AQP4 and neuromyelitis optica: In the first of these two studies, Tortorella et al. 2011 used high performance liquid chromatography-mass spectrometry/more selective (HPLC-MS/MS) to quantify NAA from CSF and sera from 32 NMO patients (13 AQP4-Ab+ and 9 AQP4-Ab−), and compared these results with 48 RRMS and 76 healthy cohorts [88].