What Is A Myeloperoxidase Assay And When Is It Used?

A myeloperoxidase (MPO) assay is a blood test that measures the level of an enzyme called myeloperoxidase, which white blood cells release when they are inflamed. Doctors most often use it to help assess a person’s risk for heart disease, especially in people who have chest pain or other signs of heart trouble but whose standard tests are not clearly abnormal. It is not a routine screening test for everyone, but rather a specialized tool that gives a clearer picture of inflammation inside your arteries.

What Exactly Does a Myeloperoxidase Assay Measure?

The test measures the amount of the MPO enzyme in your blood. This enzyme is made by white blood cells called neutrophils and monocytes. When these cells are activated by inflammation, they release MPO into the bloodstream.

MPO is part of your body’s defense system. It helps kill bacteria and other invaders. But when inflammation becomes long-term, too much MPO can damage healthy tissue. This is especially true inside your arteries. The enzyme can make LDL cholesterol more likely to stick to artery walls and form plaque.

Research published in the New England Journal of Medicine found that higher MPO levels predicted heart attack risk even when other tests looked normal. This is why doctors sometimes order it when they want more information than standard cholesterol tests provide.

When Is a Myeloperoxidase Assay Ordered?

Doctors typically order this test for people who come to the emergency room with chest pain or other symptoms that could be heart-related. If the initial tests — like an EKG and troponin blood test — are not clearly abnormal, an MPO assay can help decide if the person is having a heart problem or something less serious.

Some cardiologists also use it for people who already know they have heart disease. In these cases, the test can show whether treatment is working or if the risk of a future heart attack remains high.

The test is not for everyone. The American Heart Association does not recommend it as a general screening test for people without symptoms. It is a targeted test for specific clinical situations where the diagnosis is not clear.

One study in the Journal of the American College of Cardiology followed over 600 patients with chest pain. Those with high MPO levels were four times more likely to have a heart attack within the next month compared to those with low levels. This is the kind of real-world data that makes the test useful in emergency settings.

How Does the Myeloperoxidase Assay Compare to Other Heart Tests?

Standard heart tests measure different things. Cholesterol tests look at the fats in your blood. Troponin tests look for damage that has already happened to heart muscle. C-reactive protein (CRP) tests measure general inflammation in the body.

The MPO assay is different. It measures a specific type of inflammation that is directly involved in plaque formation and rupture. Plaque rupture is what causes most heart attacks. So MPO can give information about the activity of the disease, not just the presence of risk factors.

Each test gives a different piece of the puzzle. The MPO assay is most useful when doctors suspect active artery inflammation but other tests are still normal.

What Do the Results Mean?

MPO levels are measured in picomoles per liter (pmol/L). There is no single cutoff number that separates “normal” from “abnormal” for everyone. Different labs use different reference ranges. Your doctor will interpret your result based on your specific situation.

In general, higher levels suggest more active inflammation in your arteries. This does not mean you are definitely having a heart attack. It means your risk is higher than someone with a lower level. The test is best used as part of a larger picture that includes your symptoms, other lab results, and your medical history.

Some studies suggest that levels above 500 pmol/L are associated with increased risk. But this number varies between studies and between different test manufacturers. The most important thing is that your doctor explains what your specific result means in your case.

A normal MPO level does not guarantee you are safe from heart disease. It only tells you that one specific type of inflammation is not elevated at the moment. People with advanced heart disease can sometimes have normal MPO levels if the inflammation is not currently active.

What Are the Limitations of the Myeloperoxidase Assay?

No test is perfect, and the MPO assay has real limitations that are important to understand. First, the test is not widely available. Many smaller hospitals and clinics do not offer it. You may need to go to a larger medical center or a specialized lab.

Second, MPO levels can be temporarily elevated by things other than heart inflammation. Infections, autoimmune diseases, and even some medications can raise your MPO level. This means a high result does not always point to a heart problem. Your doctor has to rule out other causes.

Third, there is no standard treatment that specifically lowers MPO levels. If your result is high, your doctor will likely recommend treatments that reduce inflammation in general — like statins, lifestyle changes, or anti-inflammatory medications. But no drug is approved specifically for lowering MPO.

Research published in Clinical Chemistry noted that MPO testing has not been standardized across all labs the way cholesterol testing has. Two different labs might give slightly different results for the same blood sample. This is why the test is best used for trend monitoring rather than a single one-time measurement.

Common Misconceptions About the Myeloperoxidase Assay

One common myth is that the MPO assay can replace standard cholesterol tests. This is not true. The MPO test provides different information. It does not tell you your LDL, HDL, or triglyceride levels. You still need a standard lipid panel for that.

Another misconception is that a high MPO level means you will definitely have a heart attack soon. This is not accurate. A high level indicates increased risk, not a guaranteed event. Many people with high MPO levels never have a heart attack, especially if they get proper treatment.

Some people also believe that the test is only for older adults. While heart disease risk increases with age, the MPO assay can be useful for younger people with unexplained chest pain or a strong family history of early heart disease. Age alone does not determine who should get this test.

There is also confusion about whether you need to fast before the test. Unlike cholesterol tests, the MPO assay does not require fasting. You can eat and drink normally before having your blood drawn. This makes it convenient for emergency room use.

Finally, some people think a normal MPO level means they have no heart disease risk at all. That is false. Heart disease is complex and involves many factors. A normal MPO level only rules out one specific type of active inflammation. You can still have high cholesterol, high blood pressure, or other risk factors that need attention.

What to Discuss With Your Doctor

If your doctor suggests an MPO assay, ask why they think it is needed for your situation. A good answer would be that your symptoms or other test results are not clear. A less good answer would be that they just want to check everything possible.

Ask what the results will change about your treatment. If the test comes back high, will your doctor recommend a different medication or a more aggressive treatment plan? If it comes back normal, will they stop looking for heart problems? Understanding these answers helps you know if the test is worth doing.

Also ask about the cost and insurance coverage. Some insurance plans cover the MPO assay for specific indications like chest pain in the emergency room. Others may not cover it at all. You do not want an unexpected bill for a test you did not fully understand.

If you have already had the test and received results, ask for a clear explanation in plain language. What was your number? How does it compare to the reference range used by your lab? What does your doctor recommend based on this result? A good doctor will take the time to explain this without rushing.