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Editors Selection IGR 9-4

Structure and function measurements: MRA versus GPS in HRT

Comment by Felipe Medeiros on:

20517 Effect of ocular hypertension on retinal GABAergic activity, Moreno MC; de Zavalia N; Sande P et al., Neurochemistry International, 2008; 52: 675-682

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When evaluating a study of diagnostic accuracy it is essential to appraise whether the test has been evaluated on a population that is representative of the one in which the test is going to be applied in clinical practice

The Heidelberg Retina Tomograph (HRT, Heidelberg Engineering, Dossenheim, Germany) has been used for over a decade to evaluate the optic nerve head (ONH) in order to detect signs of damage from glaucoma. Evaluation of the ONH by this instrument includes a series of parameters designed to measure topographic changes in the neuroretinal rim and optic disc cup. Among the HRT parameters, the Moorfields Regression Analysis (MRA) is the most well-known and includes an evaluation of the neuroretinal rim taking into account the effects of disc size. A recent development of the HRT software provides a topographic parameter called Glaucoma Probability Score (GPS) that is independent of the contour line traced by the examiner. Moreno-Montanes et al. (64) performed a study to compare MRA and GPS in 59 eyes of normal subjects, 40 eyes of patients with ocular hypertension and 83 eyes with glaucomatous visual field defects. According to the authors, ONH characteristics were not taken into account when classifying groups. MRA Global sensitivity and specificity were 39.8% and 93.2%, respectively, when borderline results were considered as normal. For the GPS global, corresponding values were 71.1% and 69.5%. The weighted kappa coefficient for the agreement between MRA and GPS was only 0.34. No significant differences were reported on GPS values between normal subjects and patients with ocular hypertension.

The authors concluded that the GPS tended to be more sensitive but less specific than the MRA, and that the GPS would be advantageous over MRA in early glaucoma stage. An analysis of the ROC curves provided in the study, however, does not support this conclusion as when matched for the same specificity (at moderate or high values >0.8), MRA tended actually to be more sensitive than GPS. Studies such as the one by Moreno-Montanes are important to provide an initial assessment and comparison of these diagnostic parameters. However, when evaluating a study of diagnostic accuracy it is essential to appraise whether the test has been evaluated on a population that is representative of the one in which the test is going to be applied in clinical practice.

The source of normal subjects included in a diagnostic accuracy study needs to be clearly defined

The authors evaluated patients with well-defined diagnosis based on visual field loss. In these patients, the diagnosis is already confirmed by the repeatable defect on the visual field; therefore, there is no need for an imaging instrument to assist in the diagnosis. Estimates of diagnostic accuracy obtained in this study are unlikely to be applicable to the situation of applying the HRT in the evaluation of patients suspected of having glaucoma in clinical practice. Although the authors also evaluated patients with ocular hypertension, this group is likely to contain a heterogeneous population that includes normal subjects with only high pressure but no glaucoma damage as well as patients with early glaucoma. Without longitudinal follow-up to precisely define their diagnoses, diagnostic accuracy estimates obtained in this group are of little clinical relevance. Finally, the source of normal subjects included in a diagnostic accuracy study needs to be clearly defined. This is of fundamental importance when considering application of the test for diagnosing disease in clinical practice or for screening purposes.

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