Key Takeaways

  • M proteins, or monoclonal proteins, are abnormal proteins. When they’re found in the blood or urine, they can indicate the presence of certain diseases. Multiple myeloma, a cancer of the plasma cells, is the most commonly diagnosed condition.
  • Healthcare professionals often find M proteins through blood tests used to detect other health conditions; further testing, such as serum protein electrophoresis (SPEP) and immunoelectrophoresis, can help identify and classify these proteins.
  • The presence of M proteins can be a sign of monoclonal gammopathy of undetermined significance (MGUS), but most individuals with MGUS do not develop more serious conditions. However, experts recommend regular monitoring with a doctor.

Proteins are an important component of all living organisms. They can be found in all types of body tissues, including the blood. Antibodies are one example. These protective proteins attack and kill off invading disease(s).

When you’re healthy, plasma cells (a type of white blood cell) in your body’s bone marrow help your body fight infections by producing antibodies that find and attack germs. Your bone marrow is a soft tissue found inside most of your bones that produces blood cells.

Sometimes the plasma cells produce abnormal proteins. These abnormal proteins are called M proteins, or monoclonal proteins. Other common names for these proteins include:

  • monoclonal immunoglobulin
  • M-spike
  • paraprotein

Finding M proteins in the blood or urine is usually a sign of disease. Their presence is most commonly associated with a type of cancer of the plasma cells called multiple myeloma.

In other cases, M proteins could also be a sign of the following plasma cell disorders:

Plasma cells in a healthy person’s bone marrow make antibodies that fight disease when it enters the body. When multiple myeloma affects plasma cells, they begin to grow out of control, filling the bone marrow and blood with large amounts of M proteins. These cancerous plasma cells begin to outnumber healthy blood-forming cells in the bone marrow.

When M proteins begin to outnumber healthy blood cells, this can result in a low blood count and health complications, such as:

  • frequent infections
  • bone problems
  • reduced kidney function
  • anemia

Health experts aren’t sure exactly what causes multiple myeloma. But it appears to begin with one abnormal plasma cell in the bone marrow. Once this abnormal cell forms, it begins to multiply rapidly and does not die as a typical cell does. This is how multiple myeloma spreads.

Most cases of multiple myeloma start out as a usually harmless condition called monoclonal gammopathy of undetermined significance (MGUS). One sign of MGUS is the presence of M proteins in the blood. Yet, with MGUS, the level of M proteins in the body is low and does not cause damage.

The International Myeloma Foundation reports that MGUS affects between 3% and 4% of people over age 50 years in the United States. Research suggests that about 20% percent of people with MGUS go on to develop multiple myeloma or another type of cancer, indicating most people with the condition typically do not develop cancer.

Whether or not MGUS will progress into a more serious condition is difficult to determine. Some people are more at risk than others.

The more M proteins in your blood and the longer you’ve had MGUS, the higher your risk of developing one or more related conditions. Besides multiple myeloma, M proteins in your blood may result in:

  • Non-IgM MGUS (IgG or IgA MGUS): These are the most common types of MGUS that may progress into multiple myeloma, as well as immunoglobulin light chain (AL) amyloidosis or light chain deposition disease.
  • IgM MGUS: About 15% of people with MGUS have IgM MGUS. The condition can lead to a rare type of cancer called Waldenstrom macroglobulinemia and, less commonly, to lymphoma, AL amyloidosis, or multiple myeloma.
  • Light chain MGUS (LC-MGUS): LC-MGUS is a newly classified type of MGUS. It can lead to a condition called Bence Jones proteinuria, which causes a buildup of certain M proteins in the urine. It can also possibly lead to light chain multiple myeloma, AL amyloidosis, or light chain deposition disease.
  • MGUS-related complications: These may include bone fractures, blood clots, and kidney problems.

Most people receive an MGUS diagnosis during blood tests for other conditions that affect the blood’s protein levels, such as peripheral neuropathy. A doctor may notice abnormal proteins and odd levels of normal proteins during a blood test. They may also notice unusual levels of protein in your urine.

If your blood or urine test results show abnormal protein levels, a doctor will recommend further testing. Abnormal plasma cells produce M proteins in the blood that are exactly the same.

To find these identical M proteins, your doctor might order a blood test called serum protein electrophoresis (SPEP). It involves placing a sample of the liquid part of your blood (called serum) into a gel that’s exposed to an electric current. The current causes the different proteins in your serum to move and group together.

The next step is to use immunoelectrophoresis to determine the exact type of proteins in the blood. During this process, laboratory technicians measure the different antibodies in your blood. If there are M proteins in your blood, technicians will be able to identify them during this process.

If your doctor finds M proteins in your blood, they may order additional tests to rule out any conditions related to MGUS that could cause problems. These tests may include:

  • Complete blood count (CBC): A CBC measures the levels of different cells in your blood and can reveal certain MGUS-related issues, such as anemia.
  • Beta-2 microglobulin test: Abnormal plasma cells may also create a type of protein called beta-2 microglobulin. High levels of this protein suggest a more advanced state of MGUS-related disease, such as multiple myeloma.
  • Biopsies: Biopsies involve removing and then analyzing a small amount of tissue from your bone marrow, a tumor, or lymph nodes.
  • Blood chemistry tests: These tests measure creatinine, albumin, calcium, and lactic dehydrogenase (LDH). Abnormal levels of these substances may indicate complications from MGUS and a less optimistic outlook.
  • Echocardiogram (ECHO): An ECHO is used to check for any heart problems, which can develop as possible complications from amyloidosis. MGUS can cause amyloidosis.
  • Imaging tests: MRIs, CT scans, PET scans, and X-rays are used to determine how far cancer has spread or to check bones for damage from multiple myeloma.
  • Urine protein tests: Examples of urine protein tests include urine protein electrophoresis (UPEP) and urine immunofixation tests measure the level of M proteins your body produces over a 24-hour period.

Doctors often find M proteins in the blood while testing for other conditions that affect the blood’s protein levels, such as nervous system disorders. Unusual levels of proteins may also be found during routine urine tests.

The presence of M proteins in the body and a diagnosis of MGUS is not necessarily a cause for concern. Most people who have M proteins in their blood do not go on to develop further health problems. However, a small number of people with MGUS will develop serious cancers or blood conditions, such as multiple myeloma.

If you receive an MGUS diagnosis, talk with your doctor about further testing that could help you better understand your condition and its likely outcome.

Though currently you cannot minimize your risk of developing an MGUS-related condition, there are steps you can take to help manage it. You can work with your doctor to schedule routine checkups and frequent blood tests to monitor your condition.