Written by Dr. Roupen Odabashian MD, FRCPC
Oncologist | Founder, MeDucation AI | Updated October 2026
The verdict: monoclonal gammopathy of undetermined significance (MGUS) is a clonal plasma cell state found incidentally on serum protein electrophoresis, and on average it progresses to multiple myeloma or a related plasma cell disorder at a rate of roughly 1% per year[1][2]. That average hides a wide range of individual risk, and the tool fellows are expected to know cold for the ABIM exam is the Mayo Clinic risk stratification model, which uses three variables measured at the time of MGUS diagnosis: a non-IgG M-protein isotype, an M-protein concentration at or above a defined threshold, and an abnormal serum free light chain (FLC) ratio[3]. The number of these factors a patient has (0, 1, 2, or 3) sorts them into low, low-intermediate, high-intermediate, or high-risk groups, each with a different 20-year cumulative risk of progression[3]. There is no single MeDucation-endorsed or universally official online MGUS risk calculator discussed here; this article walks through applying the model by hand, the way it is actually tested and actually used at the bedside.
Conflict of interest, up front: I am a practicing hematologist-oncologist and I founded MeDucation AI, which sells a physician-written heme/onc question bank and other board-prep tools. This article is a clinical and board-prep explainer, not a product pitch, and I've tried to write it the way I'd teach it to a fellow on service.
What is MGUS, in one paragraph?
MGUS is a premalignant, asymptomatic clonal plasma cell state that precedes almost all cases of multiple myeloma[4]. It requires the absence of CRAB features or other end-organ damage attributable to the plasma cell clone[4]. It is common: a population-based study of residents of Olmsted County, Minnesota aged 50 or older found MGUS in 3.2% of that population[1], and about 80% of multiple myeloma arises from non-IgM MGUS, with the remainder arising from light-chain MGUS[4].
Ranked summary: how the risk groups compare
Here is the Mayo Clinic model's four risk groups, the number of risk factors each represents, and the 20-year cumulative risk of progression reported in the original derivation cohort[3].
Risk group | Number of risk factors present | 20-year risk of progression |
|---|---|---|
Low risk | 0 | 5%[3] |
Low-intermediate risk | 1 | 21%[3] |
High-intermediate risk | 2 | 37%[3] |
High risk | 3 | 58%[3] |
The three risk factors that are counted are: non-IgG M-protein isotype, an M-protein concentration below 15 g/L being the low-risk cutoff, and an abnormal serum free light chain ratio[3]. A patient with low-risk MGUS, by definition, has an IgG isotype, an M-protein below 15 g/L, and a normal FLC ratio[3].
What exactly counts as each Mayo risk factor?
The three inputs are simple lab values you already have at diagnosis: isotype, M-protein concentration, and the serum FLC ratio[2][3].
Isotype: IgG MGUS is the reference (lower-risk) isotype; non-IgG types (IgA, IgM, biclonal, light-chain only) count as a risk factor[3].
M-protein concentration: the low-risk cutoff is an M protein concentration below 15 g/L (1.5 g/dL)[3].
Serum FLC ratio: the low-risk range is between 0.26 and 1.65[3]; anything outside that range is an abnormal ratio and counts as a risk factor[3].
Score each patient from 0 to 3 based on how many of these three they have, then read the 20-year risk off the table above[3]. Fellows should note the overall average progression rate, approximately 1% per year across all MGUS[1][2], is a population-level figure; the Mayo model is what converts that average into a number specific to the patient in front of you.
Is there an updated version of the Mayo model I should know?
Not in the sources I reviewed. The three-factor Mayo model (M-protein above 1.5 g/dL, non-IgG isotype, abnormal free light chain ratio) is still the version the ASH MGUS pocket guide uses, with a 20-year absolute risk of progression of 5%, 21%, 37% and 58% for zero, one, two or three risk factors[5]. I did not find a newer MGUS-specific version of the Mayo model in the guidance I checked.
What does change is how those numbers are read. The IMWG consensus reports the same model with death treated as a competing risk, which lowers the 20-year estimates to 2%, 10%, 18% and 27% for zero to three risk factors[1]. For an older patient, with a high risk of dying from other causes, the competing-risk figures are the more realistic ones to quote.
Do not confuse this with the smoldering myeloma model. The IMWG describes a separate model for smoldering myeloma built on an abnormal free light chain ratio, bone marrow plasma cells of 10% or more and a serum M-protein of 3 g/dL or more, with 5-year progression rates of 25%, 51% and 76% for one, two or three of those factors[1]. Those numbers do not apply to MGUS.
How does MGUS differ from smoldering myeloma and multiple myeloma on paper?
Entity | Clonal bone marrow plasma cells | Defining feature |
|---|---|---|
MGUS | Less than 10% | No CRAB features, no myeloma-defining biomarkers, M-protein below the smoldering threshold[3] |
Smoldering multiple myeloma | 10% to 60% | Serum M-protein (IgG or IgA) ≥30 g/L and/or clonal bone marrow plasma cells 10 to 60%, with no CRAB features or amyloidosis[4] |
Multiple myeloma | ≥10% (or biopsy-proven plasmacytoma) | Any CRAB feature attributable to the clone, or any myeloma-defining biomarker: clonal bone marrow plasma cells ≥60%, involved:uninvolved FLC ratio ≥100, or more than one focal lesion on MRI[4] |
Smoldering multiple myeloma is itself a meaningfully higher-risk state than MGUS, with a risk of progression to multiple myeloma or a related disorder of about 10% per year in the first 5 years, compared to roughly 1% per year for MGUS[1]. A population-based study from Scandinavia found smoldering multiple myeloma accounts for about 14% of all patients diagnosed with multiple myeloma[4].
What follow-up testing does a patient with MGUS actually need?
Follow-up intensity should track risk group, not be identical for every MGUS patient[1]. For low-risk MGUS, patients should be followed with serum protein electrophoresis at six months and, if stable, every 2 to 3 years thereafter, or whenever symptoms suggestive of a plasma cell malignancy arise[1]. For intermediate- and high-risk MGUS, the recommendation is closer surveillance: follow-up at 6 months and then annually for life[1].
For patients who have been reclassified as smoldering multiple myeloma, the recommended testing cadence tightens further: repeat testing 2 to 3 months after initial recognition, then every 4 to 6 months for the first year, and if stable, every 6 to 12 months after that[1].
When is a bone marrow biopsy actually required in MGUS?
Bone marrow examination is not routinely required for low-risk MGUS, but it is far from optional once any red flag is present. The IMWG consensus position is that bone marrow examination should be performed if the monoclonal protein exceeds 15 g/L, if the isotype is IgA or IgM, or if the FLC ratio is abnormal[3]. A bone marrow examination is always required, regardless of risk category, if there is unexplained anemia, renal insufficiency, hypercalcemia, bone lesions, or suspicion of AL amyloidosis[3].
Is there an official online MGUS risk calculator I should be using?
No single calculator is endorsed in this article, and fellows should be comfortable doing this math by hand rather than relying on a specific external tool. The Mayo model itself is just an input count (isotype, M-protein level, FLC ratio) mapped to one of four risk tiers[2][3], something you can walk through at the bedside with three lab values in front of you. If you use an online calculator as a quick double-check, treat it as a convenience, not a substitute for knowing the three inputs and four risk tiers yourself, since that is exactly what the ABIM exam and real consult questions will ask you to reconstruct from memory.
What does imaging add to MGUS and smoldering myeloma risk assessment?
Whole-body MRI focal lesions identify a meaningfully higher-risk subset of smoldering multiple myeloma. In a study of 149 patients with smoldering multiple myeloma who underwent whole-body MRI, focal lesions were detected in 28% of patients, and more than one focal lesion was found in 15% of the total cohort[4]. The presence of more than one focal lesion was associated with a substantially increased risk of progression[4]. This is why more than one MRI focal lesion is one of the IMWG's myeloma-defining biomarkers that upgrades a patient from smoldering to active multiple myeloma requiring therapy, even without a classic CRAB feature[4].
Two other myeloma-defining biomarkers work the same way: a bone marrow plasma cell percentage of 60% or more, and a serum involved:uninvolved FLC ratio of 100 or more[4]. In the Mayo Clinic cohort used to validate the 60% bone marrow plasma cell threshold, patients meeting that criterion had a 95% rate of progression to multiple myeloma within 2 years[4], and in a separate cohort, an FLC ratio of 100 or more carried a 72% risk of progression to multiple myeloma within 2 years[4].
What does the ABIM board exam actually test on this topic?
Know the three Mayo model inputs cold: non-IgG isotype, M-protein concentration at or above the low-risk threshold, abnormal FLC ratio[3], and be able to count a vignette's risk factors and assign a risk tier.
Know when a bone marrow biopsy is and is not required in a patient presenting with an incidental monoclonal protein, including the IMWG triggers (M-protein >15 g/L, IgA or IgM isotype, abnormal FLC ratio) and the absolute requirement if there is unexplained anemia, hypercalcemia, renal insufficiency, or bone lesions[3].
Know the IMWG myeloma-defining biomarkers that upgrade smoldering myeloma to active multiple myeloma even without CRAB features: ≥60% clonal bone marrow plasma cells, FLC ratio ≥100, or more than one focal MRI lesion[4].
Know the follow-up cadence by risk group: low-risk MGUS at 6 months then every 2 to 3 years; intermediate/high-risk MGUS at 6 months then annually for life; smoldering myeloma at 2 to 3 months then every 4 to 6 months for a year[1].
If you want to drill this as spaced, board-style questions with full explanations for why each distractor is wrong, our physician-written heme/onc question bank covers plasma cell dyscrasias in exactly this format, and you can build a spaced-repetition flashcard deck straight from your own notes on MGUS risk stratification so the three Mayo inputs stay sticky through test day.
Frequently asked questions
What is the average risk of MGUS progressing to multiple myeloma?
The overall average is approximately 1% per year, based on a large Mayo Clinic population study[1][2]. That figure is an average across all MGUS patients; an individual patient's actual risk depends heavily on their Mayo model risk group[3].
What are the three risk factors in the Mayo Clinic MGUS risk model?
Non-IgG monoclonal protein isotype, M-protein concentration at or above the low-risk threshold, and an abnormal serum free light chain ratio[3]. The number of these present, from 0 to 3, determines the risk tier.
Do all patients with MGUS need a bone marrow biopsy?
No. Bone marrow examination is not routinely needed for low-risk MGUS in the absence of concerning clinical features[3]. It is recommended if the M-protein exceeds 15 g/L, the isotype is IgA or IgM, the FLC ratio is abnormal, or if there is unexplained anemia, hypercalcemia, renal insufficiency, or bone lesions[3].
How often should low-risk MGUS be followed compared to intermediate or high-risk MGUS?
Low-risk MGUS should be rechecked with serum protein electrophoresis at six months, then every 2 to 3 years if stable, or sooner if symptoms arise[1]. Intermediate- and high-risk MGUS should be followed at six months and then annually for life[1].
Is smoldering multiple myeloma the same as high-risk MGUS?
No, they are distinct entities defined by separate IMWG criteria. MGUS, including high-risk MGUS, has clonal bone marrow plasma cells below 10%, while smoldering multiple myeloma requires either a serum M-protein ≥30 g/L (IgG or IgA) or clonal bone marrow plasma cells between 10% and 60%, with no CRAB features or amyloidosis[4]. Smoldering myeloma also carries a much higher annual progression risk, roughly 10% per year in the first 5 years, compared with about 1% per year for MGUS[1].
Where this fits in your board prep
Plasma cell dyscrasias are a recurring, high-yield category on the hematology boards, and the Mayo model is one of the few pure "memorize three numbers and a table" topics in the entire blueprint, which makes it an efficient place to drill. If you are building a broader review schedule around high-yield categories like this one, our guide on building a 6-month ABIM study plan walks through how to sequence topics like this alongside question volume, and if your exam date is closer than that, the one-month-out plan covers how to prioritize reviewing weak categories like this over re-reading a textbook.
Ready to drill this?
MeDucation AI's question bank is physician-written and reviewed by board-certified hematologist-oncologists, never AI-generated, with explanations that cover why the correct answer is right and why each distractor is wrong, embedded tables, and direct PubMed links. If MGUS and smoldering myeloma risk stratification is a weak spot, it is one of the fastest categories to shore up with a focused problem set rather than another pass through a review book.
