We read with great interest the editorial by Profs Rakowsky, Papamichael and Cheifetz in response to our recently published study on therapeutic drug monitoring (TDM) of subcutaneous CT-P13. 1,2 We agree with the vast majority of the comments made. 3 Indeed, our study is a first step in demonstrating whether TDM is useful in monitoring subcutaneous biotherapies. It is now necessary to establish predictive factors of non-response for subcutaneous anti-TNF agents and to evaluate them in a proactive interventional study based on the newly established cut-off. It is likely that the immunogenicity conferred by this route will be relatively low and that we are unlikely to have to evaluate combotherapy regimens. Our cohort was highly selected by discarding pejorative factors known to increase the clearance of the biotherapy (high BMI, low albumin and high inflammation status). It seems important to study the pharmacokinetics in these specific populations. It is also essential, as the editorialists noted, to analyse using drug-tolerant assays, the early development of antibodies after subcutaneous treatment and particularly in patients already immunised after intravenous IFX. Switch to subcutaneous formulations has been associated with the reduction of anti-IFX antibodies in patients already immunised and/or with infusion reactions. 4 Moreover, in case of loss of response where we are likely to optimise dosing, it may be necessary to measure and compare the pharmacokinetics of the 120 mg ew dose vs the 240 mg eow dose as shown with adalimumab. 5 Finally, the contribution of rapid point of care (POC) tests could facilitate the TDM of these subcutaneous treatments, but it seems crucial to first establish and validate thresholds specific to these devices given that POCs tend to overestimate biotherapy trough levels as has been demonstrated with the intravenous agents. [6][7][8]