The target study of BAs used an Agilent 1290 LC system (Waters C18 column, 1.7 μm, 2.1×150 mm) coupled to a Sciex 7500 QQQ MS in multiple-reaction monitoring (MRM) mode with (-) detection, as described by Han et al. . A 10 μM standard mixture (94 BAs in total, Table ) was prepared in an IS solution of UDCA-D4 in MeCN. Next, 60 μL of each CSF sample was mixed with 140 μL of the IS solution, vortexed, sonicated and centrifuged, then dried and dissolved in 40 μL of 50% MeCN. 15 μL per sample were injected in the LC-MS system. BA concentrations were calculated by interpolating the constructed IS calibrated linear-regression curves of individual BAs, with the peak area ratios measured from injections of the sample solutions.