Smouldering myeloma (also spelled “smoldering”) is a precancerous blood condition that sits between two stages: a harmless state called MGUS and active multiple myeloma, which is a blood cancer requiring treatment. It’s defined by having abnormal plasma cells in the bone marrow and an abnormal protein in the blood, but without the organ damage that defines full-blown myeloma. About 10% of people with smouldering myeloma progress to active myeloma each year during the first five years after diagnosis, though that risk drops significantly over time.
How Smouldering Myeloma Is Defined
Plasma cells are white blood cells in your bone marrow that normally produce antibodies to fight infection. In smouldering myeloma, a group of these cells becomes abnormal and starts producing a single type of protein, called a monoclonal protein (or M-protein), that serves no useful immune function. A diagnosis requires either bone marrow plasma cells between 10% and 59%, or a serum M-protein level of 3 g/dL or higher, or both. Critically, the person must have no signs of organ damage from the abnormal cells.
This is what separates smouldering myeloma from the conditions on either side of it. MGUS, the stage before it, involves less than 10% plasma cells in the bone marrow and an M-protein level no higher than 3 g/dL. Active multiple myeloma, the stage beyond it, is diagnosed when organ damage appears or when certain biomarkers cross extreme thresholds (bone marrow plasma cells reaching 60% or higher, or a free light chain ratio of 100 or more, or more than one focal lesion of at least 5mm on MRI). In 2014, the International Myeloma Working Group updated its criteria so that patients hitting those extreme thresholds are reclassified as having active myeloma, even without traditional symptoms, because their risk of progression is so high that waiting would be harmful.
What Progression to Active Myeloma Looks Like
The shift from smouldering to active myeloma is defined by organ damage, remembered by the acronym CRAB: calcium elevation in the blood (above 11.0 mg/dL), renal (kidney) problems, anemia (low red blood cell count), and bone lesions. These are the signs that abnormal plasma cells have grown aggressive enough to cause real harm. Bone lesions, which are weak spots or holes in the bone, are detected through imaging. Anemia shows up as persistent fatigue and weakness. Kidney problems may cause no obvious symptoms early on, which is one reason regular blood work matters.
The risk of progressing isn’t constant. It’s highest in the first five years: roughly 22% of patients progress within two years, 42% within five years, and 64% within ten years. After the first five years, the annual progression rate drops to about 3%, and after ten years it falls to around 1.5% per year. Some people live with smouldering myeloma for decades without ever developing active disease.
Risk Stratification: Low, Intermediate, and High
Not everyone with smouldering myeloma faces the same odds. Doctors use a scoring system known as the 20/2/20 model, developed at the Mayo Clinic, to sort patients into risk categories based on three lab values: bone marrow plasma cells above 20%, serum M-protein above 2 g/dL, and a serum free light chain ratio above 20. Each factor present counts as one risk point.
Patients with none of these risk factors are classified as low risk, with less than a 10% chance of progressing within five years. Those with one factor are intermediate risk, facing up to a 50% chance of progression within five years. Patients with two or three factors are high risk, with more than a 50% probability of progressing within just two years. This stratification shapes everything about how a patient is monitored and whether early treatment is considered.
Symptoms During the Smouldering Phase
Smouldering myeloma has traditionally been described as completely asymptomatic, but patient-reported data tells a more nuanced story. In research published in the Journal of Patient-Reported Outcomes, every participant with smouldering myeloma reported experiencing at least one symptom. The most common were tiredness and fatigue, general weakness, and pain. These symptoms were meaningful enough to affect daily quality of life.
On top of physical symptoms, the psychological burden is significant. Living with a condition that may or may not become cancer creates a unique kind of anxiety. Patients describe the uncertainty of not knowing whether or when they’ll need treatment as one of the hardest parts of the diagnosis. This combination of subtle physical symptoms and ongoing emotional weight means smouldering myeloma is not the purely silent condition it was once assumed to be.
How Smouldering Myeloma Is Monitored
Because most patients are not treated immediately, the cornerstone of management is careful, scheduled monitoring designed to catch any signs of progression early. After the initial diagnosis, blood work is typically repeated at three months to confirm that the abnormal protein levels and blood counts are stable. If everything holds steady, follow-up shifts to every four to six months for the first year, then every six to twelve months after that.
The monitoring schedule is tailored to risk level. Low-risk patients follow a pattern similar to MGUS: visits every six months for the first two years, then annually. Intermediate-risk patients, who represent the “true” smouldering myeloma population, are seen every three to six months. High-risk patients are monitored most closely, often every two to three months, while the question of whether to start treatment is evaluated.
Imaging plays an important role. MRI is generally performed annually for at least five years to check for bone lesions or focal lesions that might signal progression. Doctors also track the trajectory of lab values over time. A rising M-protein (increasing by 25% or more), a shifting free light chain ratio, or a drop in hemoglobin of at least 0.5 g/dL within the first year are all signals that prompt more frequent monitoring. If values remain stable after two years, the schedule can relax to once a year. Annual checks for heart and kidney function are also common, since abnormal plasma cells can occasionally cause a related condition called amyloidosis.
Treatment: Watch and Wait vs. Early Intervention
For low and intermediate-risk patients, the standard approach remains active surveillance, sometimes called “watch and wait.” No treatment is given, but the condition is tracked closely. This isn’t passive neglect; it’s a deliberate strategy based on evidence that treating low-risk patients early doesn’t improve outcomes and exposes them to side effects they may never need.
For high-risk patients, the calculus is different. Clinical trials have shown that early treatment with certain medications can delay or prevent progression to active myeloma. A landmark Spanish trial demonstrated that high-risk patients treated early had significantly better outcomes than those who were simply observed. This has made early intervention for high-risk smouldering myeloma an active area of clinical practice. Whether to treat, and with what, depends on a patient’s specific risk profile, overall health, and preferences. Many high-risk patients are offered enrollment in clinical trials testing newer treatment combinations.
The evolving nature of the risk models means that patients diagnosed years ago may benefit from having their risk reassessed with current criteria. Someone previously classified as smouldering myeloma under older guidelines might now meet the updated definition of active myeloma and qualify for immediate treatment.