Inter- and intraobserver variation in the analysis of optic disc images: comparison of the Heidelberg retina tomograph and computer assisted planimetry

Aims-The development of imaging and measurement techniques has brought the prospect of greater objectivity in the measurement of optic disc features, and therefore better agreement between observers. The purpose of this study was to quantify and compare the variation between observers using two meas urement devices. Methods-Optic disc photographs and images from the Heidelberg retina tomogr aph (HRT) of 30 eyes of 30 subjects were presented to six observers for ana lysis, and to one observer on five separate occasions. Agreement between ob servers was studied by comparing the analysis of each observer with the med ian result of the other five, and expressed as the mean difference and stan dard deviation of differences between the observer and the median. Inter- a nd intraobserver variation was calculated as a coefficient of variation (me an SD/mean x 100). Results-For planimetry, agreement between observers was dependent on observ er experience, for the HRT it was independent. Agreement between observers (SD of differences as a percentage of the median) for optic disc area was 4 .0% to 7.2% (planimetry) and 3.3% to 6.0% (HRT), for neuroretinal rim area it was 10.8% to 21.0% (planimetry) and 5.2% to 9.6% (HRT). The mean interob server coefficient of variation for optic disc area was 8.1% (planimetry) a nd 4.4% (HRT), for neuroretinal rim area it was 16.3% (planimetry) and 8.1% (HRT), and (HRT only) for rim volume was 16.3%, and reference height 9.1%. HRT variability was greater for the software version 1.11 reference plane than for version 1.10. The intraobserver coefficient of variation for optic disc area was 1.5% (planimetry) and 2.4% (HRT), for neuroretinal rim area it was 4.0% (planimetry) and 4.5% (HRT). Conclusions-Variation between observers is greatly reduced by the HRT when compared with planimetry. However, levels of variation, which may be clinic ally significant, remain for variables that depend on the subjective drawin g of the disc margin.