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.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/