In our clinical experience, more than half of patients do not present a complete response to biologic drugs, or drug loses its efficacy over time. Plasma determinations of drug and anti-drug antibodies levels are an objective tool for optimisation in these patients; however, established therapeutic ranges are not suitable, so the objective of this study was to study these patients and optimise their healthcare. We have made a retrospective, observational study, using data of plasma levels of drugs and anti-drugs antibodies of infliximab, adalimumab or Etanercept, we summarise all data and make a study of sensitivity, specificity, positive and negative predictive value on current therapeutic ranges. We have found a statistically significant association between subtherapeutic levels and therapeutic failure in psoriasis treated with infliximab and adalimumab. New ranges were found with higher sensitivity than the established ones, we propose 2–10 µg/mL therapeutic range for infliximab, 3–11 µg/mL for adalimumab, and 1–7 µg/mL for etanercept. In conclusion, levels of drug and anti-drug antibodies are a decisive tool for predicting therapeutic response. The current therapeutic ranges may have minimum values that are excessively high, owing to which lowering them significantly increases the sensitivity of the test in all cases, and negative predictive value in the case of etanercept. Further prospective studies are needed to prove the usefulness of these new ranges.