ED, lab views on point-of-care cardiac troponin

Charna Albert

June 2021—Point-of-care cardiac troponin testing got a fresh look last December when an emergency medicine physician and a clinical chemist came together to talk about the use of both conventional POC troponin assays in a high-sensitivity era and high-sensitivity POC troponin testing when it becomes available.

“When we talk about the emergency department and the need for point-of-care testing, it gets down to one key factor,” and that’s time, said Deborah Diercks, MD, professor and chair, Department of Emergency Medicine, UT Southwestern Medical Center, in a 2020 AACC virtual annual meeting session. Proponents of POC troponin testing argue that a rapid result will shorten length of patient stay. But in practice, “we don’t have robust data showing that length of stay actually declines,” she says.

Whether POC cardiac troponin results drive admission and disposition decisions is unclear, she says, because other tests go into the workup of a chest pain patient and each takes time. “So when we think about point-of-care testing, we have to consider what else we’re going to do to that patient.” If POC troponin does not change a patient’s trajectory or length of stay, “it probably doesn’t add as much value as we hope,” she says. “There’s fear in the high-sensitivity troponin era that because it detects so much myocardial injury, we can’t easily differentiate a patient who has myocardial injury from myocardial infarction without more information that may come further down in the patient’s evaluation.”

To Alan Wu, PhD, co-presenter in the session, if the POC device has conventional sensitivity, “that’s a no-brainer. We shouldn’t be using it.”

“You might get a fast answer, but it might not be the best answer. And you may still have to wait. So you get a 15-minute answer only to wait another two hours for the next specimen,” says Dr. Wu, chief of clinical chemistry and toxicology, San Francisco General Hospital, and professor of laboratory medicine, University of California San Francisco.

“We need to segregate our existing point-of-care devices,” he says, “which are not high sensitivity, from the ones that are going to come on the market soon—I hope—that will fulfill the requirement of high sensitivity.” Without high sensitivity, “we’re not getting early rule-out and the accelerated protocols that we can with the central laboratory.” The one-hour sample in, say, a zero-, one-, and three-hour rule-out protocol may not produce a positive result using a POC assay with conventional sensitivity. “But if you send it to the lab and use a high-sensitivity assay and it is able to rule out, then you have gained two additional hours. Because for point-of-care [testing], you have to wait three to rule out, whereas with the central lab, even though it takes 45 minutes longer to get the result, you can at least rule out on that specimen.”

“Until we have [POC] high-sensitivity troponin, FDA cleared, and we have documentation of its efficacy,” he says, “I’m not a proponent of doing it.”

UT Southwestern stopped running its POC troponin test after the central laboratory switched to high-sensitivity troponin two years ago, Dr. Diercks says. “We felt we would get more value from each test and much more information per test by having a consistent assay using a consistent platform. And the platform we chose was high-sensitivity troponin. It wasn’t a big deal for us to make that change because it came in the process of a well-defined algorithm on how we were going to implement new testing with high-sensitivity troponin.”

Will POC and central lab testing be used in tandem when both have high sensitivity claims? Dr. Wu foresees that to be the dilemma. “What will be key is standardization between the two assays. Even if the numbers don’t match, if we have two by two concordance, that’s going to be good. But the problem is these assays won’t have the same antibodies. They’re not going to be picking up the same subunits. I can envision a time when one is positive and the other is negative, and that’s going to add to confusion.”

Still, standardization will not be an issue for the majority of patients, he says. With a high-sensitivity POC test, “even if you don’t have standardization with the central lab, you’ll be able to rule out 60 percent of patients within one to three hours. They’ll never get a central lab result, and there is no issue with standardization. And we can deal with the additional 30 or 40 percent that do have prolonged decision-making by sending it to the lab, and perhaps needing a re-baseline from their initial sample.”

Proving that implementation of a high-sensitivity POC device for troponin improves outcomes will be difficult, Dr. Wu says, because a patient’s survival depends on so many other aspects of care—how fast the patient gets to the catheterization lab, for example. But it should be possible to document economic outcomes, he says, noting that serial collection time frames can affect lengths of stay.

“Economically, I don’t think there’s any question that high sensitivity can make a big difference, particularly in the rule-out.”

Dr. Wu has evaluated the central laboratory high-sensitivity assays and says they’re superior to the conventional assays in terms of assay interference. “That in itself would be a good reason to switch,” he says. “We have a better assay than we did before.” The FDA will demand the same quality from a POC assay, he adds: “The point-of-care devices will be held to the high standard we see in the central lab.”

Many hospitals today use POC troponin testing because the central lab’s turnaround time exceeds one hour. How has the TAT in Dr. Wu’s laboratory been improved such that the initial high-sensitivity troponin result is available before the one-hour second high-sensitivity troponin has to be drawn?

“It’s a struggle,” Dr. Wu says, “and you have to try to change some of your processing to put this as higher priority over something else. That’s difficult to do. Other people who have other needs are going to say, ‘My patient and my lab test take priority.’ So it’s a juggling act.”

He acknowledges the advantage he has of having spent much of his career in this field and carrying a little more weight at his institution than some others in his position might at their institutions. “So it requires, even if you’re not an expert, believing that this is an important thing to do and being the driver for it. It’s also important to find a key opinion leader or somebody in the ED who shares the need and can help drive the need for turnaround time within hospital administration. If you don’t have an advocate within the ED, then this is not going to work.”

Dr. Diercks credits the team approach at UT Southwestern for their success. “Cardiology, emergency medicine, and our laboratories have been engaged and worked together. It’s been terrific. It has allowed us to be innovative and implement a high-sensitivity troponin pathway.”

The ideal POC high-sensitivity troponin device would have to be a handheld device, Dr. Wu says. “A benchtop reader that has to be put in some central location, even within the emergency department, I classify as near the patient, rather than point of care.” It could still be whole blood and produce good results, he says, “but you have to walk the sample to the place where testing is done, as opposed to having the device moved to the bed.” Some EDs lend themselves to an ED laboratory located near patient beds. But in a large ED “that doesn’t work, because if you have to walk a long distance to get a sample to the ED lab, you might as well send it to the central lab.”

Dr. Diercks agrees: “A test that I have to send or take somewhere doesn’t provide the time savings that a true bedside test would.”