Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is relatively common in clinical laboratories for applications in toxicology, therapeutic drug monitoring, endocrinology, and biochemical genetics. The greatest advantage of LC-MS/MS is probably the ability to isolate a specific mass to charge (m/z) ion and fragment it to another specific m/z ion, allowing high analytic specificity. Conversely, immunoassays have the advantages of shorter turnaround time and less overall expense than LC-MS/MS; however, the antibodies used in immunoassays are not always specific and false positive results are not uncommon. In addition, immunoassays are often less sensitive than LC-MS/MS.
Newer uses of LC-MS/MS in clinical labs include quantitation of peptides and proteins (recently reviewed by Scherl, reference 1). However, these applications of LC-MS/MS can be difficult to develop, so widespread adoption will likely take several years.
One current example of using LC-MS/MS for protein measurement is thyroglobulin (Tg) quantitation (2,3). This application is used to monitor patients who have had thyroid cancer and thyroidectomy. The reason for this advancement in Tg testing? - One of the known problems with using immunoassay for Tg measurement is that 10 -30% of patients being tested will have autoantibodies to Tg (TgAb). The presence of TgAb prevents accurate quantitation of Tg by causing negative interference, meaning that patients with cancer recurrence can be missed. Because methods for measuring Tg by LC-MS/MS allow for accurate quantitation of Tg even in the presence of TgAb, this approach provides a more accurate and reliable result. Thus, Tg measurements by LC-MS/MS fill a needed gap in clinical testing. But should this approach be used for everyone, or is a reflex testing strategy more appropriate?
Recently, we looked at thyroglobulin orders in our lab and performed cost-analysis by using an average list price from three different labs that offer Tg testing by LC-MS/MS. The majority of orders (89%) utilized a reflex ordering option in which the patient sample is first tested for TgAb. If samples are negative for TgAb, then Tg is measured by immunoassay. But if samples are positive for TgAb, then Tg is measured using LC-MS/MS. Cost savings for utilizing the reflex option instead of sending all samples directly to LC-MS/MS was about $40 per patient sample. While this may seem modest, one must remember that Tg is monitored regularly for patients who have had thyroid cancer. Further, since the majority of patients are TgAb negative, the savings can add up! How much money would be saved in your patient population by utilizing a reflex strategy?
Clinically, for TgAb-positive patients, LC-MS/MS is the preferred method. But questions remain. Is testing Tg with LC-MS/MS a waste of healthcare dollars? In our studies of TgAb negative patients, LC-MS/MS and immunoassay for Tg gave equivalent results. At present, LC-MS/MS does not offer improved analytic sensitivity; in fact, the immunoassay is currently more sensitive. Are there instances of false negatives in the TgAb assay? How often? What about patients whose TgAb levels fluctuate around the cutoff, sometimes being monitored with immunoassay and sometimes with LC-MS/MS? How often does this occur? These are questions clinicians and laboratorians may want to consider.
- Methods 2015, 81, 3-14.
- Kushnir, MM et al. Clin Chem 2013, 59, 982-990.
- Clarke, NJ et al. J Investig Med 2012, 60, 1157-1163