Two essential tenets of higher education—the search for truth and improving the human condition—also apply to the successful implementation of clinical mass spectrometry. The promise of mass spectrometry in the clinical lab is the ability to provide accurate results in order to improve patient care.
For clinical laboratorians accuracy equates to truth. But how do we ensure that the results of laboratory-developed mass spectrometry assays are accurate? In order to be accurate, we need to have higher order reference measurement procedures (RMP) that establish concentrations of analytes in the appropriate biological matrix. These RMPs undergo extensive review before being recognized by the Joint Committee for Traceability in Laboratory Medicine. Laboratorians need to embrace and support the continued development of higher order reference methods that can be used to value-assign concentrations of specific analytes in various biological matrices.
An excellent example of this in practice is recent work led by the Centers for Disease Control and Prevention, the National Institutes of Health, the National Institute for Standards and Technology (NIST), the College of American Pathologists (CAP), and European collaborators to standardize measurements of 25-hydroxyvitamin D (25-OH D). One important result of this effort was the NIST standard reference material (SRM) 972a, a set of frozen human serum with well-characterized 25-OH D concentrations.
For practical purposes SRMs establish truth. As such, my lab at the University of California, San Diego used the NIST SRM 972a to assign concentrations of the commercially purchased 25-OH D calibrators that we use in routine practice. Our results demonstrated that the concentrations listed on the package insert were significantly different than values obtained using the SRM. As part of our ongoing quality assurance, we participate in the Vitamin D Standardization Program and the CAP accuracy-based 25-OH D survey.
Unfortunately, for the majority of MS-based laboratory developed tests, RMPs and SRMs are not available. Tacrolimus, an immunosuppressant commonly monitored in transplant patients, is a good example. As part of our method validation we compared tacrolimus values measured by MS with the currently used immunoassay. Concentrations measured by MS were on average 20% lower than those measured by immunoassay, with
some differing by 40%.
Physicians have been making therapeutic decisions based on immunoassay results for many years, but the data from my lab showed immunoassay results were widely discrepant with values determined by MS. As clinical laboratorians engage more with physicians it becomes readily apparent that they do care about accuracy (Clin Endo Metab 2004;89:520–4). Without an RMP the true concentration of tacrolimus is difficult to ascertain, but experiments can help elucidate some of the differences between assays.
One possible explanation for our observations is that metabolites of tacrolimus cross-react with the immunoassay but not the MS assay, so we supplemented immunosuppressant free whole blood with tacrolimus. If the difference between the two assays was due to metabolites, then both assays should obtain the same result when only the parent drug is present. This experiment showed much better agreement between the methods, but until an RMP for tacrolimus is approved we continue to search for truth—knowing that accuracy matters.