Letter to the Editor regarding 'Impact of Continuous Renal Replacement Therapy with Polyacrylonitrile-Derived Filter on Caspofungin Concentration: A Retrospective Study'

polyacrylonitrile (PAN) filters [1].They report that total plasma concentrations in the PAN-CKRT group were not reduced-and in fact were higher-than in non-CKRT patients, and they interpret this as reassuring.While clinically attractive, several design and pharmacologic issues mean such reassurance should be strictly limited to the specific, late time-points and inclusion conditions they actually studied. FILTER-CHANGE TIMING VERSUS SAMPLINGA major limitation lies in the timing of sampling relative to filter changes.As Arrestier et al. themselves acknowledged, their measurements were restricted to the steady-state phase, and they noted that dedicated studies would be needed.We fully agree, but emphasize that this design choice has major implications: in their study, caspofungin levels were assessed only after several doses, never in close temporal proximity to a filter replacement.This matters because adsorption is front-loaded-it is greatest during the first hours of contact with a new membrane and then diminishes as the filter becomes saturated.Our in vitro work deliberately isolates this early phase: in a PAN model designed to track unbound drug, we observed near-complete removal of unbound caspofungin within 6 h of