Matrix effects in therapeutic drug monitoring surveys : proposed protocol to identify error components and quality improvement opportunities

Therapeutic drugs are not endogenous compounds. Therefore, differing reactivities between proficiency testing and patient samples have potential mechanisms beyond those attributable to alterations of the serum protein base, stabilizing materials, and other manufacturing components. The therapeutic drug monitoring proficiency sample is the ultimate in polypharmacy. Cross-reactivities may be uncovered that may or may not occur with any frequency in patient samples. Drugs are present in therapeutic drug monitoring proficiency samples in the absence of their metabolites, which may alter interactions with the assay systems