A clinical comparative study of tyndallometry using 2 laser flare meters

Abstract

The laser flare meter FM-500 (Kowa, diode laser, 670 nm) is a simplified version of the laser flare-cell meter FC-1000 (Kowa, He-Ne laser, 632.8 nm). the aim of this study was to compare flare measurements of the laser flare meter FC-500 with the established laser flare-cell meter FC-1000 and to determine the reproducibility of both instruments in vivo. Ten eyes of 10 normal subjects (mean age 32.2 +/- 8.0 y) and 21 eyes of 21 patients (mean age 36.7 +/- 17.2 y) with various forms of uveitis were included in the study. Aqueous flare was quantified using the laser flare-cell meter FC-1000 and the laser flare meter FM-500 by one examiner under comparable conditions. The selection of eyes as well as the order of the examinations were done in a randomized fashion. Measurements with both instruments were repeated the following day in normal eyes. Statistical analysis was done using linear regression analysis, intraclass correlation and U-test. Flare values (photon counts/ms) did not differ significantly between the instruments either for normal eyes (3.3 +/- 0.4 vs 3.5 +/- 0.6) or for eyes with uveitis (16.2 +/- 9.0 vs 16.4 +/- 8.7). A high correlation of flare counts between both instruments was observed in normal eyes (r = 0.73, p < 0.001) and a very high correlation in eyes (R = 0.99, p < 0.0001) with mildly to moderately increased counts (< 50 photon counts/ms). Differences between flare values of both instruments were -0.17 +/- 0.4, range -0.8 +/- 0.6 in the normal eyes and -0.23 +/- 1.38, range -3.1 +/- 3.4 in the patients with mildly to moderately increased flare counts. However, flare counts differed for very high aqueous flare in one patient with acute fibrinous iridocyclitis with the FM-500 indicating higher values. The intraclass correlation coefficients were 0.70 for the FC-1000 and 0.76 for the FM-500 in normal eyes and 0.99 for both instruments in eyes with uveitis. In eyes without or with mild to moderate breakdown of the blood-aqueous barrier, the two instruments give comparable results. However, measurements may differ in eyes with high flare readings.