Korean J Ophthalmol Vol.24, No.1, 2010 *P=0.041 Fig. 3. Comparison of baseline retinal thickness of panretinal photo- coagulation (PRP) (26 eyes) and No-PRP group (45 eyes). The two groups consisted of diabetic eyes with severe non-proliferative dia- betic retinopathy or proliferative diabetic retinopathy. Subsequent PRP was conducted for the PRP group within the 6 month follow-up period after baseline optic coherence tomography measurements due to disease progression. Asterisks indicate significant differ- ences between the two groups: *p<0.05. H=horizontal in macula; V=vertical in macula; S=superior to optic disc; I=inferior to optic disc; N=nasal to optic disc; D=circular con- centric to optic disc; CMT=central macular thickness at fovea pit. than the cutoff point. Shifting this cutoff point to 210 µm increased the specificity to 75.5%, but reduced the sensi- tivity to 61.8%. Among the 129 diabetics with DR, with a mean fol- low-up of 16.3±4.7 months (range, 6 to 28 months), 27 patients (20.9%) had been treated by PRP within 6 months of the baseline OCT scan. Zero percent showed mild NPDR, 2.6% moderate NPDR, 26% severe NPDR, and 61.9% PDR. We compared the baseline retinal thicknesses between the PRP group and No-PRP group. To prevent bias from the mild and moderate NPDR group that con- tained a relatively low proportion of treated patients and relatively thin retinal thicknesses, the comparison included only 71 patients with severe NPDR or PDR. The baseline retinal thicknesses of the PRP group (26 eyes) tended to be greater than that of the No-PRP group (45 eyes) in all OCT scans, but only the D scans were significantly differ- ent (independent t test, p=0.041) (Fig. 3). Table 4 shows the AUC values for each retinal thickness measurement for discrimination between the PRP and No-PRP group. The D scan produced the ROC curve with the highest AUC value (0.623, p=0.047) for discrimination between the two groups. Sensitivity and specificity at a cutoff point of 233 µm were 42.3% and 84.4%, respectively. The relative risk of the D scan≥233 µm for PRP was 1.941 (95% CI, 1.066 to 3.533). Discussion 20 In this study, the thicknesses of seven OCT scans in the macular and peripapillary retina were significantly greater in DR eyes than in normal eyes. Also detected were sig- nificant differences in retinal thickness in each of the scans when all DR groups were compared. The retinal thickness tended to be greater in more advanced stages of DR than in the early stages. The CMT measurements had the largest standard deviation among all OCT parameters through all stages of DR, and this variability of CMT miti- gated the statistical significance of the results. A key factor in the pathological process of diabetic retinal edema is the increased permeability of the BRB [17]. As evidenced by fluorescein angiography of diffuse macular edema, micro-vascular obstruction and the resultant ische- mia induce derangements in the integrity of the inner BRB [18]. The breakdown of the inner BRB results in accumu- lation of fluid within the extracellular space. Damage to the outer BRB at the level of the retinal pigment epi- thelium has also been suggested as a mechanism for the development of diffuse edema [19]. If the fluid resulting from inner or outer BRB damage is in excess of the amount that can be removed by the active pump mechanism, intraretinal and subretinal fluid continues to accumulate. Thus, if we quantify the increased retinal thickness resulting from pathological edema, we may determine the extent of altered BRB function in cases of DR [20]. Our finding that more advanced DR was associated with increased peripapil- lary retinal thickness is consistent with this assumption. The current statistical methodology indicates that our retinal thickness measurement with OCT can be employed as a diagnostic and prognostic factor in cases of DR [21]. Using ROC analysis, we evaluated the effectiveness of the measurement for DR staging, and PRP prediction within 6 months. AUCs between 0.50 and 0.70 were considered to represent low diagnostic accuracy, whereas AUC values in excess of 0.90 were suggestive of high accuracy [15]. Although other OCT parameters demonstrated comparably significant results, peripapillary circular thickness had the ROC curve with the highest diagnostic power (AUC, 0.756) with re- gard to discrimination between early (mild and moderate NPDR) and advanced (severe NPDR and PDR) stages of DR. Our results also indicate that peripapillary circular thickness in the PRP group was significantly greater than that of the No-PRP group upon baseline OCT, and that peripapillary circular thickness had the ROC curve with the highest degree of diagnostic power (AUC, 0.623) for discrimination between the two groups. Using a cut-off val- ue of 233 µm for the peripapillary circular thickness, severe NPDR or PDR patients with peripapillary circular thick- ness ≥233 µm upon baseline OCT can be considered to be 1.94 times more likely to require PRP within 6 months than patients with peripapillary circular thickness of <233 µm. This finding suggests that peripapillary retinal thickness may have prognostic value for identifying diabetic eyes at higher risk for imminent progression. The principal question is why peripapillary circular