What are the indications for Creatine Kinase-MB (CK-MB) versus troponin in an emergency setting?

From the Guidelines

CK-MB testing is no longer the preferred biomarker for diagnosing myocardial infarction in emergency settings, having been largely replaced by troponin assays due to their higher sensitivity and specificity for cardiac injury. However, CK-MB may still have niche uses, such as assessing reinfarction within 2-3 days of an initial MI, in patients with chronic troponin elevations, or when troponin testing is unavailable 1. The timing of CK-MB elevation is crucial, with levels rising within 4-6 hours after cardiac injury, peaking at 24 hours, and returning to normal within 2-3 days 1.

Key Considerations for CK-MB Testing

• The relative index (CK-MB/total CK ratio >2.5-3%) can help distinguish cardiac from skeletal muscle injury 1.
• Serial measurements of CK-MB, collected at presentation and 6-12 hours later, are recommended to detect characteristic rise and fall patterns, which can aid in the diagnosis of myocardial infarction 1.
• High-sensitivity troponin assays provide superior diagnostic accuracy for acute coronary syndromes compared to CK-MB testing, leading to their preference in emergency departments 1.

Clinical Guidelines and Recommendations

• The American College of Cardiology Foundation/American Heart Association guidelines suggest that for patients presenting within 6 hours of symptoms consistent with acute coronary syndrome, assessment of an early marker of cardiac injury (e.g., myoglobin) in conjunction with a late marker (e.g., troponin) may be considered 1.
• The European Heart Journal recommends the use of cardiac troponin T or troponin I as the preferred markers of myocardial necrosis due to their higher specificity and reliability compared to traditional cardiac enzymes like CK-MB 1.

Conclusion on CK-MB vs. Troponin

In summary, while CK-MB has specific uses, troponin assays are the preferred choice for diagnosing myocardial infarction in emergency settings due to their superior sensitivity and specificity. The decision to use CK-MB should be based on specific clinical scenarios where its use is justified, such as in the assessment of reinfarction or when troponin testing is not available 1.

From the Research

Indications for CK-MB vs Opinion in Emergency Setting

The use of CK-MB in the emergency setting is supported by several studies as a valuable tool for detecting acute myocardial infarction (AMI) in patients with nondiagnostic ECGs.

• CK-MB sampling in the emergency department can identify AMI in patients presenting with chest pain and nondiagnostic ECGs 2, 3.
• Serial CK-MB levels can provide a sensitive and specific method for detecting AMI in patients within three hours after ED presentation 2.
• The sensitivity of CK-MB ranges from 50% to 62.1% on ED presentation, increasing to 92% to 96.7% three hours later, with specificities ranging from 83.0% to 96.4% at three hours 2.
• Combining serial ED CK-MB assay results with diagnostic ECGs yields an 88.4% sensitivity for AMI detection within three hours of ED presentation 3.

Comparison with Other Markers

Other markers, such as cardiac troponin, have also been studied for their utility in detecting AMI.

• Cardiac troponin I (cTnI) is highly specific for the detection of AMI, with a specificity of 91.9% compared to CK-MB (85.6%) and myoglobin (61.4%) 4.
• The cardiac troponins are highly useful as markers of acute coronary syndromes, but abnormal values at any time following chest pain onset are highly predictive of an adverse cardiac event 5.
• Myoglobin appears promising as a marker of early cardiac ischemia, but its diagnostic window is narrow, and it is less sensitive than CK-MB at 8 hours or more after chest pain starts 5.

Clinical Application

In clinical practice, a combination of tests, including CK-MB, troponin, and ECG, is often used to diagnose AMI.

• A combination of the first ECG, CK-MB mass, and TnT has a negative predictive value (NPV) of 87.9% for AMI 6.
• If the first ECG, CK-MB mass, and TnT are all negative, the probability of having an AMI is 12% 6.
• A prudent strategy is to order CK-MB and troponin values on presentation in the hope of making an early diagnosis of AMI or unstable coronary syndrome, while recognizing that normal values obtained within 6 hours of symptom onset do not exclude an acute coronary syndrome 5.