To assess the impact of slab selection on quantitative measurements of choriocapillaris (CC) flow deficits (FDs) using optical coherence tomography angiography (OCTA).Cross-sectional study METHODS: Using a swept-source OCTA device, en face slabs to isolate the CC were first generated using the manufacturer's default setting: a 20 μm-thick slab starting 29 μm posterior to the centerline of the automatically segmented retinal pigment epithelium (RPE) band. The inner and or outer borders were then adjusted by 2 μm increments to generate CC slabs with a range of offsets relative to the center of the RPE band. FDs of the modified slabs were compared to that of the default slab.Twenty-seven eyes of healthy subjects (mean age; 42.0 years) were prospectively enrolled. FD% increased when the slab was shifted outward by ≥4μm and inward by 20μm (P < .05). Fifteen eyes (55.6%) showed large hypointense regions precluding quantification when the slab was shifted inward by 20μm. Those without hypointensity demonstrated a decrease in FD% when the slab was shifted inward by 10 to 18 μm (P < .05). When modulating slab thickness, CC FD% increased and decreased when the slab thickness became thinner or thicker by ≥8μm, respectively (P < .05).Quantitative CC parameters may be significantly influenced by small differences in the slab selection. Slab close to the RPE can be susceptible to segmentation errors. These findings highlight the importance of accurate, precise, and consistent slab definition to reliably generate quantitative CC metrics from OCTA.