Peripheral instability gradient in macular thickness measurements across five optical coherence tomography systems: a prospective cross-sectional study of inter-device agreement and clinical monitoring risk

BACKGROUND: Cross-platform variability in optical coherence tomography (OCT) macular thickness measurements poses a clinically meaningful challenge for longitudinal patient assessment, particularly when patients are imaged on different devices over time. This study characterizes intra-device repeatability and inter-device agreement across five contemporary OCT systems, and introduces the Peripheral Instability Gradient (PIG)-the progressive deterioration in cross-device measurement concordance from the foveal center outward across ETDRS-defined macular zones. METHODS: In this prospective, cross-sectional study, 38 healthy adults underwent macular imaging with five OCT devices: HRA Spectralis (Heidelberg Engineering), BMIZAR (TowardPi, 400 kHz swept-source OCTA), TOPCON Maestro 2, NIDEK RS-1 Glauvas, and HUVITZ HOCT-1/1F. Macular thickness was measured in the central 1-mm ETDRS subfield and the 3-mm and 6-mm inner rings (superior, nasal, inferior, and temporal sectors). Three consecutive scans were acquired per device during a single visit by a single experienced operator. Intra-device repeatability was assessed using ICC(3,1) and repeated-measures ANOVA with Bonferroni correction. Inter-device agreement was evaluated using ICC(2,1), Bland-Altman analysis, and absolute percentage error (APE). HRA-anchored comparisons were used for directional bias interpretation, and all 10 direct pairwise device comparisons were additionally performed as cross-validation. RESULTS: All devices demonstrated excellent within-device repeatability (ICC > 0.85 across all regions). Inter-device agreement was strongly location-dependent, with PIG: ICC values declining from the central 1-mm subfield (HRA vs. HUVITZ: 0.942; HRA vs. NIDEK: 0.902) to the 6-mm ring (ICC < 0.40 for most pairs). This represents a relative decline of 40-67%. Central thickness ranged from 227.85 μm (TOPCON) to 290.73 μm (BMIZAR). Device-specific systematic biases were identified: BMIZAR overestimated HRA by a mean relative bias of 10.98%, and TOPCON underestimated HRA by a mean relative bias 13.65%, while HUVITZ (+ 2.60%) and NIDEK (+ 3.64%) showed smaller relative deviations (all biases as HRA minus comparator). Limits of agreement widened by a relative 30-50% from central to peripheral rings. Median APE was lowest for HUVITZ (2.39%) and highest for TOPCON (12.23%). CONCLUSIONS: Macular thickness measurements are highly repeatable within individual OCT systems but are not interchangeable across devices, with disagreement amplifying progressively toward peripheral macular zones. The Peripheral Instability Gradient demonstrates that cross-device follow-up carries the greatest risk in parafoveal and perifoveal monitoring. To ensure reliable longitudinal assessment, clinicians should maintain device consistency; when switching is unavoidable, device-specific baselines should be established and calibration adjustments applied.