Retinal thickness measurement using OCT.

We read with interest the paper published by Payne et al., in the November 2011 issue, in particular the statement that the ratio of OCT thicknesses measured at different time points is machine invariant because of the cancellation of the baseline thickness. We believe that this statement is incorrect, because of the nature of the calculation of retinal thickness using OCT. We present the reasons below. The retinal thickness is the distance between the inner retinal surface and the reference plane. This distance represents interval data, as there is no natural zero, and the retinal thickness assumes a numerical value only when the reference plane is defined. As a result, the retinal thickness value contains information about the reference plane for each particular machine. Consider two OCT machines A and B, each of which measures from different reference planes, a and b, respectively. Let plane c be the position of the inner retinal plane and the retinal thickness d be measured at two different times, t1 and t2. During this time interval, let the position of the retinal layers change because of the effect of disease or intervention (Figs. 1A, 1B). If the ratio of two measurements performed at different times is independent of the OCT machine used, then these values will be equal in the same individual. For machine A we have