Introduction
Optical coherence tomography (OCT) is one of the most valuable imaging techniques for retinal assessment. It can be used to investigate different layers of the retina, especially in the posterior pole. OCT is widely used to measure macular thickness. Changes in macular thickness occur in various diseases, including diabetes, retinal vein occlusion, age-related macular degeneration (AMD), and Alzheimer disease. Accurate clinical assessment of macular thickness is not possible without advanced imaging methods, which indicates the necessity of this technique in the retinal examination of systemic diseases. The repeatability and reproducibility of imaging instruments play a vital role in their credibility in clinical applications. Since cirrus spectral doman OCT (SD-OCT) is one of the prevalent imaging instruments used in Iran, we have decided to investigate its repeatability and reproducibility in measuring the macular thickness in normal eyes before and after pupil dilation. 
Methods
A total of 44 eyes from 44 healthy subjects in the age range of 20 to 50 years with refractive error less than -6 diopters and astigmatism less than -3 diopters were enrolled in this study. Patients with the following signs or symptoms were excluded from the reserach: having a history of ocular trauma or surgery and any ocular disease, best corrected visual acuity less than 20/20 in one of the eyes, refractive error greater than 6 diopters, astigmatism greater than 3 diopters in both eyes, closed angle in one of the eyes, any retinal abnormality in the macula of both eyes, abnormal examination of the anterior and posterior segment in one of the eyes, exposure to photosensitizers in the last 14 days, contraindication or sensitivity to 1% tropicamide drops, and unable to sit behind the OCT device. All subjects underwent the macular thickness measurement covering the central 6 mm ring using the Cirrus SD-OCT machine (512×128 protocol). An examiner performed 3 scans before and after the pupil dilation using tropicamide 1%. The intraclass correlation coefficient (ICC), 95% confidence interval (CI), and the Bland and Altman tests were used for analyzing the data and assessing the repeatability and reproducibility of the Cirrus SD-OCT macular thickness measurements.
Results
According to ICC, the repeatability of macular thickness was 0.95 (95% CI, 0.95 - 0.96). The mean macular thickness before and after pupil dilation was statistically significant in the nasal quadrant of the outer 6 mm ring (N2) and the inferior quadrant of the outer 6 mm ring (I2). The highest repeatability before and after pupil dilation was seen in the nasal quadrant of the inner 3 mm ring (N1) and the central ring (C), respectively. The measurement error in 3 repetitions was 11.27±0.18 μm and in repetitions before, and after the pupil dilation, the error was 12.57±1.97 μm. The limits of agreement for macular thickness measurements were –21.92 to –22.29, and the limits of agreement for macular thickness measurements before and after pupil dilation was -26.62 to 22.62, indicating approximately equal limits of agreement.
Discussion
The findings of this study indicate that the repeatability and reproducibility of the Cirrus SD-OCT tool for measuring macular thickness are acceptable. It can be used with acceptable repeatability and reproducibility in macular thickness measurement in 20- to 50-year-old patients with refractive error of fewer than 6 diopters and astigmatism of fewer than 3 diopters. Since the absolute value of the mean difference between the three measurements in the N2 and I2 regions was statistically significant, using a single method (i.e., whether to apply the eye drop) is preferable for follow-ups in cases where the measurements are needed within 3 to 6 mm in center of the macula.

Ethical Considerations
Compliance with ethical guidelines
Ethical approval for the study was obtained by the Gilan University of Medical Sciences (GUMS) Research Ethics Committee and the study adhered to the tenets of the World Medical Association Declaration of Helsinki. The participants were allowed to leave the study whenever they wished. Also, all participants were aware of the research process and their information was kept confidential.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Study concept and design: Yousef Alizadeh, Maryam Dourandeesh; Data Acquisition, analysis, or interpretation: Yousef Alizadeh, Maryam Dourandeesh, Ehsan Kazemnazhad Leyli, Nooshin Nimasa; Drafting of the manuscript: Yousef Alizadeh, Maryam Dourandeesh, Shila Kianmehr, Nooshin Nimasa; Critical revision of the manuscript for important intellectual content: Yousef Alizadeh, Maryam Dourandeesh, Hassan Behboudi, Shila Kianmehr; Statistical analysis: Yousef Alizadeh, Ehsan Kazemnazhad Leyli; Obtained funding: Yousef Alizadeh; Administrative, technical, or material support: Yousef Alizadeh; Study supervision: Maryam Dourandeesh.

Conflicts of interest
We state that our only interest is academic and we have no financial interest in this publication and our research is not funded by any organization. The authors declare no conflict of interest.



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