Daily IOP changes undermine measurement, medication

By Sarah Guy

Day-to-day variability significantly undermines the precision of intraocular pressure (IOP) measurement and the estimation of medication effectiveness in glaucoma patients, report researchers.

Despite intraclass correlation coefficient (ICC) analysis indicating "good to excellent" correlation over a 7-week period, before- and after-treatment IOP measurements fell within a range of 20% of the mean, show their study results.

Standardizing the time of day to measure IOP did not improve correlation, remarks the research team in the British Journal of Ophthalmology.

"When a patient is started on treatment, and the IOP is checked at the same time on a later day, the assumption is that any observed drop in IOP is attributable to the treatment. However, a variability of ±20% is sufficiently large to mislead the clinician into thinking a significant response has been achieved when in fact the treatment has had no effect, or vice versa," say Alan Rotchford, from Gartnavel General Hospital in Glasgow, UK, and co-researchers.

Thirty treatment-naive individuals (mean age 64.4 years) with primary open-angle glaucoma or ocular hypertension underwent ophthalmologic examination and Goldmann applanation to measure IOP over a 7-week period.

The participants' IOP was measured at 08:00, 11:00, and 16:00 at each of the weekly visits, and after visits three and five, patients received treatment with travoprost (0.004%); first in the eye with the higher IOP, then in the remaining eye.

For the first three weeks, ICC indicated nominally good to excellent agreement, report Rotchford and colleagues. Repeatability of IOP measurements did not differ significantly by time of day or duration of treatment, they add, with coefficients of repeatability (CR) of 4.3 mmHg and 4.5 mmHg at 08:00 between weeks 1 and 2 and weeks 3 and 4 of treatment.

Repeatability of IOP was also independent of patients' particular diagnosis, note the researchers.

However, further analysis of individual patients' coefficients of variation indicated that 95% of measurements lay within ±20% of the mean IOP value at 08:00, giving a range of ±3.8 to ±4.9 mmHg between the three timepoints during no-treatment weeks, and a range of ±2.9 mmHg to ±3.4 mmHg in the weeks on treatment.

This level of uncertainty in IOP measurements is "certainly clinically important," and should be taken into consideration when interpreting isolated IOP results, the authors suggest.

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