Disclosed is a method and apparatus for determining a concentration of a glycopeptide antibiotic containing a phenol moiety such as Vancomycin in a complex sample matrix by extracting the glycopeptide antibiotic from a metered portion of the complex sample matrix by exposing said metered portion to an extraction material having an affinity with the glycopeptide antibiotic; and exposing the extraction material to a metered portion of an eluent for releasing the glycopeptide antibiotic from the extraction material; and by determining a concentration of the glycopeptide antibiotic by adding a Gibbs reagent (2,6 dichloroquinone-4-chloroimide) to the metered portion of the complex sample matrix or the eluent; activating the Gibbs reagent and, after the reaction between the activated Gibbs reagent and the antibiotic has stabilized; detecting the reaction product of the activated Gibbs reagent and the antibiotic in said eluent; and determining the concentration of the antibiotic in the complex sample matrix from the detected reaction product. A method of designing a personalized drug administration regime using the thus obtained concentration is also disclosed.