Cardiac markers explained: troponin, BNP and CK on a heart blood test

A heart blood test usually means one of two things: cardiac markers such as troponin and BNP, which show injury or strain happening to the heart now, or a lipid panel, which estimates the risk of artery disease over the coming years. They answer different questions, and the first family belongs mostly to emergency rooms and heart clinics.

The phrase cardiac enzymes is older than most of the tests it now describes. Troponin and BNP are proteins and hormones, not enzymes, but the name stuck, so a report or a clinician may still say cardiac enzymes when they mean troponin. This guide reads the whole family together and links each marker to its own page for the detail.

What do cardiac markers measure?

Cardiac markers vs a lipid panel: injury now, or risk later

People searching for a heart blood test often mean a cholesterol test. The lipid panel measures LDL cholesterol, HDL, triglycerides and total cholesterol, and those numbers estimate how fast plaque builds in the arteries over decades. Troponin and BNP measure the heart muscle itself, today. A person can have a high LDL and a perfectly normal troponin, and a person in the middle of a heart attack can have an LDL that looks fine.

What does a troponin level mean?

Troponin is part of the machinery that makes heart muscle contract. The heart's forms, troponin I and troponin T, are specific enough that troponin in the blood means heart muscle cells are leaking it. High-sensitivity assays measure troponin in nanograms per litre (ng/L) and detect small amounts in most healthy people, so the question is never whether troponin is present but whether it is above the cutoff, and whether it is moving.

Detection of an elevated cTn value above the 99th percentile URL is defined as myocardial injury.Thygesen K, et al. Fourth Universal Definition of Myocardial Infarction (2018) ²

URL in that sentence means upper reference limit: the troponin value that 99 of 100 healthy people fall below on that particular assay. The limit differs between troponin I and troponin T, between manufacturers, and often between men and women, because women's troponin runs lower. The same 20 ng/L can sit under the limit on one lab's assay and over it on another, so a troponin result can only be read against the limit printed on that report.

Why the second troponin matters more than the first

A single troponin above the limit says the heart muscle is injured. It does not say when or why. The Fourth Universal Definition calls injury acute when there is a rise and/or fall of troponin values, and defines a heart attack as acute injury with evidence that the cause is a lack of blood flow, such as symptoms, ECG changes or imaging. ² That is why emergency departments draw troponin more than once.

1. 1A sample at arrivalThe first troponin is drawn with an ECG. A very low first value in someone whose pain started more than about 3 hours earlier can be enough to rule out a heart attack under the ESC pathway. ³
2. 2A repeat 1 to 3 hours laterThe ESC 0h/1h and 0h/2h algorithms and the US chest pain pathways compare the two values; the change, called the delta, carries more information than either number alone. ^{1 3}
3. 3Read the directionA clear rise or fall points to acute injury. A raised value that stays flat points to chronic injury, from causes such as kidney disease or heart failure.
4. 4Put it with the restTroponin is read alongside symptoms, the ECG and often imaging. A result on its own does not diagnose a heart attack, and it does not exclude one when symptoms continue.

What raises troponin besides a heart attack?

Troponin rises whenever heart muscle cells are injured, whatever the cause. MedlinePlus and the Universal Definition list many causes besides a blocked coronary artery: ^{2 6}

• Heart failure and fast or irregular heart rhythms (see heart palpitations).
• Myocarditis, an inflammation of the heart muscle, often after a viral infection.
• Pulmonary embolism, a clot in the lungs (the D-dimer test is often drawn alongside).
• Sepsis and other severe illness that strains the heart.
• Chronic kidney disease, where troponin can sit above the limit for months (see kidney function).
• Stroke, some cancer treatments, and very strenuous endurance exercise.

Troponin I or troponin T: does it matter which one?

Both are heart-specific, and the 2021 AHA/ACC guideline treats high-sensitivity troponin I or T as the preferred standard. ¹ The numbers are not interchangeable, because each assay has its own scale and its own 99th percentile limit. A troponin I of 30 ng/L and a troponin T of 30 ng/L do not mean the same thing, and two results are only comparable when they come from the same assay, ideally the same lab.

What does a BNP level mean?

When the heart's main pumping chambers are stretched by high pressure or extra fluid, their muscle cells release a hormone precursor, proBNP. It splits into two pieces of equal number: BNP, the active hormone that helps the kidneys shed salt and water, and NT-proBNP, an inactive fragment that lasts longer in the blood. Labs measure one or the other. NT-proBNP numbers run several times higher than BNP numbers for the same heart, so a result has to be read against the cutoffs for the test that was run.

The strongest use of both tests is ruling heart failure out. In someone with breathlessness or ankle swelling, a low value makes heart failure unlikely. The 2021 ESC heart failure guideline puts it this way: a BNP below 35 pg/mL or an NT-proBNP below 125 pg/mL make a diagnosis of heart failure unlikely. ⁴

A value between the rule-out line and the rule-in line is a grey zone. It neither confirms nor excludes heart failure, and the usual next step is an echocardiogram, an ultrasound of the heart, which shows how well it pumps and fills.

What else moves BNP and NT-proBNP?

Natriuretic peptides rise with many things that strain the heart or slow their clearance. The 2021 ESC guideline lists advanced age, kidney dysfunction, atrial fibrillation and other fast rhythms, acute coronary syndrome, pulmonary embolism, lung disease, severe infection, anemia and an overactive thyroid among the causes. ⁴ Obesity works the other way: the same guideline notes that natriuretic peptide concentrations may be disproportionately low in obese patients, so a normal value in a person with obesity rules out less. ⁴ Kidney function is the most common reason for a high value that is not heart failure, which is why eGFR and creatinine are read beside it.

In people already diagnosed with heart failure, the level also carries prognostic weight: the 2022 US guideline recommends BNP or NT-proBNP for risk stratification in chronic heart failure and, at hospital admission, to establish prognosis, and a higher value generally signals a heavier load on the heart. ⁵ A falling value over months usually reads as the heart under less strain. A trend on the same test says more than a single result.

Do doctors still use CK-MB and myoglobin?

Before troponin, a heart attack was confirmed with CK and its heart-rich fraction, CK-MB, with myoglobin as an early signal. All three are also in skeletal muscle, so a fall, a hard workout or an injection can raise them. The Universal Definition is direct about the comparison:

Other biomarkers, e.g. creatine kinase MB isoform (CK-MB), are less sensitive and less specific.Thygesen K, et al. Fourth Universal Definition of Myocardial Infarction (2018) ²

The 2021 AHA/ACC chest pain guideline goes further: where troponin is available, CK-MB and myoglobin are not useful for diagnosing acute heart injury. ¹ Some hospitals still run CK-MB, for example after some heart procedures, but a CK-MB result is read second to troponin.

What a high CK usually means today

Outside the emergency department, a high total CK almost always reflects skeletal muscle. MedlinePlus gives a typical normal total CK of 10 to 120 mcg/L, and lists strenuous exercise, injections into muscle, recent surgery, muscle injury and medicines including statins and fibrates among the things that raise it, along with thyroid disease and muscle disorders. ⁸ A CK drawn the day after a hard leg session can read several times the upper limit in a healthy person.

How to read heart blood tests together

Each marker answers its own question, so the useful read comes from the combination. These are common patterns and what they usually point to; the real read always includes symptoms, the ECG and history.

Where cholesterol, ApoB, Lp(a) and hs-CRP fit

The risk side of heart blood testing has its own guides. LDL cholesterol and ApoB count the particles that build plaque; the lipid panel guide reads them together, and high cholesterol covers what a raised value means. Lp(a) is a mostly inherited risk particle measured once in a lifetime for most people. hs-CRP is a low-level inflammation marker sometimes added to refine risk, read in the inflammation markers guide.

The high-sensitivity in hs-CRP and the high-sensitivity in hs-troponin describe the same idea, an assay that reads very low concentrations, applied to two unrelated proteins. An hs-CRP of 3 mg/L says nothing about heart muscle injury, and an hs-troponin says nothing about inflammation in the arteries.

Questions to ask about a cardiac marker result

• Which assay was this, troponin I or T, and what is this lab's 99th percentile limit for my sex?
• Was the troponin repeated, and did it rise, fall or stay flat?
• Was this BNP or NT-proBNP, and which cutoff applies, the clinic one or the emergency one?
• Could kidney function, age, a rhythm problem or a recent infection explain a raised value?
• Does a high CK fit recent exercise, an injury or a medicine, and does it need repeating after rest?
• Is an echocardiogram or another heart test the next step, and how soon?

For how ranges are set and why one lab's limit differs from another's, see reference ranges explained. For a general method that works on any result, see how to read any lab result.

Free in LifeFromAdd the lab results you already have, and LifeFrom reads each marker next to the others it moves with, in plain language, with the sources it used. Free, on the web or in the app.

Start on the web Download the app