To check if you have a cancer called multiple myeloma, your doctor may ask you to get a test for M protein. If M protein shows up in your blood or urine, it's a sign you might have that disease or another condition.
The M stands for monoclonal. Monoclonal means that all copies of the protein, in this case, an antibody or part of an antibody, are exactly the same. You may also hear your doctor call them monoclonal immunoglobulin, myeloma, M spike, or paraprotein.
Whatever their name, they're made in your bone marrow, the spongy tissue in the middle of many bones. That's where blood cells form. One kind of white blood cell, called a plasma cell, plays a big role in your immune system -- your body's defense against germs.
To fend off sickness, plasma cells release proteins called antibodies. They find and kill viruses, bacteria, and other substances. But sometimes, plasma cells make abnormal proteins, which are the ones called M proteins. These M proteins don't have any power to fight infection.
Conditions Linked to M Proteins
A variety of conditions are related to M proteins:
MGUS (monoclonal gammopathy of undetermined significance). If you have this condition, you have both healthy plasma cells and abnormal ones. The abnormal plasma cells make M proteins that show up in your blood. Most of the time, MGUS doesn't cause any problems or symptoms. But for some people, MGUS turns into a harmful condition, such as multiple myeloma or lymphoma.
Smoldering multiple myeloma.Smoldering multiple myeloma is the pathology, but there are more monoclonal proteins than in MGUS. However, it doesn't meet the diagnostic criteria for MM because smoldering multiple myeloma usually doesn't cause any symptoms or end-organ damage. There are more abnormal plasma cells and M proteins in the blood. The risk of developing multiple myeloma is higher than seen in MGUS.
Multiple myeloma. When plasma cells are cancerous and grow out of control, it's called multiple myeloma. These cancer cells build up in the bone marrow and crowd out healthy cells. They also make M proteins. Higher levels of M protein in the blood can lead to complications.
Waldenstrom's macroglobulinemia. This cancer affects small lymphocytes (white blood cells). In this blood cancer, plasma cells produce a particular type of sticky M protein called IgM monoclonal antibody.. A buildup of these proteins can thicken the blood and lead to symptoms, such as fatigue and weight loss.
Complications of M Proteins
A lot of the time, M proteins don't cause any problems. You may not realize that you have them in your body. But as the abnormal or cancerous plasma cells spread, they may crowd out healthy blood cells. This can lead to complications, such as:
Anemia. Symptoms of anemia like shortness of breath is one of the more common presentations cause the BM is so crowded with abnormal plasma cells that red blood cells are crowded out. So are platelets that are formed in the bone marrow, so bruising is common.
Infections. When plasma cells make more M proteins, there are fewer functional antibodies to fight infections.
Peripheral neuropathy. M proteins can affect how well your nerves work. You can get numbness, tingling, or burning in your hands, feet, or lower legs.
Bone loss or damage. Bones are weakened by dysfunctional plasma cells, both cause of decreased bone density (bone loss) and because of actual collections of cancer cells in the bone. You can suffer from bone fractures from minimal activity (like a sneeze). This is called a pathologic fracture.
Kidney problems. Your kidneys have to work overtime to filter out the M proteins and calcium in your blood. Your kidneys might not work as well as they should, or you might even have kidney failure.
Types of M Proteins
M proteins are made of pieces called chains. There are usually two "light" chains and two "heavy" chains.
M proteins are grouped by what kind of heavy and light chains they have. There are five kinds of heavy chains, called IgG, IgA, IgD, IgM, and IgE. There are two types of light chains, called kappa and lambda.
The most common type of M protein in myeloma is IgG. High levels of IgM suggest you may have Waldenstrom's macroglobulinemia.
Tests for M Proteins
If your doctor suspects a disease or notices abnormal proteins on another test, they may want to test for M proteins. There are several types of tests:
Serum and urine protein electrophoresis (SPEP and UPEP). A small sample of urine and blood are collected. The sample is run through an electric charge that separates proteins based on their charge. If the results look suspicious for a monoclonal protein (an M spike is seen), a follow-up test is automatically done. It is called immunofixation. It is done to identify what sub-type of monoclonal protein is present. It can also identify an M protein that is a piece of antibody, called a light chain M protein.
Quantitative immunoglobulin testing (Qlg). This test measures the total amount of immunoglobulin proteins in your blood. It is useful to follow if you are found to have a monoclonal protein in your blood or urine. An increasing amount of M protein over time is one of the criteria for treatment or a change in treatment.
If you already know that you have M proteins, your doctor may use these tests to learn more about them. This may help better diagnose and treat your condition.
Immunofixation electrophoresis (IFE) of blood or urine. In this test, an electric charge separates the M proteins by type. This helps your doctor find out what kind of M proteins you have.
Serum-free light chain assay. Sometimes, the light chains that are normally part of your M proteins instead float freely in your blood. This test looks for and measures these light chains.
