearity, and sensitivity. Ascorbic acid is stable for at least 7 days at moderately acidic pH (pH 3.6) and low temperature. Total antioxidant activity and ascorbic acid and uric acid concentrations (mean ± SD) in reflex tears were 409 ± 162, 23 ± 9.6, and 68 ± 46 µM, respectively. Ascorbic acid and uric acid constituted around half the total antioxidant activity measured. There was a significant correlation between uric acid and total antioxidant activity (r = 0.754; P < 0.0001). Men had significantly (P = 0.0045) higher tear ascorbic acid concentrations than women.

CONCLUSIONS. FRASC is suitable for measuring total antioxidant activity and ascorbic acid in human tears. Further clinical study is needed to investigate the male–female difference seen, to characterize the remaining 50% antioxidant activity, and to investigate the effects of environmental conditions, antioxidant supplementation, age, and ocular disease on tear antioxidant status.




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