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. 2008 Feb;145(2):381-5.
doi: 10.1016/j.ajo.2007.09.012. Epub 2007 Nov 28.

Defining real change in prism-cover test measurements

Jonathan M Holmes  1 , David A Leske, George G Hohberger
Affiliations

Defining real change in prism-cover test measurements

Jonathan M Holmes et al. Am J Ophthalmol. 2008 Feb.

Abstract

Purpose: To describe the interobserver test-retest variability of both simultaneous prism and cover testing (SPCT) and alternate prism and cover testing (APCT) in horizontal deviations, and to calculate 95% limits of agreement that might be used to define real change.

Design: Prospective cohort study.

Methods: Twenty-three patients with sixth nerve palsy and three controls were independently examined by two experienced strabismus surgeons. SPCT and APCT were performed at distance and near fixation. Test-retest variability and agreement between tests were evaluated using Bland-Altman plots and 95% limits of agreement were calculated.

Results: For SPCT, the 95% limits of agreement half-widths were 6.3 prism diopters (pd) at distance fixation and 6.9 pd at near. For APCT, the 95% limits of agreement half-widths were 10.2 pd at distance and 9.2 pd at near.

Conclusions: Based on 95% limits of agreement half-widths between two examiners, a change in strabismus measurements of less than 10 pd may be attributable to test-retest variability. Changes of 10 pd or more are likely to represent real change and might be used as the threshold for management decisions.

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Figures

Figure 1
Figure 1
Test-retest variability of simultaneous prism and cover test at distance fixation. Bland-Altman plot, showing 95% limits of agreement half-widths (± 1.96 × standard deviation of the difference) to be ± 6.3 pd. Dotted line represents the mean of test-retest differences; 0.8 pd ± 3.2 pd. There appears to be no relationship between magnitude of deviation and test-retest variability. pd = prism diopters
Figure 2
Figure 2
Test-retest variability of simultaneous prism and cover test at near fixation. Bland-Altman plot, showing 95% limits of agreement half-widths (± 1.96 × standard deviation of the difference) to be ± 6.9 pd. Dotted line represents the mean of test-retest differences; −3.0 pd ± 3.5 pd. There appears to be no relationship between magnitude of deviation and test-retest variability. pd = prism diopters
Figure 3
Figure 3
Test-retest variability of alternate prism and cover test at distance fixation. Bland-Altman plot, showing 95% limits of agreement half-widths (± 1.96 × standard deviation of the difference) to be ± 10.2 pd. Dotted line represents the mean of test-retest differences; 0.2 pd ± 5.2 pd. There appears to be no relationship between magnitude of deviation and test-retest variability. pd = prism diopters
Figure 4
Figure 4
Test-retest variability of alternate prism and cover test at near fixation. Bland-Altman plot, showing 95% limits of agreement half-widths (± 1.96 × standard deviation of the difference) to be ± 9.2 pd. Dotted line represents the mean of test-retest differences; −3.9 pd ± 4.7 pd. There appears to be no relationship between magnitude of deviation and test-retest variability. pd = prism diopters
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References

    1. Holmes JM, Hohberger GG, Leske DA. Photographic and clinical techniques for outcome assessment in sixth nerve palsy. Ophthalmology. 2001;108:1300–1307. - PubMed
    1. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1:307–310. - PubMed
    1. Johns HA, Manny REF K, Hu YS. The intraexaminer and interexaminer repeatability of the alternate cover test using different prism neutralization endpoints. Optom Vis Sci. 2004;81:939–946. - PubMed
    1. Rainey BB, Schroeder TL, Goss DA, Grosvenor TP. Inter-examiner repeatability of heterophoria tests. Optom Vis Sci. 1998;75:719–726. - PubMed
    1. Holmes JM, Droste PJ, Beck RW. The natural history of acute traumatic sixth nerve palsy or paresis. J AAPOS. 1998;2:265–268. - PubMed

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