Normal Values for CK-MB
The normal CK-MB value is defined as below the 99th percentile of a healthy reference population, with sex-specific reference limits required—typically CK-MB mass should be <5 ng/mL in most assays, though the exact cutoff varies by laboratory and must be established using at least 120 healthy individuals. 1
Reference Range Standards
The 99th percentile cutoff represents the upper limit of normal for CK-MB, and values exceeding this threshold suggest myocardial injury when accompanied by appropriate clinical context 1
Men typically have higher baseline CK-MB values than women due to greater muscle mass, making sex-specific reference limits mandatory in clinical practice 1
The assay's total imprecision (coefficient of variation) should be ≤10% at the 99th percentile reference limit to ensure reliable clinical decision-making 1
Units of Measurement
CK-MB is reported as CK-MB mass in ng/mL (nanograms per milliliter), which is the standard unit for modern immunoassays 2
Studies reference diagnostic thresholds ranging from ≥5 ng/mL to ≥8 ng/mL depending on the specific assay and clinical context 2
CK-MB Relative Index
The CK-MB relative index (CK-MB mass/total CK activity ratio) is expressed as ng/U (nanograms per unit of total CK activity) 3, 4
A CK-MB index >5.0 is diagnostic of myocardial infarction, while a value <3.0 virtually excludes this diagnosis 3
For differentiating myocardial necrosis from skeletal muscle injury, a ratio of ≥80 ng/U strongly indicates myocardial damage rather than muscular injury 4
The typical CK-MB index in myocardial infarction exceeds 200 ng/U, with most patients demonstrating at least one ratio ≥110 ng/U within 8-32 hours of symptom onset 4
Critical Clinical Context
Two consecutive measurements of CK-MB above the 99th percentile are required to establish sufficient biochemical evidence of myocardial necrosis 1
Serial measurements documenting a characteristic rise and/or fall pattern are essential to maintain diagnostic specificity and distinguish acute myocardial injury from chronic elevations or skeletal muscle sources 1
CK-MB begins to rise within 3-4 hours after myocardial injury onset and returns to normal within 24-72 hours 1, 5
Important Caveats
Total CK measurement alone is not recommended for diagnosing myocardial infarction due to poor specificity, as it is abundantly present in skeletal muscle 1
CK-MB can be elevated in skeletal muscle damage, particularly from diaphragm and other muscles, though the CK-MB/total CK ratio helps differentiate cardiac from skeletal muscle sources 4
In trauma and burn patients without myocardial involvement, maximum CK-MB mass/total CK activity ratios average 7-18 ng/U, with the highest ratio after muscular damage typically not exceeding 38 ng/U (except in rare cases) 4