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We thank Huang and associates for their interest in our recent article. By “within-rater repeatability and between-rater reproducibility,” we presume that the correspondents are referring to intraobserver and interobserver repeatability. As clearly outlined in the Introduction, the study was intended to be purely a validity study, rather than a reliability study. In the study, each instrument was operated intentionally by a single examiner; therefore, interobserver variability was not a relevant outcome measure.


Power calculations determined a sample size of 24 subjects for 80% power to discriminate a difference of 3 mm Hg. The sample size therefore had sufficient statistical power to detect clinically relevant differences in intraocular pressure (IOP) measures.


The Goldmann applanation tonometer (GAT; Haag-Streit USA, Mason, Ohio, USA) has long been accepted as the gold standard for IOP measurement and is reported to have acceptable repeatability in normal eyes and in those that have undergone penetrating keratoplasty (PK). However, it should be noted that the reported standard deviation for GAT IOP measurement in eyes that have undergone PK has been reported to be ± 4 mm Hg, giving a 95% confidence range of almost 8 mm Hg, which is clearly less than ideal for clinical practice. Indeed, for irregular or edematous corneas, the degree of error with any type of tonometer can be expected to be higher than normal, and therefore warrants caution in such cases. The study clearly did not endorse routinely using other devices interchangeably with GAT in eyes that have undergone PK; rather, it suggested that in cases where GAT measurements cannot be obtained (a common occurrence in clinical practice), Tono-Pen (Mentor O & O Inc, Santa Barbara, California, USA) or Pascal dynamic contour tonometer (PDCT; Swiss Microtechnology AG, Zurich, Switzerland) are 2 alternative techniques that have the greatest degree of agreement with GAT measurements. Pascal dynamic contour tonometer may have the added advantage in assessing IOP in eyes that have undergone PK of being independent of central corneal thickness, as highlighted in the earlier work of our group. As always, consistent use of 1 technique and 1 instrument, in the knowledge of known limitations, will always provide the most useful measurements of IOP for intrapatient and interpatient comparison—because even instruments with similar repeatability cannot be used interchangeably.

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Jan 12, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on Reply

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