CK-MB Index Calculator
CK-MB Index Calculator
The CK-MB relative index — CK-MB mass divided by total CK. Read it knowing that CK-MB is no longer the test for diagnosing a myocardial infarction, and that the index answers a narrower question: is this CK-MB coming from heart or from skeletal muscle?
CK-MB Index
CK-MB + total CK → indexCK-MB mass 12 ng/mL with a total CK of 600 U/L — a marathon runner with chest discomfort
The relative index
Below 3 favours skeletal muscle · 5 or above favours myocardium · 3 to 5 does not discriminate
- the units are not interchangeable
- the published cut-offs are defined for CK-MB as a MASS in ng/mL over total CK as an ACTIVITY in U/L. A ratio built from a CK-MB activity result is a different quantity on a different numerical scale, and these cut-offs do not apply to it
- why it is quoted as a percentage
- by convention, and because the original version of the index used CK-MB activity over total CK activity, which really was a dimensionless percentage. The modern mass-over-activity form keeps the per-cent sign without being a true proportion of anything
- one draw, not two
- CK-MB and total CK rise and fall on different timetables, so an index assembled from samples taken hours apart is arithmetic without meaning. Both values must come from the same specimen
- the cut-offs are not agreed
- 3 and 5 are the commonest published pair, but other sources use a single cut-off around 2.5 to 3 with no indeterminate zone. A number that changes meaning between textbooks is not a number to decide on
Worked example
CK-MB mass 12 ng/mL with a total CK of 600 U/L — a marathon runner with chest discomfort
12 ÷ 600 × 100 = 2.0
2.0 is below 3, which favours a skeletal-muscle source
The absolute CK-MB of 12 ng/mL is raised, and read on its own it looks cardiac. The index says it is a small fraction of a very large skeletal release, which is what running does
Had the total CK been 150 U/L instead, the same CK-MB would give an index of 8.0 — the cardiac band. The CK-MB has not changed; the context has
This is the whole use of the index, and it is still not the test: a high-sensitivity troponin on the same sample answers the question this is only gesturing at
What the index can and cannot tell you
| Question | Answer |
|---|---|
| Is this CK-MB from heart or skeletal muscle? | This is the question the index was built for, and roughly what it does |
| Has this patient had a myocardial infarction? | No. A cardiac-range index is not a diagnosis, and a skeletal-range index does not exclude one |
| Is the index better than a troponin? | No, on any axis — sensitivity, specificity, or how early it turns positive |
| Can I compute it from a CK-MB activity result? | No. The published cut-offs are for CK-MB mass in ng/mL over total CK activity in U/L |
| Can I use results from different draws? | No. Both markers must come from the same specimen |
| What if total CK is normal and CK-MB is raised? | Consider macro-CK, which is a benign immunoglobulin-bound complex that many CK-MB methods read as CK-MB. A CK-MB exceeding total CK is an analytical result, not a biological one |
Marker timings after myocardial injury
| Marker | Starts to rise | Peaks | Back to baseline |
|---|---|---|---|
| Myoglobin | 1 – 3 hours | 6 – 9 hours | Around 24 hours |
| CK-MB | 4 – 6 hours | 12 – 24 hours | 48 – 72 hours |
| Total CK | 4 – 8 hours | 24 hours | 3 – 4 days |
| Cardiac troponin (high-sensitivity) | 1 – 3 hours | 12 – 48 hours | 7 – 14 days |
A calculation worth doing, for a test not worth ordering
CK-MB is the MB isoenzyme of creatine kinase. It is concentrated in myocardium, but it is not confined to it: skeletal muscle contains a few per cent CK-MB, so a large skeletal release raises the absolute CK-MB while contributing far more to the total CK. The relative index exploits that difference. Divide CK-MB mass in ng/mL by total CK activity in U/L and multiply by 100; below about 3 the proportion is what skeletal muscle produces, and at 5 or above it is what myocardium produces.
The honest framing has to come first. CK-MB and its index have been superseded by high-sensitivity cardiac troponin for diagnosing myocardial infarction, and are no longer recommended for that purpose. Troponin is more cardiac-specific, turns positive earlier, and is measured by assays that are more sensitive; the 2026 Fifth Universal Definition of Myocardial Infarction observes that CK-MB may no longer even be widely available. If the question is whether this patient is having a myocardial infarction, the answer comes from a troponin measured now and repeated, read against the reporting laboratory’s own 99th-centile limits, not from this ratio.
The other thing usually said in CK-MB’s defence needs correcting too. Because CK-MB clears within two or three days while troponin stays elevated for a week or more, it is often claimed that CK-MB is the marker for a re-infarction occurring days after an index event. The kinetics are real; the recommendation is not. The Fourth Universal Definition of Myocardial Infarction states that CK-MB analysis is not useful or cost-effective in the differential diagnosis of re-infarction where troponin can be measured, and defines re-infarction instead by an immediate troponin with a repeat three to six hours later, a rise of more than 20 per cent above the still-elevated value counting as a new event.
So what is the index actually for? Two situations. The first is a CK-MB that has already been reported and now has to be explained — most often alongside an enormous total CK after exercise, a seizure, a fall, an intramuscular injection, myositis or rhabdomyolysis, where a low index shows the CK-MB is skeletal collateral rather than a cardiac signal. The second is a setting where high-sensitivity troponin is not available, which the 2026 definition explicitly addresses. Outside those, calculating this index changes nothing. And in every case the band is a description of a ratio, not a decision about a patient: nobody should be admitted, discharged, anticoagulated or taken to a catheter laboratory because a number crossed 3 or 5.
Frequently asked questions
What is a normal CK-MB relative index?
An index below 3 is the pattern of a skeletal-muscle source and 5 or above favours a cardiac source, with 3 to 5 not discriminating. Some sources instead use a single cut-off of about 2.5 to 3 and have no indeterminate zone, so the figures are not universally agreed — which is one reason the index is not a decision-making tool.
Is the CK-MB index still used to diagnose a heart attack?
No. High-sensitivity cardiac troponin has replaced CK-MB for diagnosing myocardial infarction, and CK-MB is no longer recommended for that purpose. The index is a way of attributing an already-reported CK-MB to heart or skeletal muscle, not a diagnostic test for infarction.
Can I calculate the index from a CK-MB activity result?
No. The published cut-offs are defined for CK-MB measured as a mass concentration in ng/mL over total CK measured as catalytic activity in U/L. A ratio built from a CK-MB activity result in U/L is a different quantity on a different numerical scale, and these cut-offs do not apply to it.
Is CK-MB better than troponin for detecting a second heart attack?
It is often said to be, because CK-MB clears in 48 to 72 hours while troponin stays raised for one to two weeks. The Fourth Universal Definition of Myocardial Infarction does not agree: it states CK-MB is not useful or cost-effective here when troponin can be measured, and defines re-infarction by a repeat troponin rising more than 20 per cent above the still-elevated value.
Why is my CK-MB higher than my total CK?
That is analytically impossible biologically, so it points to interference — most often macro-CK, a benign complex of CK with an immunoglobulin that many CK-MB methods read as CK-MB. Adenylate kinase released from red cells can also falsely raise CK activity. The laboratory should be asked to investigate rather than the result being acted on.
Related calculators
References
- Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation. 2018;138(20):e618–e651.
- Joint ESC/ACC/AHA/WHF Task Force for the Universal Definition of Myocardial Infarction. Fifth Universal Definition of Myocardial Infarction (2026). Circulation. 2026. doi:10.1161/CIR.0000000000001477.
- StatPearls. Creatine Kinase MB: Diagnostic Utility and Limitations. StatPearls Publishing; NCBI Bookshelf NBK557591.
- Aydin S, Ugur K, Aydin S, Şahin İ, Yardim M. Biomarkers in acute myocardial infarction: current perspectives. Vasc Health Risk Manag. 2019;15:1–10.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.