Abstract: We included 10 studies (830 participants, 1387 eyes), published between 1998 and 2012. Prevalence of CSMO was 19% to 65% (median 50%) in nine studies with CSMO as the target condition. Study quality was often unclear or at high risk of bias for QUADAS 2 items, specifically regarding study population selection and the exclusion of participants with poor quality images. Applicablity was unclear in all studies since professionals referring patients and results of prior testing were not reported. There was a specific 'unit of analysis' issue because both eyes of the majority of participants were included in the analyses as if they were independent. In nine studies providing data on CSMO (759 participants, 1303 eyes), pooled sensitivity was 0.78 (95% confidence interval (CI) 0.72 to 0.83) and specificity was 0.86 (95% CI 0.76 to 0.93). The median central retinal thickness cut-off we selected for data extraction was 250 µm (range 230 µm to 300 µm). Central CSMO was the target condition in all but two studies and thus our results cannot be applied to non-central CSMO. Data from three studies reporting accuracy for detection of DMO (180 participants, 343 eyes) were not pooled. Sensitivities and specificities were about 0.80 in two studies and were both 1.00 in the third study. Since this review was conceived, the role of OCT has changed and has become a key ingredient of decision-making at all levels of ophthalmic care in this field. Moreover, disagreements between OCT and fundus examination are informative, especially false positives which are referred to as subclinical DMO and are at higher risk of developing clinical CSMO. Using retinal thickness thresholds lower than 300 µm and ophthalmologist's fundus assessment as reference standard, central retinal thickness measured with OCT was not sufficiently accurate to diagnose the central type of CSMO in patients with DR referred to retina clinics. However, at least OCT false positives are generally cases of subclinical DMO that cannot be detected clinically but still suffer from increased risk of disease progression. Therefore, the increasing availability of OCT devices, together with their precision and the ability to inform on retinal layer structure, now make OCT widely recognised as the new reference standard for assessment of DMO, even in some screening settings. Thus, this review will not be updated further.

Plain-language summary: Our review included 10 studies (830 participants, 1387 eyes) published between 1998 and 2012. Nine of these studies investigated the ability of OCT to diagnose CSMO. Study funding sources There were no overt declarations of potential conflicts of interest in terms of the manufacturer of the OCT device being involved in funding the research. Key results We found that OCT retinal thickness measurement is not sufficiently accurate to detect CSMO, involving the centre of the macula, using clinical fundus examination as the reference standard. Of 10 patients with diabetic retinopathy, 5 of whom have CSMO, 1 of 5 with no CSMO would be wrongly diagnosed as having CSMO, and about 1 of 5 with CSMO would be missed. However, researchers have found that disagreements between OCT and clinical examination occur because OCT can detect early, subclinical retinal thickening in people without CSMO and more advanced retinopathy. They suggested that such cases of subclinical macular oedema are followed more closely, since they are at increased risk of progression to CSMO. Furthermore, OCT is an essential tool to manage antiangiogenic therapy in patients with DMO and is believed by many to be a new reference standard for its diagnosis. Quality of the evidence Study quality was often unclear because of incomplete reporting or because it was at risk of bias. Specifically, this concerned how patients were selected in the study, who referred them and how, and exclusion of those for whom poor quality images were obtained. Furthermore, many studies included both patient's eyes, which is a problem in data analyses.