Troponin triple crown: diagnosis, risk, Rx
William Check, PhD
"The king is dead. Long live the king." While the CK-MB assay has not yet been deposed in practice as the reigning authority for diagnosis of acute myocardial infarction and other acute coronary syndromes, it is, like Elvis in his last years, no longer the undisputed top performer. That title has shifted rapidly in the last few years to troponin assays. Many experts believe that soon troponin will largely replace CK-MB for adjudicating the status of persons presenting with ischemic chest pain.
Troponin’s coronation as coronary arbiter par excellence was marked by a September 2000 report, "Myocardial Infarction Redefined—A Consensus Document of The Joint European Society of Cardiology/American College of Cardiology [ESC/ACC] Committee for the Redefinition of Myocardial Infarction" (J Am Coll Cardiol. 2000;36:959-969). In the article, a positive troponin assay was placed on a basis of apparent equality with a positive CK-MB result for defining or excluding acute, evolving, or recent MI. According to the document, MI is present when there is "a typical rise and gradual fall" in troponin or a "more rapid rise and fall" in CK-MB, along with at least one of three other criteria:
- Ischemic symptoms.
- Development of pathologic Q waves on the EKG.
- EKG changes indicative of ischemia (ST-segment elevation or depression).
But the biochemical equality was only apparent. Committee members wrote, "The most recently described and preferred biomarker for myocardial damage is cardiac troponin (I or T), which has nearly absolute myocardial tissue specificity, as well as high sensitivity . . . . If cardiac troponin assays are not available," they continued, "the best alternative is CK-MB (measured by mass assay). This is less tissue-specific than cardiac troponin . . . ." Not so much a redefinition as a coup d’état.
Simultaneously, another expert committee published a companion document called "ACC/AHA Guidelines for the Management of Patients with Unstable Angina and Non-ST-Segment Elevation Myocardial Infarction" (Circulation. 2000;102:1193-1209). In this guideline, troponin’s supremacy for diagnosis was extended to risk stratification and guiding the choice of patients without ST-segment elevation who could benefit from aggressive medical therapy. In patients with ischemic symptoms but no ST-segment elevation, a positive troponin result plus other EKG changes (for example, ST-segment depression, T-wave inversion) identifies patients who have non-ST-segment elevation myocardial infarction (NSTEMI) and who should receive aggressive medical therapy with a glycoprotein IIb/IIIa receptor antagonist (GP IIb/IIIa inhibitor) plus low-molecular-weight heparin. Patients who do not have an elevated troponin level are considered to have unstable angina and treated conservatively.
In this guideline, too, troponin is preferred over CK-MB. The authors note that "about 30 percent of patients who present with rest pain without ST-segment elevation and would otherwise be diagnosed as having UA because of a lack of CK-MB elevation actually have NSTEMI when assessed with cardiac-specific troponin assays." Moreover, among patients without ST-segment elevation and with normal CK-MB levels, "elevated [cardiac troponin] concentrations identify those at an increased risk of death."
Even before its official recognition, troponin had entered widespread use. "Troponin has become the gold standard and its use has increased significantly over the last several years," says Mark Lifshitz, MD, director of clinical laboratories at New York University Medical Center. "If you had only one cardiac marker to choose, you would choose troponin."
In Dr. Lifshitz’s experience, most hospitals, if they haven’t already started offering troponin, have at least considered it. "Most places I know offer it with CK-MB or have replaced CK-MB with troponin," he says. More and more, hospitals and practitioners are looking at troponin as the primary or at least preferable biochemical cardiology marker. "I think there is a feeling that CK-MB is also an acceptable marker," Dr. Lifshitz says, "but that it doesn’t provide a lot of troponin’s benefits, especially helping to diagnose those individuals who have minor myocardial damage." He predicts that troponin will to a large extent replace CK-MB.
Troponin would already have replaced CK-MB if it weren’t more costly in some settings at the laboratory level, he believes. For a large tertiary care center with 25,000 cardiac patients per year, that excess cost becomes a major consideration. "Not to say that people shouldn’t use it despite its cost," Dr. Lifshitz clarifies, "but some of us initially tried to be more selective in its use as a marker for AMI." However, he says, troponin’s rapidly expanding role in selecting patients for therapy and risk stratification will change this.
"Troponin is now being routinely done" for diagnosis and stratification of acute coronary syndrome patients, agrees Rajendra Mehta, MD, clinical assistant professor of internal medicine at the University of Michigan School of Medicine, Ann Arbor. Unlike CK-MB, a higher level of troponin predicts worse outcomes, so that troponin provides both diagnostic and prognostic information. A survey of a Health Care Financing Administration database revealed that, in 1998-1999, 7.1 percent of acute myocardial infarction diagnoses were made primarily on the basis of a positive troponin result and that in-hospital mortality was significantly more closely related to a positive troponin test than to a positive CK-MB result. In fact, cases with an elevated troponin but a negative CK-MB had the highest in-hospital mortality (13.2 percent).
"Most important," Dr. Mehta says, "troponin prevents missed diagnosis of heart attack and undertreatment and allows appropriate use of life-saving strategies to improve outcomes."
Christopher Cannon, MD, cardiologist at Brigham and Women’s Hospital and the principal investigator for several large trials evaluating GP IIb/IIIa inhibitors and troponins in acute coronary syndrome patients, calls troponin "a terrific cardiac marker. It has been adopted by everyone as the number one triage factor and has been highlighted as critical by recent guidelines in choosing therapies for ACS patients." But Dr. Cannon notes that issues have come up in the clinical use of troponin assays. Most important, many high-risk patients have a negative troponin result. Conversely, in real-world practice, outside the setting of a clinical trial, some patients have false-positive troponin values. "This is where using clinical factors to add to and incorporate with troponin can be useful in making sure you have the whole picture," says Dr. Cannon, who has reported on the value of this approach.
One drawback to using troponin assays in their current forms is raised by Fred Apple, PhD, medical director of clinical laboratories at Hennepin County Medical Center and professor of laboratory medicine and pathology at the University of Minnesota School of Medicine. Dr. Apple was a member of the biochemical subcommittee of the joint ESC/ACC committee that redefined AMI and in fact was the only noncardiologist on that panel. He notes that the document uses a single cutpoint for troponin positivity, defined as the 99th percentile of a normal reference population. "To use a single cutpoint, the imprecision of the assay has to be 10 percent or less," Dr. Apple points out. But no manufacturer of troponin assay kits can meet that goal at the present time. Dr. Apple hopes this stringent definition will put the onus on manufacturers to improve their assays at the low end. "The last thing you want to do is to misclassify a patient because the assay is not precise," he says.
Troponin’s clinical value comes from its higher sensitivity to smaller myocardial injury, so-called minor myocardial damage (MMD), and its virtually total cardiac specificity. In the past, when CK-MB was the only marker being run and it was negative, it would have been interpreted as no AMI. With the increasing appreciation of troponin’s sensitivity and specificity, says Dr. Lifshitz, "We are now able to identify smaller amounts of myocardial damage and to start to extract additional information from these markers." He cites substantiating evidence from pathologic studies that show focal necrosis of myocardial cells—MMD—associated with small elevations in troponin.
Identifying patients with MMD is important because their clinical course and prognosis are worse than those of persons who are truly normal and who don’t have any cardiac damage. The corollary is that patients with MMD need, and should benefit from, early aggressive intervention. Several studies have borne out this expectation. In the PURSUIT trial, for example, in which patients with acute coronary syndromes were treated with standard therapy (heparin and aspirin) or standard therapy plus an infusion of the GP IIb/IIIa inhibitor eptifibatide begun in the emergency department, the 30-day risk of death or AMI in the total study population was significantly reduced, by 11 percent. In PRISM-Plus, early intervention with the GP IIb/IIIa inhibitor tirofiban produced a 30 percent relative risk reduction. In both trials, significant benefit was seen only in the subset of troponin-positive patients.
However, the most recent trial, GUSTO-IV ACS, which tested the monoclonal antibody GP IIb/IIIa inhibitor abciximab in acute coronary syndrome patients, did not show any benefit. More relevant to the new status of troponins, in GUSTO-IV ACS abciximab didn’t even work in troponin-positive patients. "But that is the fault of abciximab, not troponin," Dr. Cannon says. "The short answer is that dose matters," he explains. The abciximab dose in GUSTO-IV ACS was an extrapolation from angioplasty trials, where the drug was given for a shorter time. In GUSTO-IV ACS, by the end of 24 hours platelet inhibition was down to 50 to 60 percent. "That’s too low," Dr. Cannon says. On the other hand, both tirofiban and eptifibatide had doses selected to maintain a high level of platelet inhibition.
One consequence of troponin’s higher sensitivity and specificity is that more persons will be diagnosed with AMI as the new criteria come into wider use. Dr. Mehta and colleagues have measured the approximate magnitude of this increase from a database at the University of Michigan Hospitals into which all patients presenting with suspected ACS are entered and their outcomes followed. Using patients entered into this database from May through December 1999, they also determined whether there was any difference in these patients’ prognosis.
Using the traditional definition—CK-MB-positive and either troponin-negative or -positive—264 patients were identified; an additional 41 patients were CK-MB-negative but troponin-positive, for a 16 percent increase in AMI patients identified using the new criteria. In-hospital complications-heart failure, ventricular tachycardia, shock, death, etc.-were numerically but non-significantly lower among patients who were troponin-positive and CK-MB-negative. "You could say that they had similar in-hospital outcomes," Dr. Mehta says.
However, at six-month followup, mortality was significantly higher among patients who had a diagnosis made solely on the basis of a positive troponin assay. Perhaps the worse outcomes at six months were due to the fact that procedures were used much less in patients diagnosed solely by troponin positivity. "At least these data show that the newer criteria appropriately pick up patients at higher risk for long-term adverse outcomes," Dr. Mehta points out. "A positive troponin result in the absence of a positive CK-MB result is suggestive of micronecrosis," Dr. Mehta concludes. "But this micronecrosis should alert the clinician to the impending doom of larger heart attacks in the future" and should trigger more aggressive therapy.
While more sensitive than CK-MB, troponin still does not identify all high-risk patients. For instance, in the PRISM trial, 42 (13 percent) cardiac events (death or AMI) occurred among 324 troponin-positive patients treated with heparin (the standard baseline therapy), while 39 (4.9 percent) events occurred among the 801 troponin-negative patients. Clearly, troponin selected a population at higher risk. But it identified only about half of the high-risk patients.
Dr. Cannon has showed that high-risk patients can be identified more easily by combining troponin results with a clinical score called the TIMI Risk Score, or TRS. ("TIMI" stands for Thrombolysis in Myocardial Infarction, the name of a series of ACS trials.) TRS includes seven independent risk factors: age =65 years; prior coronary artery disease (CAD); >3 CAD risk factors (increased cholesterol, hypertension, diabetes, smoking, family history); use of aspirin in the last seven days; ST deviation; =2 anginal events in the last 24 hours; and elevated cardiac markers. In one trial, among patients with an intermediate troponin I value (0.1-1.5 µg/L), those with a TRS of 3-4 had almost a threefold increased risk of death or MI in 30 days, while those with a 5-6 TRS had a sevenfold increased risk. Even the few patients with a "negative" troponin I value (=0.1) but a TRS of 5-6 had a 3.3-fold elevated risk.
One source of this problem is that patients with intermediate troponin levels are considered negative in many clinical practice settings. "In clinical trials troponins perform beautifully because only patients with good clinical histories are included," Dr. Cannon says. "In clinical practice, histories are fuzzier so intermediate troponin values tend to be discounted." As clinicians become more familiar with these assays and lower cutoffs are adopted, the problem will resolve somewhat. "If people start paying more attention to minor elevations of troponin, there will be a huge increase in the number of high-risk patients who are identified," Dr. Cannon says.
For laboratorians planning to add troponin assays to their menu, Dr. Lifshitz proposes a few basic steps to make things go more smoothly. Even if you are planning eventually to convert completely to troponin, he advises, it is helpful to provide clinicians with both CK-MB and troponin results for a time. "When we converted," he says, "we first did a clinical correlation study in about 500 patients coming to the ED. We ran both assays on all samples to determine what type of results you got with troponin relative to CK-MB." If a patient has a massive MI, what does that look like on a troponin assay? What do small elevations of troponin mean? Clinicians need these comparisons to be able to make management decisions based on the new assay.
Coordinating with emergency department physicians is also a must. "The problem with algorithms," Dr. Lifshitz notes, "is that they can be difficult to implement in the clinical arena and the laboratory. They require a close working relationship between the laboratory and emergency room physicians." For example, optimal benefit from troponin testing—indeed, from measuring any cardiac marker—derives from taking two timed samples. If the assay is negative at five hours after onset of symptoms, another sample should be taken four to six hours later. Because troponin may remain positive for seven to 10 days, an initial minimal elevation with a negative CK-MB assay could indicate an acute minor episode or something that happened several days ago.
Coordination is mandatory to meet another requisite for cardiac marker testing—rapid turnaround time. "It is as important to be timely as accurate," Dr. Lifshitz says. In the new guidelines, a TAT of 30 to 60 minutes is specified for cardiac marker results. Is that achievable running tests in a central laboratory on immunoassay platforms? "It is achievable," Dr. Lifshitz replies, "but it requires understanding all elements of workflow." Even for a 60-minute TAT, actually running the test on the instrument is probably the shortest element. Other important factors are how long it takes from draw to delivery of the sample to the laboratory, whether the instrument will be able to take on stat samples, and whether there will be a technician present at all times to handle samples. "It can be done," Dr. Lifshitz reiterates, "but it can be a challenge to a laboratory to set up its workflow to meet that guideline."
To achieve a satisfactory TAT for troponin and several other biochemical markers (urine-hCG, urinalysis, stat glucose, liver function tests), Kent Lewandrowski, MD, associate director of the Clinical Laboratory Division in the Department of Pathology, Massachusetts General Hospital, and associate professor at Harvard Medical School, set up a small satellite laboratory in the emergency department. Because the ED was clogged with patients, newcomers were being diverted to other hospitals, compromising patient care and the ability to admit patients. One contributing factor was unacceptable turnaround times. Dr. Lewandrowski set up what he calls an "ED kiosk" staffed by laboratory personnel who gather specimens and run tests. One reason he adopted this approach is that the hospital’s emergency department is very large, so intra-ED logistics are a challenge.
In a pilot study, average within-laboratory TAT for cardiac markers dropped from 110 minutes to 17 minutes, and ED length of stay for those tested decreased by 45 minutes. Based on the overall benefits of the ED kiosk shown in the pilot study, Dr. Lewandrowski convinced MGH to fund five additional FTEs. "You need a pathway to do this process," Dr. Lewandrowski says. For example, a point-of-care device is used for cardiac marker testing, which gives qualitative results. When the troponin test is positive, then a quantitative result that matches the laboratory is useful to facilitate serial testing. The majority of the POC tests are negative and reported as such. However, to achieve consistency with the central laboratory, all positive tests are reflexed to the central lab for a quantitative result to determine whether the marker is rising or falling.
Richard Summers, MD, associate professor of emergency medicine at the University of Mississippi Medical Center, Jackson, adopted a similar approach-using a point-of-care device in the ED to shorten TAT. "We have been using the bedside troponin I marker to do some rapid diagnostics for cardiac chest pain patients," Dr. Summers says. "We are trying to implement some of the therapies a little faster." Dr. Summers, too, sends positive samples to the central laboratory for a quantitative result, but says the device "still saves us 30 minutes to an hour in getting results." Point-of-care troponin testing "has worked very well for us," he adds.
From a laboratory perspective, says Dr. Lifshitz, point-of-care testing for cardiac markers "has pros and cons." The major advantage of POC testing is that, in theory at least, it gives a result faster than doing the test in a central laboratory. Major drawbacks are that POC testing requires strict quality control, which means laboratory staff have to train ED personnel and oversee QC procedures, which can be difficult in an ED. "You have a lot of people working different shifts in a sometimes chaotic environment," Dr. Lifshitz says. Performing tests in the ED using laboratory staff is the structure that yields the fastest TAT and highest quality, but it is the most expensive. "You might be able to justify it in a busy ED, especially if the laboratory is far removed," he notes.
Sometimes the biggest concern is that POC testing is much more expensive on a test basis alone than doing a test in the central laboratory. "Then you have to take into account the potential value of getting a quicker result," Dr. Lifshitz notes. But do you always get the result quicker? If ED personnel are attending to patients, they may not run the test faster. And, Dr. Lifshitz finds, some ED personnel don’t like to do laboratory testing.
In the end, he observes, "The turnaround time that the laboratory can offer to the ED and the systems in place to deliver samples to the central laboratory have a lot to do with ED physicians’ desire to do cardiac markers and POC tests." At New York University hospitals, Dr. Lifshitz says, ED physicians do not want to do POC cardiac testing. NYU has a tube delivery system connecting the ED to the laboratory. "For the most part, we meet the 60-minute guideline," he says, "and there hasn’t been much interest in moving cardiac testing down to the ED."
Commenting on the potential role of POC troponin devices, Alan H.B. Wu, PhD, director of clinical chemistry at Hartford (Conn.) Hospital, says, "If they had high sensitivity, which they do not, they could be used for risk stratification." Dr. Wu cites the study led by German cardiologist Christian Hamm, MD, of the University Hospital Hamburg, that compared laboratory-based troponin testing to Spectral’s troponin I POC device and Roche’s troponin T POC device (N Engl J Med. 1997;337:1648-1653). "Dr. Hamm’s data showed that those POC devices could be used for risk stratification," Dr. Wu says, "but it is my opinion that you will see more cases with higher sensitivity assays, those using a lower cutpoint, such as the second- and third-generation central laboratory assays are now achieving."
The full utility of POC assays will be realized when they, too, become more sensitive, Dr. Wu believes. "Qualitative results should be at a very low cutoff," he says. "I don’t believe any now available meet the cutoff concentration criteria set by ESC/AHA. Their cutpoint is higher because they are not as precise."
Setting cutoffs is also an issue now for troponin assays performed in the central laboratory. "You have to work through with ED physicians the type of assay and cutoff that you will use," Dr. Lifshitz says. A range of cutoffs is still being used depending on the specific assay and platform. "One of the real issues is variability among vendors," he emphasizes. "What people are reading about risk stratification is based on a specific method, so you need to translate that cutoff to your method."
Dr. Lifshitz set his laboratory’s cutoff based on his own correlation study with CK-MB. He selected a cutoff value that balanced the need for sensitivity and specificity of the assay. "The value we chose was very similar to the vendor’s cutoff. However, initially we chose to use a slightly higher value because we felt we were still exploring the reproducibility of the assay," he says.
Regarding setting cutpoints, Dr. Apple says, "Those are calls individual laboratories will have to make." But laboratories and cardiologists have to work together on these decisions. Recently he met with the chief of cardiology to discuss lowering the troponin cutpoint at his institution from 0.6 µg/L to 0.3, with 0.4 or greater indicative of an AMI as well as a risk stratification cutpoint. "Based on the assay we use, data in the literature suggest that those with troponin values above 0.1 µg/L are at risk," Dr. Apple says, "but the coefficient of variation on our assay is not totally acceptable at that 0.1 µg/L level."
After the publication of the ESC/ACC document redefining AMI and the guidelines for management of unstable angina and NSTEMI, Drs. Wu and Apple wrote an editorial discussing laboratory medicine (analytical) issues in cardiac troponin testing (Clin Chem. 2001;47:377-379). Both had sat on the National Academy of Clinical Biochemistry panel that wrote the initial practice guidelines for cardiac markers; Dr. Wu chaired that panel (Clin Chem. 1999;45:1104-1121). Both also took part in molding those initial guidelines into the International Federation of Clinical Chemistry guidelines on use of biochemical markers in ACS (Clin Chem Lab Med. 1999; 37:687-693).
As background, it is important to recognize that the notion that there is no troponin in the blood of healthy persons is erroneous. "To say there is no troponin at all in normal blood," Dr. Wu says, "means either that heart tissue doesn’t degrade, which is absolutely ridiculous, or that it degrades apoptotically, which would be different from ischemic death where we clearly see release of enzymes and proteins. Since the blood of healthy individuals contains measurable concentrations of skeletal muscle enzymes and proteins, we assume there is cardiac troponin in normal blood as well. The reason we haven’t seen it previously is because we didn’t have sensitive enough tools," Dr. Wu says. Next-generation assays will be sensitive enough to show normal turnover, he adds.
With an assay that is sensitive enough, it is possible to see a distribution of troponin values in normal healthy individuals, which makes it possible to calculate percentiles. "To talk about the 99th percentile makes no sense if all normal values are below the lower limit of detection of the assay," Dr. Wu points out. And yet that is just what the new cardiology document proposes—a single cutpoint for troponin positivity, defined as the 99th percentile of a normal reference population. "No manufacturer today has a 99 percent cutpoint with a 10 percent CV," Dr. Wu says, "so no one can follow this recommendation."
That is why he and Dr. Apple, along with cardiologist Allen Jaffe, MD, of the Mayo Clinic, have proposed that, since you can’t compute a percentile on most available assays, it would be best to use the concentration with <10 percent coefficient of variation as a surrogate for troponin positivity. The onus should be on manufacturers to provide that information. Drs. Apple and Jaffe have submitted to the FDA Good Guidance Program a Guidance Document Submission to require manufacturers and in vitro diagnostics companies to provide that information in their package inserts when applying for 510(k) or revised 510 approval. Laboratories can confirm this value but won’t have to reinvent it.
In the interim, Drs. Apple and Wu propose that each laboratory find the lowest concentration with <10 percent CV for its system and make that the cutpoint. "You will still find that is lower than what the majority of manufacturers are recommending for the AMI cutpoint, which is usually based on the ROC curve cutpoint," Dr. Apple says. "Probably cutpoints will slowly drift down as manufacturers improve their assays."
Cardiologists proposed using the 99th percentile cutoff recommended in the consensus document because it reduces the fraction of false-positive results and the number of patients who will be treated without benefit, Dr. Wu notes. But because of the lower precision at higher percentiles, this creates a tension between clinical accuracy and analytical accuracy. Ordinarily cutoffs are set at the 95th percentile, Dr. Wu says, and some false-positive results are accepted. "But the cardiologists felt that was not appropriate," he says.
Further tension arises from the opposite trend—attempts by research cardiologists to push the cutoff lower. "We are seeing more clinical decisions being made for diagnosis, risk stratification, and therapeutic intervention based on low concentrations of cardiac troponin, which is a lot different than we do for CK-MB," Dr. Apple says. This is because GP IIb/IIIa inhibitor therapy is not beneficial in patients with normal troponin values. So many cardiologists are trying to use lower troponin concentrations to identify more patients who are likely to benefit from aggressive therapy.
For instance, in the FRISC study, intervention with low-molecular-weight heparin significantly reduced cardiac events in ACS patients who had troponin T values <0.03 µg/L by a third-generation assay, lower than the conventional cutoff of <0.1, according to data presented at the recent meeting of the American College of Cardiology. Risk actually starts at lower values, around 0.01 µg/L, but FRISC investigators recommend using the cutoff of <0.03 µg/L, since imprecision is too great at lower values. Dr. Apple notes this leaves a "tough call" for clinicians—should patients with troponin T values between 0.01 and 0.03 be treated?
Improvements in the analytical performance of troponin assays in the future may raise new and different, but equally vexing, questions. For instance, Dr. Lifshitz raises the possibility that decisions may be based on a change in the concentration of troponin in an individual patient between two readings, even if values are within the normal range. "But that will demand higher reproducibility and even more precise assays," he says.
William Check is a freelance medical writer in Wilmette, Ill.