Sir,

The authors of the letter made several points regarding the validity of our results. We agree that a correlation coefficient plot does not necessarily exclude systematic bias or disagreement between measurements obtained by the two methods being evaluated. This is the reason for quantifying agreement using the Bland–Altman graphical method.1 In our paper we elected to use analogue measurements on the horizontal axis as this was regarded as the gold standard.2 Re-plotting the graph using an average of analogue and digital on the horizontal axis did not make any difference to the limits of agreement.

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