BlandeAltman plots were generated to validate the
agreement between the measured 24-h urinary sodium
(gold standard) and each of the reported methods. The
difference between the gold standard and each alternative
method was calculated and plotted against the mean of
the two measurements. Overall, 95% limits of agreement
were calculated as the mean difference 1.96 SD, where
SD is the standard deviation of the differences in paired
measurements. Agreement between methods was also
examined by Pearson’s correlation coefficients and by
receiver operating characteristic (ROC) area under the
curve with a cut -off point of 100 mmol/l for sodium.
These levels were chosen as it is the upper tolerable limit
for sodium intake in Ireland and the UK.