Updated 4/9/98

NACB Committee: Alan Wu, Ph.D., Chair, Fred S. Apple, Ph.D., Brian Gibler, M.D., Robert Jesse, M.D., Myron Warshaw, Ph.D., and Roland Valdes, Jr., Ph.D.

American Association for Clinical Chemistry 1998 National Meeting, Chicago, IL

1. The triage of patients with chest pain from the emergency department is one of the most difficult challenges that face emergency physicians today. Admission of patients with a low probability of acute coronary disease often lead to excessive hospital costs (2). A strategy that is too liberal with regard to ED discharges will lead to higher numbers of patients released with AMIs. The ED release rate for AMI have been estimated to be between 2 to 5% (3,4).

Patients with chest pain and negative results for biochemical markers do not have myocardial necrosis as the cause for their pain represent a low risk group for myocardial infarction. However, such patients may have coronary artery disease, which may require additional testing such as echocardiology, exercise or pharmacologic stress tests, nuclear imaging, etc. Establishment of a clinical practice guideline for the evaluation of patients with chest pain will reduce the variability of practice among physicians and institutions (5).

2. Although the onset of chest pain in AMI is sometimes known, for the majority of patients with unstable angina and other cardiac diseases, this information is either unknown or unreliable. It is not uncommon for these patients to report multiple episodes of chest pain over the hours and days prior to admission. Intermittent closure and spontaneous reperfusion of coronary arteries with ruptured plaques reflect the dynamic nature of these diseases. In the elderly, or in patients with insulin-dependent diabetes, there may be altered thresholds or a blunted response to pain. Indeed, there are many patients with ACS who experience no pain during occlusive episodes (silent ischemia) (6).

3. The ideal biochemical marker is one that has high clinical sensitivity and specificity, appears early after AMI to facilitate early diagnosis, remains abnormal for several days after AMI, and can be assayed with a rapid turnaround time (7,8). As currently there is no single marker that meets all of these criteria, a multi-analyte approach has the most merit.

Because the interval between the onset of pain and ED presentation is variable from patient to patient, two markers are needed to enable detection of patients who present early and late. Currently, myoglobin is the marker that most effectively fits the role as an early marker. Myoglobin is detectable in blood as early as 1-2 h after onset and can be highly effective for AMI ruleout (9). Automated immunoassays for myoglobin are commercially available. Myoglobin is not cardiac specific, and patients with renal failure, skeletal muscle injury, trauma, or disease can have abnormal concentrations in the absence of AMI. CK-MB isoforms have also been shown to be an early marker for AMI (10). However, several investigators have questioned the early diagnostic utility and convenience of isoforms measurement (11,12). Nevertheless, automated stat isoform measurements are being used in many hospitals as an early measure of myocardial injury. Other early markers are also being investigated (see above recommendation) (13,14).

Cardiac troponin T or I are currently the best markers for definitive AMI diagnosis. Troponin appears in the searm relatively early after symptoms onset (6-9 h) and remains abnormal for 4-10 days. Results are not increased in the presence of skeletal muscle troponin (15,16). Early studies questioned the clinical specificity of cardiac troponin T assays in patients with chronic renal failure (17). With the development of a second generation ELISA assay for cTnT, the frequency of positive results in these patients is lower than the first generation assay, although still higher than for troponin I (18).

4. About 50-70% of AMI patients will present to the ED with evidence of acute myocardial injury on the electrocardiogram (ECG). Acute intervention with thrombolytic therapy or angioplasty should be considered in these patients who present within 6-12 hours after the onset of symptoms (19,20).

The basis for this recommendation is the fact that specific ECG changes, when interpreted by competent physicians, is highly specific (21). Retrospective testing at a reduced frequency of blood collection (e.g., once per day) may be useful to qualitatively estimate the size of the infarct in the absence of reperfusion, or to detect the presence of complications such as an reinfarction. In the latter case, a marker that returns to normal quickly, such as myoglobin, may be the most effective. As in the case of the initial AMI, serial measurements of myoglobin will be necessary to rule in reinfarction as well.

5. Cardiac markers play an essential diagnostic role in non-Q wave or subendocardial AMIs. There is great variability from hospital to hospital in the frequency of blood collections. The American College of Physicians (ACP) recommend a conservative testing guideline based on total CK and CK-MB for blood collected on admission, 12, and 24 h, and using lactate dehydrogenase isoenzymes when admission is more than 24 hours after onset (22). This committee feels that this strategy is not adequate.

A single positive results for the definitive marker would trigger a diagnosis of AMI and triage of the patient to the appropriate level of care (23,24), the without the need for necessarily completing this algorithm (25). Myocardial necrosis for most patients will be effectively ruled in or ruled out within 6-9 h after admission.

The need to perform the 2-4 hour blood collection for the late marker can be questioned. In particular, negative results at admission and 2-4 hours for the early marker and negative results at admission for the late marker would largely obviate the need for measuring the 3 hour sample. The Committee felt that the laboratory does not currently have a mechanism for "reflexing testing." Reflex testing involves the ordering or cancellation of followup tests on a given sample based on the results of preliminary testing. Use of reflex testing involves pre-approval of physicians to computerized algorithm that makes reflex decisions. These algorithms would ideally be preloaded onto immunochemistry analyzers so that sequential testing can automatically be performed with a minimum amount of turnaround time delay. Until reflex testing becomes common practice, if ever, it may be more convenient for a laboratory to perform myoglobin and troponin on all samples.

Among chest pain centers, there are many variations to the protocol for blood sampling. Some centers use intervals of every 3 hours, while others use every 4 hours. In one study, chest pain patients were triaged with only two samples were collected: one at admission and one at 4 hours (26), (with a third sample collected only on patients presenting with <2 h history of chest pain). Because of the unreliability of the chest pain history, the committee has taken a more conservative approach of recommending the collection of at least three blood samples during the early triage period. A blood collection at 12-24 h may be useful for detection of reinfarction or myocardial extension, or for risk stratification of patients with unstable angina.

6. Some emergency departments are slow to develop a rapid rule out chest pain center because of financial expenses, space, and/or knowledge of the potential benefits. In these centers, the extra laboratory tests bring additional costs without a reduction in lengths of stay or frequence of missed AMIs.

1. The acute coronary syndrome is a pathophysiologic continuum that results from rupture of an atherosclerotic plaque and an associated thrombus (27). It can ultimately result in clinical presentations ranging from entirely asymptomatic to unstable angina to massive acute myocardial infarction. Advances in the diagnostic assays for myocardial necrosis have led to some confusion as to the performance of newer assays have exceeded existing "gold standards." Regardless of the standards set for diagnostic thresholds, even small elevations of markers of myocardial necrosis define patients at high short and long term risk for adverse cardiac events including AMI and death. Patients with minor myocardial injury, those with elevations of markers between the lower limit of detectability and upper limit of normal, should be acknowledged as being at high risk even in the absence of AMI. The degree of risk is directly related to the extent of marker elevations for both cTnT (28) and cTnI (29).

The first cutoff is determined as the upper 2.5 percentile (one-tail test) of results from a normal healthy population (30). The second cutoff is determined as the lower 2.5 percentile (one-tail test) of results from marker concentrations collected within the established diagnostic window for that marker, on patients with confirmed AMI (WHO criterion).

2. The short term risk for death or AMI for patients with minor myocardial injury may be similar to death and reinfarction rate of patients who suffer their first AMI. However it is important to not classify these patients as such, because they may be disadvantaged from a social, psychological, and socio-economic standpoint. With reference to CK-MB, results between the upper limit of normal and the AMI cutoff has been termed a "gray zone." This practice was appropriate because CK-MB is not specific to the heart, and there are normal subjects who have measurable CK-MB concentrations from skeletal muscle release within this range, and would have been incorrectly classified as having high cardiac risk. For cTnT and cTnI, the term gray zone should not be used, as it connotes uncertainty in the measurement and/or clinical interpretation.

3. The World Health Organization (WHO) has defined the diagnosis of acute myocardial infarction (AMI) as a triad (1). Two of which must be present for diagnosis:

a) The history is typical if severe and prolonged chest pain is present;

b) Unequivocal electrocardiographic (ECG) changes are the development of abnormal, persistent Q or QS waves, and evolving injury lasting longer than 1 day; and

c) Unequivocal change consisting of serial enzyme change, or initial rise and subsequent fall of levels. The change must be properly related to the particular enzyme and to the delay time between onset of symptoms and blood sampling.

With the development of biochemical markers that are not themselves enzymes, such as cardiac troponin T and I, and myoglobin, etc., the third criterion of the WHO triad should be revised.

4. The analysis of blood for lipids such as cholesterol, and lipoproteins such as low and high density lipoproteins is well established in the assessment of coronary artery disease risk (33). As such, these markers are being used to screen asymptomatic individuals. Because sensitive cardiac markers have also been shown to provide risk stratification, there may be an impetus to use these markers as part of a biochemical panel for routine health screening to detect the presence of silent ischemia, or after exercise stress testing to detect presence of ischemic injury.

Studies of biochemical markers before and after nuclear ventriculography of chest pain patients have shown that neither cTnT or cTnI are increased after stress testing, even in patients with documented evidence of flow defects (34). These markers are not sufficiently sensitive enough to detect myocardial ischemia that is not associated with irreversible injury or myocardial necrosis.

1. Acute revascularization is now standard practice for patients with ST-segment elevation AMI. The objectives for thrombolytic therapy and/or emergent percutaneous transluminal coronary angioplasty (PTCA) are to recanalize occluded arteries and to reduce mortality. Cardiac markers can be used to assess the success or failure of such therapy. AMI patients who develop patent coronary circulation will release a bolus amount of enzymes and proteins into the circulation ("washout phenomenon") when compared to AMI patients with permanent occlusions (35). The gold standard measurement of reperfusion status is coronary angiography. Blood flow is assessed according to a scale determined by the Thrombolysis in Myocardial Infarction Investigators (TIMI)(19):

a) Grade 0: no antegrade flow beyond the point of occlusion,

b) Grade 1 (penetration without perfusion): contrast media passes beyond the area of obstruction but fails to opacify the entire coronary bed distal to the obstruction for the duration of the filming,

c) Grade 2 (partial perfusion): contrast media passes across the obstruction and opacifies the coronary bed distal to the obstruction, however, the rate of entry into the vessel distal to the obstruction or its rate of clearance from the distal bed (or both) are perceptibly slower than its entry into or clearance from comparable areas not perfused by the previously occluded vessel (i.e., the opposite coronary artery or coronary bed proximal to the obstruction), and

d) Grade 3 (complete perfusion): antegrade flow into the bed distal to the obstruction occurs as promptly as antegrade flow into the bed proximal to the obstruction, and clearance of contrast material from the involved bed is as rapid as clearance from an uninvolved bed in the same vessel or the opposite artery.

Grades 0-2 indicate various stages of occluded blood flow while grade 3 indicate reperfusion. The time interval of collection of samples is important for the proper interpretation of results.

When reperfusion is successful, it is produced in the majority of cases within 90 minutes after the initiation of therapy (36,37). Sampling blood prior to 90 minutes may be helpful in the early determination of successful reperfusion, but cannot be used in place of the 90 minute blood sample. Some investigators have suggested a 120-minute sample (38). This time interval is also acceptable, however, this will delay any subsequent prospective management decision. Other investigators have used the time to peak as the discriminating factor. This requires more blood sampling, and a delay particularly in the non-reperfused group, before an final assessment can be made.

2. Cardiac markers have also been used to detect presence of perioperative AMI, in patients undergoing surgical procedures (39). Use of nonspecific cardiac markers such as CK, CK-MB, and LD have limited usefulness as they are released from non-cardiac tissues as a result of the procedure itself (40).

The performance of cardiac troponin for detection of perioperative AMI has been shown to be superior to other cardiac markers such as CK-MB (41,42). However, a protocol for frequency of blood collection and interpretation of results will require more clincial studies before further recommendations can be made. Studies will also be required to determine the proper cutoff concentration for perioperative AMI. If cardiac troponin is highly specific, then can the existing AMI cutoff concentrations be used? If the surgical procedure involves the heart, e.g., coronary artery bypass graft, some injury to the myocardium itself is expected. Should a higher AMI cutoff concentration be used in open heart surgeries? In a study by Metzler et al., a cTnT concentration of 0.6 µ/L had a positive predictive value for an adverse outcome of 87.5% with a negative value of 98% (43).

3. Cardiac markers have been used in other monitoring roles such as myocardial infarct sizing (44). Infarct sizing involves serial collection of cardiac markers and integrating the area under the curve of a plot of enzyme activity or protein concentration vs. time. For cardiac markers that exhibit the washout phenomenon, infarct sizing estimates are innaccurate when reperfusion is successful (45). Other markers that are not sensitive to reperfusion status, such as myosin heavy chains (46), may provide more accurate estimates but not at an early time period.

Assessment of infarct sizing may be useful in clinical trials on new drugs designed to limit the extent of myocardial injury, such as those following IV thrombolytic therapy or angioplasty (e.g., glycogen IIb/IIIa inhibitors).

4. The establishment of the normal ranges and cutoff concentrations for new biochemical markers is essential for the interpretation of results. Often, studies from the literature are quoted for normal ranges, and such studies are only carried out for the investigational marker in question.

The committee recognizes that the exclusive release of new markers may be in their best interests in terms of profitability, and therefore they may be reluctant to share ideas and needs with their colleagues. Nevertheless, implementation of new tests is more easily integrated into the laboratory when these markers are available on a wide spectrum of analyzers, and it is in the best interests of the medical community and the in vitro diagnostic industry that assays correlate to one another.

6. New biochemical markers have and will continue to be developed over the next few years. Much of the focus has been on the development of tests that can detect earlier steps within the pathophysiology of acute coronary syndromes, such as inflammation, thrombus formation, platelet aggregation, and reversible ischemia. Some of the markers examined for these processes include C-reactive protein, amyloid protein A, thrombus precursor protein, p-selectin, and glycogen phosphorylase isoenzyme BB. Other markers are designed to be used to improve the specificity of myoglobin including heart fatty acid binding protein and carbonic anhydrase III isoenzyme. For research studies involving these new markers, time of admission is not useful when comparing results against conventional markers of myoglobin, CK-MB, and cardiac troponin, because the admission time is variable from institution to institution (48,49).

1. Creatine kinase-MB is currently considered the "gold standard" for the laboratory diagnosis of AMI (50). The development, characterization, and clinical interpretation of cardiac troponin T and I seriously challenges the role of CK-MB. Cardiac troponin T and I appears in the blood at or near the same time as CK-MB, but remains abnormal for 7-10 days.

Use of CK-MB should be phased out over the ensuing years as more cTnT and cTnI assays become available, and the cost for such assays becomes competitive to CK-MB mass assays (51). Measurement of lactate dehydrogenase isoenzymes and ß-hydroxybutyric dehydrogenase should be immediately discontinued (16,52).

2. New biochemical markers will continue to be developed especially in the area of early AMI diagnosis. Tests that detect the presence of early pathophysiologic events of acute coronary syndrome such as inflammation, platelet activation and aggregation, thrombus formation, and early ischemia, are being studied. Standardization of clinical protocols which incorporate these tests will be important. The accuracy of these new biochemical markers may be unnecessarily compromised if the diagnosis of AMI is based on laboratory procedures that are themselves have limitations (e.g., total CK and CK-MB).

3. It is recognized that testing cardiac markers on a continuous random-access basis will likely increase costs for providing this service (53,54). However, when considering the potential costs for delaying critical management decisions that might occur if testing were performed on an infrequent (batched) basis, the overall hospital costs for management chest pain patients is likely to be higher.

The factors that affect turnaround times include the delay in the delivery of the sample to the laboratory, the pre-analytical steps necessary to prepare the sample, the analysis time itself, and the effort it takes to deliver results to the ordering physician. The committee recognizes that the time taken for the delivery of samples to the laboratory is not under the control of the laboratory. Nevertheless, laboratory personnel should work closely with hospital administration and nursing units to minimize the delays. This could be accomplished with the implementation of pneumatic tubes that delivery samples directlyl to the central laboratory. Use of satellite laboratories is another mechanism to reduce delivery times. The pre-analytical time include the time required for blood clotting and centrifugation.

When the laboratory staff recognizes a situation of non-compliance they should have the authority and exercise their right to remove the POC testing devices from the affected units until the deficiencies have been satisfactorily corrected.

7. Assays for cardiac markers for early diagnosis, rule out, triaging of patients from the emergency department, or determination of successful reperfusion requires markers that have a short assay time.

The Committee recognizes the importance of establishing objective goals for imprecision of assays for new cardiac markers. This will assist manufacturers in the construction of new assays. The total precision required for a particular assay is dependent on the biological variation of the analyte. The biologic variation for cardiac troponin has not been established. As such this recommendation for total precision was arbitrarily set at 10% without a scientific basis. Prior to the publication of the final recommendations, the committee will seek guidance from the NACB membership and attendance of the SOLP, in an attempt to reach some consensus.

While outcome studies have shown that stat reporting of results for cardiac markers reduces hospital length of stay and laboratory costs for cardiac patients, there are no outcome studies to validate the specific need for a 1 hour turnaround time. It is clear, however, that early treatment of Q-wave AMI patients with thrombolytic therapy is important for the success in terms of reducing mortality and increasing the rate of coronary artery patency. With the development of new therapeutic strategies for unstable angina and the non-Q-wave AMI, the committee anticipates that early detection of minor myocardial injury will also be beneficial in the management of these patients. For those patients who do rule out for acute coronary syndromes, it is expected that early reporting of laboratory data will also reduce overall hospital costs. The committee encourages prospective outcome studies to examine the putative advantage of reporting turnaround times within 1 hour.

Schedule for the 1998 NACB SOLP on Recommendations for use of Cardiac Markers     [ Top ]

The NACB Standards of Laboratory Practice will be presented as a 2-day Edutrak at the AACC 50th Annual Meeting and Clinical Laboratory Exposition to be held August 2-6, 1998 in Chicago, IL.

Tuesday, August 4, 1998

10:30-noon Session I: Recommendations for markers in triage of patients with chest pain.

Brian Gibler, M.D., Department of Emergency Medicine, University of Cincinnati,, Cincinnati, OH

Rebuttle: James W. Hoekstra M.D., Department of Emergency Medicine, Ohio State University, Columbus, OH (to be confirmed)

2:45-5:00 Session II: Recommendations for markers in acute coronary syndromes.

Robert Jesse, M.D., Ph.D., Division of Cardiology, Medical College of Virginia, Richmond, VA

Rebuttle: Allan Jaffe, M.D., Department of Medicine, State Universityi of New York, Syracuse, NY

8:45-10:00 Keynote address: Pathophysiology of unstable angina pectoris.

Eugene Braunwald, M.D., Harvard Medical School, Boston, MA

10:30-noon Session III: Recommendations for markers in clinical applications other than AMI and research.

Fred S, Apple, Ph.D., Department of Pathology and Laboratory Medicine, Hennepin County Medical Center, Minneapolis, MN

Rebuttle: Michael Salinger, M.D., Department of Medicine, Evanston Northwestern Healthcare, Evanston, IL.

2:45-5:00 Session IV: Recommendations for assay platforms and markers of acute myocardial infarction.