Diagnostic accuracy of the standard and alternative periodic leg movement during sleep indices for restless legs syndrome

Objective To evaluate the diagnostic accuracy of the standard periodic leg movement during sleep (PLMS) index, a recently introduced alternative one, and the periodicity index (PI) for restless legs syndrome (RLS). Subjects and methods A total of 107 patients with RLS were retrospectively identified and included (47 males and 60 females, mean age 56.9 years), along with 48 normal controls (24 males and 24 females, mean age 52.0 years). The standard PLMS index, the alternative PLMS index, and the PI were calculated from polysomnographic recordings. In order to establish the best performing threshold for the diagnosis of RLS, sensitivity and specificity were calculated with a series of threshold values ranging from 5 to 20/hour for the two PLMS indices, and from 0.2 to 0.9 for the PI. Results The optimal threshold was found to be between 15 and 16/hour for the standard PLMS index, close to 13/hour for the alternative index, and around 0.5 for the periodicity index. With these thresholds, all methods showed high values of sensitivity and specificity, with corresponding high positive and negative predictive values and accuracy; the receiver operating characteristic (ROC) curve areas were similar, and the Kappa coefficient of agreement indicated a ‘very good’ agreement between the PLMS indices. Conclusions This study assessed the cut-off values of the standard (15–16/hour) and alternative (13/hour) PLMS indices, and of the PI (0.5). The diagnostic accuracy of all indices for RLS vs controls was found to be acceptably high, with the new alternative PLMS index performing slightly higher than the other two indices.