Cardiac Blood Tests (Cardiac Enzymes & Cardiac Biomarkers)
Cardiac blood tests help assess damage to the heart muscle, cardiac stress and heart failure. They are commonly requested in patients presenting with chest pain, shortness of breath, palpitations, syncope or suspected acute coronary syndrome (ACS). Cardiac biomarkers can help identify myocardial injury, support the diagnosis of heart failure and guide treatment decisions. Cardiac blood tests should never be interpreted in isolation. An abnormal result does not automatically diagnose a cardiac condition, just as a normal result does not always exclude one. Results should be interpreted alongside the patient's symptoms, ECG findings, physical examination and other investigations.
What You Need to Know
The heart is a muscular pump that relies on a constant supply of oxygenated blood to function normally. When the heart muscle is damaged, placed under excessive strain or unable to pump effectively, changes occur within the cardiac cells. Some cells become injured and release proteins (cardiac enzymes) that are normally contained within the muscle, while others release hormones in response to increased pressure or stretching of the heart walls. These substances enter the bloodstream, where they can be measured using cardiac blood tests.
Each cardiac biomarker provides different information about the heart. Troponin is the most sensitive and specific biomarker for detecting myocardial injury because it is released when cardiac muscle cells are damaged. Even relatively small amounts of myocardial injury can be detected using modern high-sensitivity troponin assays (a laboratory procedure to measure or analyse a substance). BNP and NT-proBNP are released when the ventricles are stretched due to increased pressure or volume overload, helping assess heart failure. Other biomarkers, such as CK and CK-MB, may provide additional information in selected clinical situations, although they have largely been replaced by high-sensitivity troponin.
One of the most important principles when interpreting cardiac blood tests is understanding that an abnormal result does not automatically diagnose a specific condition, just as a normal result does not necessarily exclude myocardial injury. For example, troponin may be normal when blood is collected shortly after symptom onset but become elevated on repeat testing several hours later. An elevated troponin confirms myocardial injury but does not identify its cause. Likewise, an elevated BNP indicates increased cardiac wall stress but does not confirm heart failure without considering the patient's symptoms, examination findings and other investigations. Looking at the overall clinical picture, rather than relying on a single blood test, is essential for accurate interpretation.
Key points:
Cardiac blood tests assess myocardial injury and cardiac stress.
Troponin is the most sensitive and specific marker of myocardial injury.
Elevated troponin does not always indicate myocardial infarction.
BNP and NT-proBNP help assess heart failure.
Cardiac biomarkers should always be interpreted alongside symptoms, ECG findings and the clinical presentation.
Trends over time are often more informative than a single result.
Components of Cardiac Blood Tests:
Troponin (High-sensitivity Troponin I or T)
B-type Natriuretic Peptide (BNP)
N-terminal pro-B-type Natriuretic Peptide (NT-proBNP)
Creatine Kinase (CK)
Creatine Kinase-MB (CK-MB)
Beyond the Basics
Troponin (High-sensitivity Troponin)
Reference range
Varies depending on the assay used. Most laboratories report values below the 99th percentile as normal.
Troponin is a structural protein found almost exclusively within cardiac muscle cells. It forms part of the contractile apparatus responsible for cardiac muscle contraction. When myocardial cells become damaged, troponin is released into the bloodstream, making it the most sensitive and specific biomarker of myocardial injury.
High-sensitivity troponin assays can detect extremely small amounts of myocardial injury and have largely replaced older troponin assays. They allow earlier diagnosis of myocardial infarction but have also increased the recognition of non-ischaemic causes of myocardial injury.
Decreased troponin: Troponin is normally either undetectable or present at very low concentrations. Low values are not clinically significant.
Elevated troponin: Increased troponin indicates myocardial injury but does not identify the cause. Common causes include acute myocardial infarction, myocarditis, heart failure, tachyarrhythmias, pulmonary embolism, sepsis, chronic kidney disease, severe hypertension and cardiac trauma.
The pattern of troponin elevation is often more important than the absolute value, which is why serial troponin measurements are commonly performed. Rather than relying on a single result, blood samples are usually collected at presentation and repeated according to local protocols (for example, at 3 or 6 hours after the initial sample) to identify whether troponin levels are rising or falling.
An ECG is also repeated with each troponin because changes in cardiac electrical activity, together with changes in troponin, provide a more complete picture of myocardial injury. A rising and/or falling troponin pattern suggests acute myocardial injury, whereas chronically elevated but stable levels are commonly seen in chronic kidney disease and chronic heart failure. Troponin should always be interpreted alongside the patient's symptoms, ECG findings and clinical presentation.
B-type Natriuretic Peptide (BNP)
Reference range
Generally <100 pg/mL
B-type natriuretic peptide (BNP) is a hormone released primarily by the ventricles when they become stretched due to increased pressure or volume overload. This commonly occurs when the heart is unable to pump blood efficiently, causing blood to back up and increasing the volume of blood within the ventricles. In response to this increased wall stretch, the ventricles release BNP as a protective mechanism to reduce the workload on the heart. BNP promotes the excretion of sodium and water by the kidneys (natriuresis and diuresis), relaxes blood vessels (vasodilation) and suppresses the renin-angiotensin-aldosterone system (RAAS). Together, these effects reduce circulating blood volume, lower blood pressure and decrease the amount of work required by the failing heart.
Decreased BNP: Low BNP levels make significant heart failure less likely, although obesity may suppress BNP concentrations.
Elevated BNP: Increased BNP commonly occurs with heart failure, acute coronary syndrome, atrial fibrillation, pulmonary hypertension, chronic kidney disease and advancing age. Higher levels generally correlate with greater cardiac stress, although they do not necessarily reflect the severity of symptoms.
BNP is most useful for assessing suspected heart failure rather than diagnosing myocardial infarction. It should always be interpreted alongside the patient's symptoms, physical examination and echocardiography where appropriate.
N-terminal pro-B-type Natriuretic Peptide (NT-proBNP)
Reference range
Reference ranges vary according to age.
NT-proBNP is an inactive fragment released alongside BNP when ventricular muscle fibres are stretched. It provides similar clinical information to BNP but remains in the circulation longer, making it a commonly used marker for diagnosing and monitoring heart failure.
Decreased NT-proBNP: Low levels make significant heart failure unlikely.
Elevated NT-proBNP: Increased NT-proBNP commonly occurs with heart failure, atrial fibrillation, pulmonary hypertension, chronic kidney disease and advancing age.
NT-proBNP should always be interpreted using age-specific reference ranges because levels naturally increase with age, even in the absence of heart failure. Elevated values support, but do not independently diagnose, heart failure and should always be interpreted alongside the patient's clinical presentation.
Creatine Kinase (CK)
Reference range
Approximately 30–200 U/L(varies between laboratories)
Creatine kinase is an enzyme found in skeletal muscle, cardiac muscle and the brain. It is released whenever muscle cells are damaged. Although CK was once widely used to diagnose myocardial infarction, it has largely been replaced by high-sensitivity troponin because it lacks cardiac specificity.
Decreased CK: Low CK levels are generally not clinically significant.
Elevated CK: Increased CK commonly occurs with skeletal muscle injury, strenuous exercise, rhabdomyolysis, seizures, intramuscular injections, myocarditis and myocardial infarction.
Because CK is found in many tissues, elevated levels should never be assumed to represent cardiac injury without considering other clinical findings.
Creatine Kinase-MB (CK-MB)
Reference range
Varies depending on the laboratory.
CK-MB is an isoenzyme (a different form of the same enzyme that performs the same function but is found predominantly in specific tissues) found predominantly in cardiac muscle, although smaller amounts are present in skeletal muscle. Before the introduction of high-sensitivity troponin, CK-MB was widely used to diagnose myocardial infarction.
Decreased CK-MB: Low CK-MB levels are not clinically significant.
Elevated CK-MB: Increased CK-MB may occur with myocardial infarction, myocarditis, cardiac surgery, cardiac trauma and severe skeletal muscle injury.
Although troponin has largely replaced CK-MB for diagnosing myocardial infarction, CK-MB may still be used in selected situations, such as detecting reinfarction because it returns to normal more rapidly than troponin.
In Practice
Scenario 1: Acute ST-elevation myocardial infarction (STEMI)
A 59-year-old man presents with crushing central chest pain radiating to his left arm. He is pale, diaphoretic and nauseated. His ECG demonstrates ST-segment elevation in leads V2–V4, consistent with an acute anterior STEMI.
Cardiac blood tests
Troponin: 2,840 ng/L
BNP: 68 pg/mL
CK: 340 U/L
Interpretation
The markedly elevated troponin confirms significant myocardial injury. However, the diagnosis of STEMI is not made from the troponin alone. In this scenario, the patient has classic ischaemic chest pain and ST-segment elevation on ECG, which strongly suggests acute coronary artery occlusion. The troponin supports the diagnosis, but urgent reperfusion therapy should not be delayed while waiting for troponin if the ECG and clinical presentation are consistent with STEMI.
Scenario 2: Elevated troponin, but not myocardial infarction
A 79-year-old woman presents with fever, hypotension and confusion due to severe urosepsis. She has no chest pain. Her ECG shows sinus tachycardia but no acute ischaemic changes.
Cardiac blood tests
Troponin: 96 ng/L
BNP: 112 pg/mL
Interpretation
The elevated troponin indicates myocardial injury, but it does not automatically diagnose myocardial infarction. In this patient, there are no ischaemic chest pain symptoms and the ECG does not show acute ischaemic changes. Severe sepsis can cause myocardial injury by increasing oxygen demand while reducing oxygen delivery through hypotension, hypoxia and systemic inflammation. This is why troponin must be interpreted alongside the ECG and clinical presentation rather than treated as a standalone diagnosis.
Scenario 3: Acute heart failure
A 74-year-old woman presents with worsening shortness of breath, bilateral leg oedema, orthopnoea and fine crackles on auscultation. She is unable to lie flat without becoming breathless.
Cardiac blood tests
Troponin: 18 ng/L
NT-proBNP: 5,860 pg/mL
CK: 88 U/L
Interpretation
The markedly elevated NT-proBNP indicates significant ventricular stretch, which supports a diagnosis of heart failure in this clinical context. Her symptoms and examination findings suggest fluid overload and impaired cardiac pumping function. The mildly elevated troponin may occur due to myocardial strain rather than acute coronary occlusion. This scenario shows why NT-proBNP is useful for supporting heart failure assessment, but still needs to be interpreted alongside symptoms, examination findings, ECG and imaging such as echocardiography.
Scenario 4: Severe skeletal muscle injury
A 32-year-old man is brought to the emergency department after lying unconscious for several hours following a drug overdose. He complains of severe muscle pain when he regains consciousness, and his urine is dark brown. His ECG is normal.
Cardiac blood tests
Troponin: 8 ng/L
CK: 18,600 U/L
CK-MB: Mildly elevated
Interpretation
The profoundly elevated CK indicates significant skeletal muscle injury and is highly suggestive of rhabdomyolysis. Although CK-MB is found predominantly in cardiac muscle, smaller amounts are also present in skeletal muscle and may increase following extensive muscle damage. The normal troponin and normal ECG make acute myocardial injury unlikely in this patient.
While the elevated CK is not indicative of myocardial infarction, it is far from insignificant. Severe rhabdomyolysis can lead to life-threatening complications including hyperkalaemia, acute kidney injury and metabolic disturbances as damaged muscle cells release large amounts of intracellular contents into the bloodstream. This scenario demonstrates why CK should never be interpreted as a cardiac marker in isolation, while also highlighting that markedly elevated CK requires prompt recognition and management.
Scenario 5: Serial troponins
A 65-year-old woman presents one hour after the onset of central chest tightness and shortness of breath. Her initial ECG does not show ST-segment elevation, but symptoms remain concerning.
Initial troponin
Troponin: 7 ng/L
Repeat troponin, 3 hours later
Troponin: 84 ng/L
Interpretation
The initial troponin was within the reference range because the blood sample was taken early, before enough troponin had entered the bloodstream to be detected. The repeat result shows a clear rise, indicating acute myocardial injury. This demonstrates why serial troponins are performed when clinical suspicion remains high. A single normal troponin early after symptom onset does not necessarily exclude myocardial injury, especially when the history remains concerning.