The aim of this study is to determine whether the boosting of darunavir by cobicistat results effectively in lower darunavir concentrations in the CSF as compared to a boosting by ritonavir. The study will be performed in HIV infected patients presenting HIV associated neurocognitive disorders (HAND) and requiring a lumbar puncture for clinical reasons. In addition, the patients will be only eligible if the are treated or if they qualify for a darunavir/ritonavir (800/100 mg once daily) containing regimen. Darunavir concentrations will be measured simultaneously in the CSF and plasma (CSF/plasma ratio) first with the ritonavir boosting and subsequently with the cobicistat boosting.