What is MGUS and smoldering myeloma?

MGUS stands for monoclonal gammopathy of undetermined significance. It is a long name for a common finding. It means a blood test found a small amount of an abnormal protein called M-protein (monoclonal protein). This protein comes from a small group of plasma cells in the bone marrow. Plasma cells are white blood cells that normally make antibodies to fight infection. In MGUS, the group of abnormal plasma cells is small, and it is not causing harm. MGUS is not cancer. It usually causes no symptoms and is most often found by chance, when blood tests are done for another reason. About 3 in 100 people over age 50 have MGUS, and it becomes more common with age. It is about twice as common in Black Americans.

When should I call my care team?

If you are getting treatment: fever of 100.4 F (38 C) or higher, or shaking chills; New or sudden back pain, or new numbness, weakness, or trouble walking or controlling your bladder or bowels (this can mean pressure on the spinal cord); Confusion, extreme sleepiness, strong thirst, or severe constipation (signs of high calcium); Passing much less urine than usual, very foamy urine, or new swelling in the legs or around the eyes; Sudden, severe bone pain after a minor fall or movement (possible broken bone); New or quickly spreading numbness, tingling, or weakness in your hands or feet; Shortness of breath, chest pain, or feeling faint, especially with leg swelling; Swelling, pain, or warmth in one leg, or sudden shortness of breath (possible blood clot, especially if you take lenalidomide); Bleeding that will not stop, or many new bruises.

MGUS and smoldering myeloma

MGUS and smoldering myeloma are quiet, early plasma cell conditions that usually need watching, not treatment, and most people with MGUS never develop myeloma.

Hi, I'm Rosa. Today we'll go through MGUS and smoldering myeloma together, one step at a time. I'll keep it simple, and you can pause me, go back, or skip ahead whenever you like. Just so you know, this is for learning only. It is not medical advice, so please check everything with your own doctor and care team. Okay, let's get started.

What these conditions are

Let's start with the basics. MGUS is short for a long medical name. It means a blood test found a small amount of an abnormal protein. This protein comes from a small group of plasma cells. Plasma cells are white blood cells in your bone marrow. In MGUS, this group is small and is not causing harm. MGUS is not cancer. It is common, especially as people get older. Smoldering myeloma is a middle step. There are more abnormal cells, but still no damage to your body. Most people with MGUS never have a problem from it.

Types and related conditions

Let's talk briefly about the types. The most common type of MGUS makes a protein called IgG or IgA. If it ever changes, it most often turns into myeloma. Another type makes a protein called IgM. If it changes, it more often turns into a slow-growing lymphoma. Some people only make small protein pieces called light chains. Smoldering myeloma has more protein or more abnormal cells. Rarely, the protein harms the kidneys or nerves, even when the cell group is small. Those cases are treated. Your care team will tell you which type you have.

Tests you may have

Now let's talk about tests. Blood tests find the abnormal protein and measure how much there is. Another blood test measures light chains and compares the two kinds. Blood counts, calcium, and kidney tests check that there is no damage. You may give a urine sample. Some people need a bone marrow biopsy. A small sample is taken from the hip bone after numbing medicine. Many people with low-risk MGUS do not need one. People with smoldering myeloma usually have a scan to look at the bones. You can read more about each test in the guide.

Risk groups

Next up, risk groups. These conditions are not staged like cancer. Instead, doctors sort people into low, middle, or high risk. They look at how much protein there is, the type of protein, and the light chain test. For smoldering myeloma, they also look at the bone marrow. Risk tells your team how likely a change is over time. It also helps decide how often you need tests. A risk group is an estimate, not a promise. Many people, even at higher risk, stay stable for years.

Lab tests and risk

Before we move to treatment, there are some things to know. The amount of protein is followed over time. A number that stays steady is reassuring. A number that keeps rising may need a closer look. The light chain test and bone marrow results add more detail. Gene tests on the marrow can also help in smoldering myeloma. Each test has a different role. Your doctor will tell you what your results mean. You can read more in the guide. The Questions to ask page can help you prepare.

Watching and treatment

Now let's talk about the plan. MGUS is not treated. Instead, you have regular blood tests. For low-risk MGUS, tests can often be spaced a few years apart. Most smoldering myeloma is watched more closely, every few months at first. This is a real plan, not doing nothing. For high-risk smoldering myeloma, a medicine shot called daratumumab can delay myeloma. Some people choose a clinical trial instead. Clinical trials are research studies that may offer new options. If myeloma ever develops, finding it early means it can be treated before serious damage.

Side effects and help

Let's look at side effects. Waiting and watching can feel stressful. Many people feel worried before each blood test. Talking with your team and with others in the same situation can really help. A bone marrow biopsy can be sore for a few days. If you get daratumumab, the first shot can cause chills or a stuffy nose. You get medicine beforehand to prevent this. Infections can be more likely on treatment. Vaccines and hand washing matter. We have a whole side effects guide on this site.

Living well

Now let's talk about living well. Most people with MGUS live a normal life and never need treatment. Keep your follow-up visits. That is the most important thing. Ask for copies of your results, so you can see the trend. Stay active, and take care of your bones. Keep up with your vaccines and regular health care. Call your care team for new bone or back pain. Call for foamy urine, leg swelling, or numbness in your feet. Call for confusion or strong thirst. If it feels very bad, call 911. Also visit our new Screening, prevention and survivorship section.

You did great today. Thank you for learning with me. Keep your follow-up visits, and bring your questions to your next one. You can call your care team any time you are worried. Most people with these conditions live full lives. You are not alone.

Call your care team right away if you have

If you are getting treatment: fever of 100.4 F (38 C) or higher, or shaking chills
New or sudden back pain, or new numbness, weakness, or trouble walking or controlling your bladder or bowels (this can mean pressure on the spinal cord)
Confusion, extreme sleepiness, strong thirst, or severe constipation (signs of high calcium)
Passing much less urine than usual, very foamy urine, or new swelling in the legs or around the eyes
Sudden, severe bone pain after a minor fall or movement (possible broken bone)
New or quickly spreading numbness, tingling, or weakness in your hands or feet
Shortness of breath, chest pain, or feeling faint, especially with leg swelling
Swelling, pain, or warmth in one leg, or sudden shortness of breath (possible blood clot, especially if you take lenalidomide)
Bleeding that will not stop, or many new bruises

Side effects: what helps

Sources

Quick answers

What is MGUS and smoldering myeloma?: MGUS stands for monoclonal gammopathy of undetermined significance. It is a long name for a common finding. It means a blood test found a small amount of an abnormal protein called M-protein (monoclonal protein). This protein comes from a small group of plasma cells in the bone marrow. Plasma cells are white blood cells that normally make antibodies to fight infection. In MGUS, the group of abnormal plasma cells is small, and it is not causing harm. MGUS is not cancer. It usually causes no symptoms and is most often found by chance, when blood tests are done for another reason. About 3 in 100 people over age 50 have MGUS, and it becomes more common with age. It is about twice as common in Black Americans.
When should I call my care team?: If you are getting treatment: fever of 100.4 F (38 C) or higher, or shaking chills; New or sudden back pain, or new numbness, weakness, or trouble walking or controlling your bladder or bowels (this can mean pressure on the spinal cord); Confusion, extreme sleepiness, strong thirst, or severe constipation (signs of high calcium); Passing much less urine than usual, very foamy urine, or new swelling in the legs or around the eyes; Sudden, severe bone pain after a minor fall or movement (possible broken bone); New or quickly spreading numbness, tingling, or weakness in your hands or feet; Shortness of breath, chest pain, or feeling faint, especially with leg swelling; Swelling, pain, or warmth in one leg, or sudden shortness of breath (possible blood clot, especially if you take lenalidomide); Bleeding that will not stop, or many new bruises.

Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.