There is major concern regarding the pharmacokinetics of drugs under continuous renal replacement therapy (CRRT), including anti-infectious agents and more especially antifungal agents. From a regulatory viewpoint, only dialysis and filtration are considered meanwhile there is growing evidence that adsorption may also significantly alter the pharmacokinetics of anti-infectious agents. Adsorption results from a complex drug-filter interaction and might be considered an unexpected adverse effect induced by CRRT. Measurement of total plasma concentrations instead of the unbound, free, active concentrations in in vitro as well as in clinical studies hides this major adverse effect, which may jeopardise the therapeutic effect and even result in treatment failure. Noteworthy, minimal inhibitory concentrations (MIC) of anti-infectious agents are performed using solid and liquid medium without proteins testing only the antimicrobial activity of the free fraction of drugs. In a new in vitro model using crystalloid solution instead of blood, we report data supporting the assumption that the assessment of the disposition of the free fraction of caspofungin and micafungin unveils adverse effects of ST150® filter, which might eventually result in non-detectable drug concentrations and treatment failure. From a technical viewpoint, we conclude the measurement of the free fraction of drugs that largely bound to plasma proteins, including caspofungin and micafungin, should be considered instead of total plasma concentrations to assess all effects induced by filters used in CRRT.