Author response: retinal thickness measurement in OCT.

which is the case only if the reference planes do not change position during the time interval or if the difference between the two planes is negligibly small. It may be that for some diseases, such as diabetic retinopathy and other retinal vascular diseases, that only the inner retinal layer moves and the reference planes (e.g., the tips of the outer segment or the first bright RPE reflection) do not move, in which case the linear difference between measurements will be machine invariant [as (b2 b1) is zero and (a2 a1) is zero]. However, in florid cystoid macular edema, where the photoreceptor layer itself detaches from the RPE or where the position of the RPE layer moves (e.g., RPE detachment reducing after intravitreal ranibizumab), then the linear difference between different machines will not be invariant. In summary, retinal thicknesses derived from OCT measurements are interval data. There is no natural zero, and the numerical value of the thickness always depends on the choice of reference plane. The ratio of two thickness measurements made at different time intervals and the logarithm of this ratio varies from machine to machine. The linear difference between thicknesses at these different times may be machine independent, but only if the position of the reference plane remains unchanged with the passage of time. We therefore warn the retinal community against performing studies in which different OCT machines are used and ratios of thicknesses are calculated and treated as comparable. Such comparisons would introduce errors in the data that are collected. We hope that our analysis explains this concern. Sahar Parvizi Nigel P. Davies