To evaluate the effect of optical coherence tomography (OCT) grid decentration on macular thickness measurements in healthy subjects and patients with diabetic macular edema.

In this prospective study, 3D spectral-domain OCT images of 51 eyes with clinically significant diabetic macular edema and 29 healthy eyes were assessed. In each eye, the macular Early Treatment Diabetic Retinopa- thy Study grid was evaluated for decentration. After grid adjustment, changes in central subfield thickness (CST) and central subfield volume (CSV) measurements were recorded. Changes >1 μm and >8.5 μm in CST were con- sidered as grid decentration and clinically significant grid decentration, respectively.

Grid decentration was found in 10 normal eyes (34.6%) and 32 clinically significant macular ede- ma (CSME) eyes (62.7%, p = 0.01). Clinically significant CST changes were found in 3 normal eyes (10.3%) and 23 CSME eyes (45%, p = 0.001). The mean change in CST after grid adjustment was 6.4 ± 5.8 μm and 24.7 ± 27 μm in normal and CSME eyes (p = 0.04). Age, sex, and CST had no statistically significant effect on grid decentration in normal and CSME eyes. Best-corrected visual acuity was significantly worse in eyes with grid decentration (0.75 ± 0.55 vs 0.42 ± 0.22 logMAR, p = 0.02) and clinically significant grid decentration (0.81 ± 0.6 vs 0.45 ± 0.27 logMAR, p = 0.01). Change in CSV was detected in 3 normal eyes (10.3%) and 24 CSME eyes (47%, p = 0.001).

Grid decentration occurs in a large number of OCT thickness maps and leads to significant error in macular thickness measurements. The effect is more prominent in patients with CSME.