Barriers to Wider Use of TDM
“The biggest barrier with TDM is simply just a lack of what to do with the data,” Balevic said. “The clinician needs clear-cut guidance on what to do with the drug level. So, in other words, what is the target concentration for the drug? And if that target is not the goal, how should that dose be adjusted?”
The optimal drug levels, particularly for the older conventional synthetic DMARDs, simply have not been validated by clinical trials, he said.
“Different studies may report different target drug levels, and this could be due to different underlying population, or a different matrix — a measure of whole blood vs plasma — or even the timing of the sample,” he said. Balevic led a pharmacokinetic study earlier this year that proposed an algorithm for determining the number of missed hydroxychloroquine doses.
“This really goes back to the clinician needing to draw on a lot of pharmacology training to interpret the literature,” Balevic added.
That gets to the need for more education among rheumatologists, as Brun pointed out. “The physician needs to be educated about therapeutic ranges, when to assess concentrations of drug antibodies, and how to react to the results,” Brun said.
Which ADAs to identify is also problematic. “For antidrug antibodies, it’s especially challenging because there are so many assay formats in use, and it’s a bit complicated to analyze these antidrug antibodies,” Brun said. “There’s no consensus on what calibrators to use, and there’s no standardization of how to report the results, so you can’t really compare results from different assays. You need to know what your laboratory is using and how to interpret results from that particular assay, so that’s a challenge.”
Variability in drug tolerance also exists across assays, Wallace noted. “One of the challenges that have come up in the discussion of therapeutic drug monitoring is understanding what the target level is,” he said. “Defining what the target level might be for a specific condition is not something that’s well understood.”
Breaking down the science, he noted that an ADA can bind to a monoclonal antibody, forming an immune complex that avoids detection. Drug-sensitive assays may detect high concentrations of ADAs but miss low or moderate concentrations. Drug-tolerant assays may be more likely to detect low concentrations at ADAs, but the clinical significance is unclear.
Cost and Patient Trust as Barriers
“The costs vary a lot from assay to assay,” Brun said. “Some commercial assays can be really expensive.” In Norway, a dedicated lab with its own in-house assays helps to keep costs down, she said.
But that’s not the case in the United States, where insurance coverage can be a question mark, Shivani Garg, MD, a rheumatologist at the University of Wisconsin (UW)-Madison and director of the UW-Madison Health Lupus and Lupus Nephritis Clinics, told Medscape Medical News. “A lot of insurances are covering therapeutic drug monitoring, but for the high-deductible plans, there should be a way to offer these important tests to patients at a lower cost or figure out a way for coverage for those patients so that they can show that there are benefits of therapeutic drug monitoring without being sent a really big bill,” she said.
UW Health
Dr. Shivani Garg
Patient trust could be another potential barrier, Garg said. “A lot of times there is not shared decision-making involved in why this test is being done, how those tests will help us as clinicians, and [patients’ understanding of] the use of the medicine,” Garg said.
“If the shared decision-making to build trust is not there, a lot of times patients worry that they’re being under surveillance or they’re being watched, so that might add to the lack of trust in the core issues that are critical threats to patients with chronic diseases because this is a lifelong partnership,” she said.
Convenience is another issue. “Particularly with mycophenolate levels, a lot of studies have used area under the curve, so getting an area under the curve level over a period of 12 hours would require several samples,” Garg said.
Testing protocols are also uncertain, Garg added. “A few data points ... are missing, like how we use the data over time,” she said. “If you do it for a given patient over several years, how often should you do it? How often do the levels fluctuate? How are the data used to inform dosing changes or monitoring changes?
“When those pieces are put together, then we are more likely to build up an intervention that clinicians can use in clinical practice, so they know how to order it and how frequently do it — every 6 months, 3 months, or every month. And then, over a period of time, how to adjust the dosing. That’s the big question.”