Ferric reducing ability of tears in healthy subjects-diurnal variation and dependence on defined demographic data

The assay of the ferric reducing ability of tears (FRAT) can be useful for monitoring ocular antioxidant capacity in clinical settings. FRAT diurnal variation was evaluated in healthy subjects and its relation with age, sex, body mass index, and ferric reducing ability of plasma (FRAP) were studied.

FRAT of 10-microl tear samples collected with capillary tubes from 68 healthy subjects (age 10 to 92 yrs, 46 women, 22 men) were measured after 0, 3, 5, and 10 min of incubation with Fe(3 +). FRAT diurnal variation was estimated in 11 subjects, with tear samples collected every 4 hr during the day. Comparison of FRAT versus FRAP was determined in 20 subjects.

Mean FRAT after 10-min incubation reached 207.7 +/- 136.8 micromol/l. No differences were found between FRAT of men and women. Only donor age correlated with FRAT (rho = 0.40, p < 0.01). Subjects 59 yrs (179.4 +/- 95.4 versus 237.2 +/- 151.7 micromol/l, p < 0.01). For all investigated times of incubation, FRAT was approximately two times lower than FRAP (p < 0.01). FRAT revealed diurnal variation, with the highest value at 08:00 hours, upon awakening.

The significance of age-matched controls and the specific time of the day for tears collection should be considered in the trial design and investigations with FRAT as a marker of the ocular antioxidant defense system.