Smoldering Multiple Myeloma: The Middle Ground Between MGUS and Myeloma

In the last post, we talked about MGUS — a silent, usually benign condition where the body produces abnormal M proteins, and most people go on to live completely normal lives. But what happens when things progress a little further, but not quite to full-blown multiple myeloma? That's where smoldering multiple myeloma (SMM) comes in.

Think of the plasma cell disorders as a spectrum. On one end, you have MGUS — low levels of M protein, very few abnormal plasma cells, no symptoms. On the other end, you have active multiple myeloma — a cancer requiring treatment. Smoldering multiple myeloma sits in the middle: more abnormal cells than MGUS, but without the end-organ damage that defines active myeloma. It's an intermediate state — and understanding it matters.

Smoldering multiple myeloma is a precursor plasma cell disorder, meaning it has the potential to evolve into active multiple myeloma over time, but has not yet caused organ damage.

Like MGUS, it typically causes no symptoms. It is most often discovered incidentally on blood work, when a serum protein electrophoresis (SPEP) or free light chain test is ordered for another reason.

SMM is diagnosed when both of the following are present, and there is no evidence of end-organ damage (no hypercalcemia, renal insufficiency, anemia, or bone lesions attributable to the plasma cell disorder — also known as CRAB criteria):

This is what sets SMM apart from MGUS: the numbers are higher. More M protein, more abnormal plasma cells — but still no end-organ damage.

If the bone marrow plasma cells reach ≥ 60%, or if there are CRAB features, the diagnosis upgrades to active multiple myeloma, which requires treatment.

SMM is less common than MGUS. It affects roughly 0.5–1% of adults over 40, and like most plasma cell disorders, it becomes more prevalent with age. It is also more common in people of African descent.

This is the key question with SMM. The average risk of progression to active multiple myeloma is about 10% per year for the first 5 years after diagnosis — significantly higher than the 1% per year seen with MGUS. After 5 years, the risk does decrease, but monitoring remains important throughout.

Not all SMM is the same, though. Researchers have identified several features that help stratify risk:

The Mayo Clinic and the IMWG (International Myeloma Working Group) have developed risk stratification models to help identify patients at high vs. low risk of progression. One of the most widely used is the 2/20/20 rule, which flags high-risk SMM based on three factors:

The more of these criteria a patient meets, the higher the risk of progressing to active myeloma. A patient with all three has a substantially higher short-term risk than someone with none or just one. This rule helps guide the conversation about whether active surveillance alone is appropriate, or whether early treatment should be considered. High-risk SMM patients are increasingly being referred for clinical trials or offered treatment, while low-risk patients may be managed with observation alone.

This is an evolving and exciting area of research. Traditionally, SMM was managed with active surveillance — regular monitoring without treatment, since treating it early did not consistently show a survival benefit. However, that is changing.

Recent clinical trials, including the QUIREDEX and GEM-CESAR trials, have shown that early treatment in high-risk SMM may delay progression and, in some cases, improve overall survival. As a result, many major cancer centers now offer treatment to carefully selected high-risk SMM patients, ideally in the context of a clinical trial.

For low- and intermediate-risk SMM, watchful waiting with regular monitoring remains the standard of care.

For patients with SMM on surveillance, monitoring typically involves:

The goal is to catch progression to active myeloma early, so treatment can begin before significant end-organ damage occurs.

Smoldering multiple myeloma is a precancerous condition that occupies the middle ground between MGUS and active myeloma. Most patients have no symptoms, and many live for years — even decades — without ever progressing. But because the risk of progression is real and meaningful, it requires more vigilant monitoring than MGUS and, in high-risk cases, a conversation about early intervention.

If you or someone you know has been diagnosed with SMM, the most important thing is to be followed by a hematologist or oncologist with expertise in plasma cell disorders, to understand your personal risk category, and to know the warning signs that might signal progression.

To learn more about the building blocks of this conversation, check out the previous posts on SPEP, light chains, and MGUS.

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