Reproducibility of Manifest Refraction in Patients With Keratoconus Compared With Healthy Subjects: A Prospective Cohort Study

Purpose: To measure and compare the inter-examiner reproducibility of manifest refraction in patients with keratoconus. Design: Prospective, inter-examiner reliability analysis comparing cases and controls. Methods: Patients with keratoconus (KC) and healthy subjects had undergone manifest refraction by the same 2 skilled optometrists; each was masked to the refraction of the other, on the same day. The KC group comprised patients with KC, who were recruited from the cornea clinic. The control group consisted of healthy individuals who wore spectacles and did not have KC. Participants for the control group were recruited from the clinic's staff, including doctors, technicians, nurses, and medical students. The study took place in 1 tertiary medical center in Israel from April 2021 to May 2022. The results of the manifest refraction and achieved corrected distance visual acuity (CDVA) were compared between groups. Results: A total of 120 eyes of 60 patients were enrolled in the study, 30 in the keratoconus group and 30 in the control group. A difference of 0.67 ± 0.83 diopters (D) and 0.19 ± 0.21 D in the absolute manifest cylinder was observed between the optometrists in the keratoconus (95% LoA, −0.96, 2.30) and control group (95% LoA, −0.22, 0.61), respectively (P < .001). Multivariate analysis revealed a 22-fold higher likelihood of an error exceeding 0.75 D in cylinder measurements (odds ratio = 22.24; 95% CI = 2.39-206.95) and a 10-fold likelihood of a difference of at least 1 row on the Snellen chart for corrected distance visual acuity (odds ratio = 10.32; 95% CI = 2.39-44.44) in the KC group. Conclusions: When compared to healthy subjects, patients with KC exhibited greater variability in manifest refraction. This discrepancy has the potential to influence the decision-making process when managing patients with KC.