A heart attack, or myocardial infarction (MI), happens when not enough oxygen-rich blood reaches the heart, decreasing its oxygen supply. This loss of blood supply occurs when one or more of the arteries supplying blood to the heart become blocked, often due to a clot, or when the demand for oxygen-rich blood exceeds the supply.
These clots develop due to atherosclerosis, which occurs when plaques of fatty deposits, cholesterol, and other substances accumulate in the arteries. When plaques rupture, blood clots form and may lead to a heart attack.
Prolonged lack of oxygen leads to cell death in the heart muscle, causing a heart attack. Symptoms include tightness or pain in the chest that can spread to the neck, shoulders, and arms.
To limit damage to the heart and begin suitable treatment, doctors must diagnose heart attacks as quickly as possible. Blood tests form part of the diagnostic process.
In this article, we examine blood tests for a heart attack and what the results mean. We also look at other ways to diagnose a heart attack and what happens if a doctor confirms a heart attack diagnosis.
Is there a blood test for diagnosing a heart attack?
Doctors use various blood tests to help them diagnose heart attacks.
When the heart becomes injured during a heart attack, the muscle releases specific proteins into the blood, increasing their concentration. If a doctor suspects someone has had or is having a heart attack, they measure levels of these proteins using a blood test.
Historically, doctors tested for such proteins as creatinine kinase (CK), CK-MB, and myoglobin. However, since they can be elevated non-specifically and offer less diagnostic information, doctors no longer rely on them to confirm a person has had a heart attack.
Currently, doctors only measure the levels of cardiac troponin, which is an enzyme present solely in the heart. Most people with an acute heart attack have raised troponin levels within 2–3 hours of arriving at the hospital.
There are many types of troponin, including troponin C, troponin I, and troponin T. Only troponin I and T are present in the heart, making them ideal markers for diagnosing a heart attack.
Moreover, doctors use other diagnostic tools besides blood tests to verify a heart attack diagnosis, including an electrocardiogram (ECG).
How long after a heart attack can doctors use the test?
Doctors can test for troponin immediately when an individual presents with symptoms. Troponin levels rise as early as 4 hours after a heart attack and peak between 24 and 48 hours. Increased levels can persist for 7 days or longer, depending on kidney function.
The American College of Cardiology recommends troponin testing when a person exhibits symptoms and again 3–6 hours later.
What do troponin test results mean?
According to a 2017 diagnostic evaluation, a troponin level that exceeds the 99th percentile of the upper reference level indicates a heart attack. This means that 99% of people have a value of troponin below the upper reference range.
Additionally, an increase or decrease of 20% or more from the norm may also be consistent with a heart attack.
When considering the accuracy of troponin tests, it helps to understand the definitions of sensitivity and specificity.
Sensitivity refers to the likelihood a troponin test yields a positive result when an individual experiences a heart attack. Health experts refer to this as true positive.
By contrast, specificity refers to how likely it is for a troponin test to yield a negative result when an individual is not experiencing a heart attack. Health experts term this true negative.
Troponin T blood tests have a sensitivity of 79% and a specificity of 93%. Troponin I blood tests have a sensitivity of 83% and a specificity of 95%.
While it is possible for tests to yield false positive or negative results, troponin tests are usually highly accurate.
High-sensitivity troponin can have a false positive with a few conditions that are not heart attacks. This can happen with conditions that can damage the heart muscle.
Some conditions that can elevate troponin levels include:
- pericarditis, which is inflammation of the heart lining
- aortic dissection, which is when a tear occurs in the inner layer of the aorta
- coronary artery spasm, which is the sudden tightening of the muscles in the arteries of the heart
- myocarditis, which is inflammation of the heart muscle
Elevated troponin levels alone do not indicate that someone has had a heart attack. Doctors also perform a physical examination, obtain a medical history, and evaluate an ECG to diagnose a heart attack.
During a physical examination, a doctor will evaluate a person for heart attack symptoms, such as:
- chest pain
- upper body pain
- shortness of breath
They will also ask about an individual’s medical history, focusing on heart attack risk factors. These include:
- preexisting cardiovascular disease
An ECG measures the heart’s electrical activity and is an essential tool for heart attack diagnosis. Doctors should perform an ECG within 10 minutes of a person presenting with heart attack symptoms.
To perform an ECG, a doctor will place sticky electrode patches on an individual’s chest and limbs to measure the rate, rhythm, and timing of the heart’s electrical impulses.
ECG results can indicate disrupted electrical signals that make the heartbeat, and show whether someone has had a heart attack.
The troponin test and ECG results indicate whether a person has an ST segment elevation MI (STEMI) or non-ST segment elevation MI (NSTEMI). An NSTEMI is a type of heart attack that is typically less severe than STEMI, as it causes less damage.
If a person has a STEMI, they may receive thrombolytic therapy to dissolve the blood clot blocking the blood supply to the heart.
They may also undergo percutaneous transluminal coronary angioplasty, which doctors also refer to as percutaneous coronary intervention. This is a nonsurgical procedure during which a doctor inserts a tiny tube called a stent to keep the blood vessels open.
If an individual has an NSTEMI heart attack, a doctor can suggest anticoagulant and antiplatelet medications to stop blood clots from forming.
A heart attack can be a life-altering occurrence for many people. After a heart attack, individuals should fully understand their treatment plan to lower their risk of complications and another heart attack.
A person should ask a doctor about:
- the lifestyle changes they should make
- how to take their medications
- when to follow up with a cardiologist and primary care physician
- how to reduce the risk of another heart attack
- what to do if they experience chest pain again
- how much physical activity they can do and how often they can do it
- when they can return to work
A heart attack occurs when a clot decreases blood and oxygen supply to the heart.
Diagnosis of a heart attack involves a combination of blood tests, an ECG, and a comprehensive physical examination, including medical history.
Blood tests measure levels of important biomarkers known as troponin I and T. When these elevate significantly, this may indicate a heart attack. Likewise, an unusual ECG may also be indicative of a heart attack.
Treatment depends on the type of heart attack a person has, but typically, it includes a combination of medications, surgical procedures, and lifestyle changes.