Measurement procedures in confocal choroidal laser Doppler flowmetry

Abstract

Purpose. To estimate a practical number of measurements within a series of recordings with a confocal choroidal laser Doppler flowmeter, capable of providing an adequate statistical power for future studies. Methods. Ten independent measurements of choroidal blood flow were obtained within 30 minutes in one randomly selected eye of 24 subjects. In 12 subjects, subsequent series of 10 measurements were also obtained 2 and 7 days after baseline assessment. Average and median values of these series were calculated for the choroidal blood flow parameters velocity, volume, and flux, using either the first 3, the first 4, the first 5, the first 6, the first 7, the first 8, the first 9, or all 10 measurements. The variance between the series was analyzed using non-parametric tests. Results. The average values of a series of independent measurements decreased with an increasing number of independent measurements. More than 5–7 independent recordings did not markedly influence the estimate of the average value of choroidal blood flow parameter for an individual. Base-line measurements showed markedly higher values than subsequent series. Average values obtained with 6 independent measurements seem to provide a reasonable level of statistical power for paired comparisons, 13 subjects providing a 80% power at a probability level of 0.05 to find a significant difference of 15% between two means. Conclusion. It seems recommendable that experimental measurements with a confocal choroidal laser Doppler flow-meter are preceded by a session of familiarization and that average values of series of at least 6 measurements are used.