Urine drugs of abuse screening is among the most widely used procedure in the clinical laboratory. Immunoassays are typically used as an initial screen often followed by confirmation with liquid chromatography coupled to unit resolution tandem mass spectrometry (LC-MS/MS). LC-MS/MS is commonly used with multiple reaction monitoring (MRM) where identifications are typically based on monitoring retention time and one or more pairs of precursor and product ion(s). A primary limitation of MRM based analytical platforms is that they can only detect a set list of targeted analytes and are unable to perform non-targeted screening. Recently, high resolution mass spectrometry (HRMS) such as time of flight-mass spectrometry (TOF-MS) has been proposed as an alternative for non-targeted drug screening.[3,4]
TOF-MS offers several advantages to the traditional low-resolution tandem MS, some of which include: greater specificity and the ability to retrospectively analyze data without the need to have a reference standard. Since most HRMS techniques capture full scan data across a range of mass to charge (m/z) values and essentially capture all the data all the time they have the ability to perform broad spectrum screening in an untargeted manner. As compared with clinical immunoassay drugs of abuse screening , HRMS has superior specificity with the ability to detect a wide range of compounds as a broad spectrum screen in an untargeted manner.
However; as with many clinical assays, rigorous method validation is required prior to implementing a HRMS in clinical practice. The number of false positives can be reduced by increasing the stringency of parameters required for a positive identification. Unfortunately as you increase the rigor for identifications, more false negative results are obtained. Another difficulty in this area has been the challenge of performing a rigorous validation.
From our previous study we determined that retention time, precursor ion, and at least one fragment ion information was necessary for positive identification of an analyte. However, our study was limited on the fact that we only looked at single spiked concentrations and patient comparison studies. It is important to perform within run and between run validation studies prior to patient comparison. In addition, studies such as matrix effects, sample stability and carry over should also be performed. The challenge in doing these studies is when you are dealing with multiple analytes (over 100) and have to test several concentrations (at least 3). This leads to very large data sets and lengthy experiments which require thorough investigation. Another challenge is the criteria for reporting results which can vary depending on the analyte in question. All of these challenges require skilled personnel, time to validate and investigate. So the question become is it worth pursuing this path to drug screening? At UC San Diego, TOF-HRMS has been of great help to the emergency department and the active clinical toxicology residency program. In such hospital settings where patients could be taking a variety of drugs not commonly detected by the standard immunoassay screening, the TOF-HRMS has been useful in identifying several new cases of abuse.[2,3]
- Chindarkar NS, Wakefield MR, Stone JA, Fitzgerald RL. Liquid chromatography high-resolution TOF analysis: investigation of MSE for broad-spectrum drug screening. Clin Chem. 2014; 60: 1115-1125.
- O’Connel CW, Schricker AA, Schneir AB, Metushi IG, Brigersdotter-Green U, Minns AB. High-dose loperamide abuse-associated ventricular arrhythmias. HeartRhythm Case Rep. 2016; 2(3):232-236.
- Schneir A, Metushi IG, Sloane C, Benaron DJ, Fitzgerald RL. Near Death From a Novel Synthetic Opioid Labeled U-47700: Emergence of a New Opioid Class. Clin Toxicol. 2017; 55(1):51-54