Therapeutic drug monitoring in dried blood spots using liquid microjunction surface sampling and high resolution mass spectrometry.

Dried blood spots (DBS) are a versatile and stable tool for direct clinical blood analysis. Ambient high-resolution mass spectrometry is emerging as a method of choice for their quantitative analysis, for instance in therapeutic drug monitoring. Here, we coupled liquid microjunction surface sampling technology, a so-called Flowprobe, with an Orbitrap mass spectrometer and demonstrated the utility of this set-up for direct quantification of multiple drugs in DBS on filter paper. A three-layer set-up that we had introduced earlier enabled introduction of internal standards into DBS. We furthermore took an established point-of-care test system a step further and analyzed disposable test fields for blood glucose monitoring also for Flowprobe-based acetaminophen screening without additional sample preparation. Using as little as 2 μL blood, the method had an LOD of 1 μg mL(-1) (coefficient of variation of ≤15%) and acetaminophen recoveries of 82 to 119% for blinded samples, as assessed by LC-MS/MS. Half an hour after ingestions of a single 1000 mg acetaminophen dose, indistinguishable drug levels were measured in three healthy volunteers by LC-MS/MS and Flowprobe-Orbitrap MS analysis of DBS. Flowprobe analysis of DBS was 6- to 100-times more sensitive than corresponding desorption electrospray ionization MS analysis for four drugs. For instance, the LOD for salicylic acid analysis was 0.07 ng mL(-1) with Flowprobe measurement. Furthermore, we showed that multi-component analysis of five different substances, which may mimic polypharmacy in diabetes patients, in one blood sample for screening purposes was feasible. Taken together, our study suggests that microjunction surface sampling of DBS on filter paper and disposable point-of-care test fields may be developed into routine methods for near-patient multi-compound therapeutic drug monitoring that may advance blood screening analysis for patients with polypharmacy.