When a person develops multiple myeloma, a cancer of the plasma cells, the cancer produces large amounts of abnormal antibodies called monoclonal proteins, or M proteins. Some call this an M spike. Other conditions can also cause high M protein levels.

Plasma cells, which are a type of white blood cell, normally produce antibodies that fight germs. Instead of fighting infections like antibodies should, these abnormal proteins grow out of control.

Having lots of M proteins in the blood indicates that a person may have multiple myeloma, which is a type of blood cancer. This condition accounts for 1.6% of all cancers in the United States.

In this article, learn about what M proteins do, how they develop, and how they contribute to multiple myeloma.

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M proteins are antibodies that are present in the blood. People with multiple myeloma or another plasma cell disorder can have high levels of M proteins in the blood.

Unlike infection-fighting antibodies, these M proteins are not helpful to the body. If they are related to a cancerous condition, they can multiply and build up in the bloodstream, depositing in the tissues and leading to organ dysfunction.

M proteins are known by several different names. They are also called:

  • monoclonal immunoglobin
  • paraproteins
  • monoclonal proteins
  • M spike

High levels of M proteins can indicate that a person has multiple myeloma. However, high M protein levels may also be a sign of several other plasma cell disorders, including:

Some people have M proteins in their blood but do not have multiple myeloma. A doctor will monitor them for changes, and they will advise treatment if the person starts showing symptoms.

People with M proteins in their blood may have MGUS, which is a noncancerous condition. In some cases, MGUS can lead to multiple myeloma or other cancers.

However, M proteins can also indicate SMM. This condition is usually a precursor to multiple myeloma. Although people with SMM may not have any other symptoms, a doctor will monitor them closely and advise treatment as the condition progresses.

Checking for high M protein levels is one of the first things a doctor will do to diagnose multiple myeloma. They will also look for other symptoms of the condition, such as anemia, reduced kidney function, high calcium levels, and bone lesions.

Learn more about the signs and symptoms of multiple myeloma here.

M proteins are made up of chains. These are pieces of the protein, and they can be heavy chains or light chains. Typically, antibodies are made up of two heavy and two light chains.

In myeloma, the M proteins can be made up of one type of heavy chain and one type of light chain.

The heavy chains are:

  • immunoglobulin A (IgA)
  • immunoglobulin M (IgM)
  • immunoglobulin E (IgD)
  • immunoglobulin G (IgG)
  • immunoglobulin D (IgE)

The light chains are kappa and lambda.

M proteins always produce the same types of cells. So, if a person has IgA heavy chains and kappa light chains, all of their proteins will be IgA kappa.

This is important because the types of M proteins can tell doctors what kind of myeloma a person has, and this can inform treatment decisions. For example, people with IgM myeloma may have a rare type of cancer called Waldenstrom macroglobulinemia.

IgG and IgA are the most common types of myeloma. IgG occurs in about 50% of people. Occasionally, traditional methods may not detect IgA myeloma, so it may not show up as an M spike.

Occasionally, M proteins only make light chains. This causes light chain myeloma, which affects 15% of people with multiple myeloma.

Having M protein in the blood is related to multiple myeloma. However, it can also occur in people with other conditions, such as those that can precede multiple myeloma. Such conditions include MGUS and SMM.


Most cases of multiple myeloma start as MGUS, which is a noncancerous condition. It affects about 3% of people over 50 years of age.

Although people with MGUS can have M protein in their blood, it does not cause any harm, and it usually causes no symptoms.

Many cases of MGUS will never progress to multiple myeloma.


This condition is a progression from MGUS, but it is not considered to be cancer. It occurs when plasma cells increase in the individual bone marrow.

Usually, people with SMM have no symptoms. Some doctors may treat SMM to delay progression to multiple myeloma.

Because multiple myeloma does not cause any symptoms until it progresses, it can be hard to diagnose early. Doctors often find high M protein counts incidentally during routine blood tests.

Some tests that a doctor may use to check for M protein in the blood include:

  • Serum and urine protein electrophoresis: This is a blood test that measures all proteins in the blood and looks for any protein present in abnormally high concentrations. This is the definitive test for an M spike, which gets its name for the characteristic spike that the M protein creates on the test result. A doctor will follow this up with a test called immunofixation, which can identify the abnormal protein.
  • Blood tests: One example is a quantitative immunoglobin test, which measures the amount of each heavy chain present in the blood.
  • Urine tests: These tests look at how much M protein has filtered through the kidneys.
  • Serum free light chain assay test: Normally, the light chains kappa and lambda have about equal numbers of each in the blood. If one is higher than the other, however, it can be a sign of myeloma.
  • Serum heavy/light chain assay test: The test measures both light and heavy chains bound together. A doctor may perform this test to detect the amount of M proteins in a person’s bloodstream.

If the tests show M protein in the blood, a doctor may recommend further tests.

Multiple myeloma is a condition that affects the plasma cells. It occurs when plasma cells produce abnormal M proteins.

However, the presence of M proteins does not necessarily mean that a person has cancer. They can also indicate MGUS, which is a noncancerous condition.

A doctor can check for M proteins with blood and urine tests. If M proteins are present in the blood, a doctor may perform further tests.