September 2009 Clinical Laboratory News: Cardiac Ischemia

 

September 2009: Volume 35, Number 9


Cardiac Ischemia
Where No Biomarker Has Gone Before
By Fred S. Apple, PhD


 

With the intense focus on predictive diagnostics in cardiac healthcare today, no doubt every major and minor diagnostic company is trying to figure out which biomarker will be the future homerun for early detection of myocardial infarction (MI) and/or prediction of an impending cardiac event. Whether obtaining intellectual property rights to published biomarkers or biomarkers in the early stages of proteomic exploration, diagnostic companies are pursuing all avenues to find the next superstar.

However, it appears that no one has yet to locate a novel and reliable biomarker for cardiac ischemia. Since the introduction of cardiac troponin (cTn) 20 years ago at a small conference in Bavaria, Germany, researchers have reported on numerous biomarkers to complement or replace cTn, currently acknowledged as the ‘gold standard’ of cardiac biomarkers. In both prospectively and retrospectively collected acute coronary syndrome (ACS) patient cohorts, emerging biomarkers listed in Table 1 have added little value to the diagnostic and risk stratification power of cTn.

Table 1
Candidate Biomarkers

These biomarkers have been studied as potential biomarkers of cardiac ischemia.

  • glycogenphosphorylase BB
  • myoglobin
  • fatty acid binding protein
  • CD40 ligand
  • choline
  • ischemia-modified albumin
  • unbound free fatty acids
  • placental growth factor
  • pregnancy-associated plasma protein A
  • cytokines
  • C-reactive protein
  • adhesion molecules
  • natriuretic peptides
  • serum amyloid
  • phospholipase A2
  • oxidized LDL
  • monocyte chemotactic protein
  • isopentanes

Despite these discouraging results, I’m reminded of the place that CK-MB held in diagnosis of MI in the early 1990s. If someone were to have told me back then that another biomarker would replace CK-MB as the preferred diagnostic biomarker for MI detection, I would have just smiled. But with cTn taking the place of CK-MB in clinical practice, news is circulating in the lab community that the first round of labs is starting to retire this stalwart. In fact, effective June 1, 2009, our lab at Hennepin County Medical Center has removed CK-MB from our test menu and has relegated this dinosaur to a send-out test by approval only.

Figure 1 shows the continuum from vascular inflammation to myocardial dysfunction along with biomarkers associated with each stage of ACS. As the search for the next superstar cardiac biomarker continues, the goal is to identify one that will become abnormal prior to the onset of an index cardiac event, such as MI. Consequently, researchers frequently refer to this yet-identified marker as a biomarker of cardiac ischemia, in other words, a marker that can be detected before myocardial cell death. In an ideal world, this biomarker would also allow clinicians to initiate therapy without delay, thereby preventing, and ultimately reversing, coronary damage in patients with ACS.

Recently described biomarkers that look promising for both ACS and heart failure include: ST2; galactin; GDF-15; copeptin; choline; and MR-proANP. Whether these biomarkers prove to be independent or covariate tools in diagnostics or risk assessment along with cTn will require substantial clinical and laboratory medicine resources for developmental and applied studies in varied patient groups. Therefore, it could take years before any new cardiac biomarker makes its way onto lab test menus.

Figure 1
Biochemical Profile in ACS Patients

Markers associated with the various stages of ACS are shown.

Validation: A Two-path Process

Clinical utilization of cardiac biomarkers for ruling in and ruling out MI has been a success story in laboratory medicine. Currently, all specialties in medicine endorse cTn as the standard biomarker for detection of MI and myocardial injury. Furthermore, in the appropriate clinical setting, cTn makes the diagnosis of MI (Table 2, below).

Table 2
Criteria for Universal Definition for Acute Myocardial Infarction*

This table outlines the criteria for the universal definition of MI as established by the Global Task Force on the Universal Definition of MI in their recent guideline. In addition, the National Academy of Clinical Biochemistry (NACB) Laboratory Medicine Practice Guidelines recommend that patients with suspected ACS undergo early risk stratification based on an integrated assessment of: an ECG finding; physical exam findings; symptoms; and a cardiac biomarker test result, with cTn as the preferred biomarker.

Definition

The term MI should be used when there is evidence of myocardial necrosis in a clinical setting consistent with myocardial ischemia. The following criteria meet the diagnosis for MI:

Detection of a rise and/or fall of cardiac troponin with at least one concentration above the 99th percentile value, together with evidence of myocardial ischemia with at least one of the following:

Symptoms of ischemia

  • ECG changes indicative of new ischemia (new ST –T changes, new left bundle branch block);
  • Development of pathological Q waves in ECG; and/or
  • Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.

Analytical Considerations

  • Optimal precision (%CV) at the 99th percentile for each assay should be defined at < 10%.
  • Blood samples for measurement of cTn should be drawn at first assessment and at least 6 hours later.

*Adapted from: J Am Coll Cardiol 2007;50:2173-95.

The knowledge gained from labs’ experience with CK-MB and cTnI and T suggests that validation of any new biomarker should be a two-path process. The first path would define the analytical sensitivity and precision needed at an appropriate reference cutoff concentration. Along with such studies, new biomarkers must also be adapted to technologies and instrumentation that are currently available in clinical labs so that they can provide the results to clinicians in a timely manner. The second path queries whether the biomarker will be an earlier signal for disease processes, have sufficient clinical specificity, and whether the results will enable physicians to select therapies that improve patient management.

Going Lower: A New, Improved cTn Assay

At present the biomarker that appears to be most promising for detection of cardiac ischemia is none other than cTn. However, to detect ischemia a high sensitivity (hs) version of the cTn assay is required. Over the past several years, proof-of-principal analytical and clinical studies have been published describing hs-cTn assays that are able to measure cTn in the low pg/mL range in contrast to contemporary cTn assays that measure in the low ng/mL range.

Table 3 presents the analytical characteristics of the hs-cTn assays in comparison to contemporary assays that are FDA-cleared. These hs-cTn assays have three important advantages over current cTn assays. hs-cTn assays can measure almost 100% of concentrations in normal subjects; they demonstrate guideline-acceptable total precision with <10% CV at the 99th percentile reference value; and they detect small concentration changes ( < 5pg/mL) within the normal range of the assay, but less than the detection limit of contemporary assays.

Table 3
Cardiac Troponin Assay Scorecard

Company/
platform/assay
a

99th
µg/L

%CV 99th
µg/L

10%CV
µg/L

Acceptance
designation

Assay
designation

FDA-cleared Assays

Abbott/AxSYM/ADV

0.04

15.0

0.16

Clinically usable

Level 1

Abbott/Architect

0.028

15.0

0.032

Clinically usable

Level 1

Abbott/i-STAT

0.08

16.5

0.1

Clinically usable

Level 1

Beckman/Access/Accu

0.04

14.0

0.06

Clinically usable

Level 2

bioMerieux/Vidas/Ultra

0.01

27.7

0.11

NA

Level 1

Inverness Biosite/Triage

0.05

NA

NA

NA

Level 1

Inverness Biosite/Triage (r)

0.056

17.0

NA

Clinically usable

Level 1

Mitsubishi/PATHFAST

0.029

5.0

0.014

Guideline
acceptable

Level 1

Ortho Clinical Diag/Vitros ES

0.034

10.0

0.034

Guideline
acceptable

Level 1

Radiometer/AQT90

0.023

17.7

0.039

Clinically usable

Level 1

Response Biomedical/RAMP

0.1

18.5

0.21

Clinically usable

Level 1

Roche/Elecsys 2010

0.01

18.0

0.03

Clinically usable

Level 1

Siemens/Centaur/Ultra

0.04

10.0

0.03

Guideline
acceptable

Level 1

Siemens/Dimension RxL

0.07

20.0

0.14

Clinically usable

Level 1

Siemens/Immulite 2500 STAT

0.2

NA

0.42

NA

Level 1

Siemens/Stratus CS

0.07

10.0

0.06

Guideline
acceptable

Level 1

Siemens/VISTA

0.045

10.0

0.04

Guideline
acceptable

Level 1

Tosoh/AIA II

0.06

8.5

0.09

Clinically usable

Level 1

Research High-sensitivity Assaysb

Beckman Access hs-cTnI

0.0086

10.0

0.0086

Guideline
acceptable

Level 4

Roche Elecsys hs-cTnT

0.013

8.0

0.012

Guideline
acceptable

Level 4

Nanosphere hs-cTnI

0.0028

9.5

0.0005

Guideline
acceptable

Level 3

Singulex hs-cTnI

0.0101

9.0

0.00088

Guideline
acceptable

Level 4

aPer manufacturer’s package insert.
bAs reported in published literature.

Abbreviations used: 99th = 99th percentile; 10% CV = lowest concentration giving a 10% CV; NA = insufficient information to designate. Acceptance criteria defined as %CV at 99th percentile: guideline acceptable < 10%; clinically useable >10 to < 20%; not acceptable > 20%; Assay designation based on measureable normal values below 99th percentile: level 4 >95%; level 3, 75% to <95%; level 2, 50% to <75%; level 1 <50% ; (r) revised assay submitted to FDA.

Source: Adapted from Clin Chem published 5/28/09 doi:10.1373/clinchem.2009.128363.

Where Are We Now?

Every year in the U.S., approximately 8 to 9 million patients arrive in emergency medicine departments with non-traumatic symptoms, including chest pain. Triaging these patients to the appropriate level of care presents both a medical and financial challenge. In this context, an ideal cardiac biomarker would detect cardiac ischemia and indicate whether myocardial cell damage is reversible or irreversible. Furthermore, the marker would need to be highly specific: it should not be increased in normal or healthy individuals, nor should it rise in organs other than the heart.

Although researchers have investigated numerous novel biomarkers, to date none has proven to be a standout as a specific marker of cardiac ischemia. Currently, research suggests that high-sensitivity assays for cTn may provide the sought-after properties of this elusive biomarker. For example, in the clinical setting of stress test-induced myocardial ischemia, researchers detected 3.1- 8.6 pg/mL cTnI in a group of 120 patients before the stress test was started. At 4 hours, cTnI levels were unchanged in patients without ischemia but increased in patients with moderate and severe ischemia. Furthermore, when added to clinical features, a >2-pg/mL change in cTnI concentration was an independent predictor of ischemia (Eur Heart J 2009;30:162–9).

Such encouraging findings have prompted further investigations, which will take several years to complete on a larger scale.

Cardiac marker experts have proposed that clinical adoption of hs-cTn assays would necessitate a new interpretation of the currently accepted normal cTn values, as individuals outside the clinical setting of MI would no longer have undetectable values. This clinical challenge will also entail an analytical challenge in the validation of hs-cTn assays.

Recently, I proposed a two-tier approach—a scorecard system—to assist in implementing new cTn assays into clinical practice. As shown in Table 3, the approach designates assays based on their ability to: measure normal subjects; and meet the guideline-recommended 10% CV at the 99th percentile concentration for each assay. The ideal assay would be considered ‘guideline acceptable’ if it were able to measure >95% of normal. A level 4 designation would be for an assay having <10% CV at the respective 99th percentile. In my opinion, for assays to perform with the proposed characteristics of a biomarker of cardiac ischemia, these criteria must be met in order for an assay to reliably and precisely measure 1 to 2-pg/mL changes. Keep in mind that such concentrations are lower than the current limit of detection of all contemporary FDA-cleared assays.

Another challenge in the implementation of hs-cTn assays is for diagnostic manufacturers to adapt these assays so that labs can perform them on currently existing instrument platforms that will be provide < 20-minute turnaround time. Our clinical partners are accustomed to these parameters and will expect any new cardiac marker to perform similarly.

Additional studies of the diagnostic and prognostic use of hs-cTn assays, along with new findings for other potentially valuable biomarkers, will help the lab community determine whether new biomarkers will add to the prognostic value of cTn or whether another marker will replace cTn. Currently, diagnostic companies expect that these high sensitivity assays will be ready for clinical use within the next 1 to 2 years. I envision that the guideline-acceptable, level 4, hs-cTn assays will be the last generation of cTn assays that will significantly contribute to improved clinical sensitivity.

As laboratorians, we need to prepare now to implement this new generation of hs- cTn assays by initiating discussions with our clinical colleagues. When the assays are introduced to the market, it will be our job to ensure the highest level of quality and to help clinicians interpret the results. Together, we can improve care of ACS patients.  

Suggested Readings

  • Apple FS, Jesse RL, Newby LK, Wu AHB, et al. National Academy of Clinical Biochemistry and IFCC Committee for Standardization of Markers of Cardiac Damage Laboratory Medicine practice guidelines: analytical issues for biomarkers of acute coronary syndromes. Clin Chem 2007;53:547–51.
  • Apple FS. Opinion: A new season for cardiac troponin assays: it’s time to keep a scorecard. Clin Chem 2009;55:1303-6.
  • Eggers KM, Jaffe AS, Lind L, Venge P, et al. Value of cardiac troponin I cutoff concentrations below the 99th percentile for clinical decision-making. Clin Chem 2009; 55:85–92.
  • Manolio T. Novel risk markers and clinical practice. N Engl J Med 2003;349:1587–9.
  • Morrow DA, Cannon CP, Jesse RL, Newby LK, et al. National Academy of Clinical Biochemistry practice guidelines: clinical characteristics and utilization of biomarkers in acute coronary syndromes. Clin Chem 2007;53:552–74.
  • Rosalki S, Roberts R, Katus HA, Giannitsis E, et al. Cardiac biomarkers for detection of myocardial infarction: perspectives from past to present. Clin Chem 2004; 50: 2205–13.
  • Sabatine MS, Morrow DA, de Lemos JA, Jarolim P, et al. Detection of acute changes in circulating troponin in the setting of transient stress test-induced myocardial ischaemia using an ultrasensitive assay: results from TIMI 35. Eur Heart J 2009;30:162–9.
  • Thygesen K, Alpert JS, White HD, Jaffe AS, et al. On behalf of the Joint ESC/ACCF/AHA/WHF Task Force for the redefinition of myocardial infarction. Universal definition of myocardial infarction. J Am Coll Cardiol 2007;50:2173–95.
  • Vasan RS. Biomarkers of cardiovascular disease: molecular basis and practical consideration. Circulation 2006;113:2335–62.
  • Wu AHB, Jaffe AS. The clinical need for high-sensitivity cardiac troponin assays for acute coronary syndromes and the role for serial testing. Am Heart J 2008;155:208–14.

Fred S. Apple, PhD, is medical director of clinical laboratories at Hennepin County Medical Center in Minneapolis, Minn., and professor of laboratory medicine and pathology at the University of Minnesota School of Medicine. Dr. Apple’s research interests have been centered in the areas of cardiac biomarkers in acute coronary syndrome and heart failure, fetal protein expression in injured skeletal muscle, and forensic toxicology. He directs the Cardiac Biomarkers Trials Lab at the Minneapolis Medical Research Foundation.

Disclosures: Dr. Apple receives grant support1, serves as a paid consultant2, and sits on the scientific/medical advisory board3 from the following companies: Abbott (1–3); Beckman Coulter(3); BioRad(1); Biosite/Inverness(3); BRAHMS(1); Nanosphere(1–3); Ortho Clinical Diagnostics(1–3); Roche(1); Response Biomedical(1); Radiometer(1); Sansera (2,3); and Siemens(1). 

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