Defining real change in prism-cover test measurements.
OBJECTIVE
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.


METHODS
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.
Department of Ophthalmology, Mayo Clinic, College of Medicine, 200 First Street SW, Rochester, MN 55905, USA. holmes.jonathan@mayo.edu
Jonathan M
JM
Holmes
David A
DA
Leske
George G
GG
Hohberger
D020434:Abducens Nerve Diseases / Q000175:diagnosis*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D009801:Oculomotor Muscles / Q000473:pathology*; D011446:Prospective Studies; D013285:Strabismus / Q000175:diagnosis*