Only assess glaucoma drug efficacy after multiple testing

Nov 30 2012

By Sarah Guy, medwireNews Reporter

Intraocular pressure (IOP) measurements may be insufficient to determine treatment effectiveness, even when repeated multiple times, show study results.

Using a single set of IOP readings in eyes before and after treatment initiation, the precision with which a therapeutic effect was able to be detected lay within 73.2% of the mean effect. Precision among a set of eight readings in these eyes was narrower, at 25.9%, but still "relatively poor," say the researchers.

Indeed, in one pair of visits before and after initiation of glaucoma treatment, the reduction in IOP would need to exceed 7 mmHg to achieve 95% certainty that there was any therapeutic benefit at all, report the researchers.

"This clearly represents a significant level of uncertainty which is likely to result in incorrect clinical decision making, that is, to assume incorrectly that the medication is working or, conversely, to switch medications unnecessarily," say Alan Rotchford (Gartnavel Hospital, Glasgow, UK) and team.

The team assessed 30 untreated individuals with primary open-angle glaucoma or ocular hypertension who had mean IOP measurements of more than 21 mmHg at an initial examination.

Participants completed seven further examinations at 3-week intervals, where IOP measurements were taken at three time points during the day. The first three examinations were without treatment, travoprost medication was initiated at the fourth examination, and treatment continued for the remainder of the study.

The average therapeutic effect between the three pretreatment and three posttreatment examinations varied between 6.7 and 7.9 mmHg, report Rotchford et al in the British Journal of Ophthalmology.

The team calculated coefficients of repeatability (CR) for the three sets of pre- and posttreatment measurements. A CR for right eyes at 8:00 h of 7.8 mmHg indicated that 95% of repeated measurements taken at this time should match this figure. Therefore, after starting treatment, any reduction in IOP of less than 7.8 mmHg would be indistinguishable from measurement error, explain the researchers.

The corresponding difference in IOP from within-subject sets of paired measurements would be expected to lie at a slightly lower measurement of 5.5 mmHg.

The precision of this measurement improved over time, however, dropping to 4.0 mmHg with two sets of pre- and posttreatment IOP readings, and 2.0 mmHg with eight sets.

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