Can Monoclonal Gammopathy (MGUS) Cause Anemia?

By its official definition, MGUS does not cause anemia, because anemia is one of the criteria that would push the diagnosis toward something more serious like multiple myeloma. Yet anemia shows up in MGUS patients at rates far higher than expected. A large study found that roughly 43 percent of people diagnosed with MGUS also had anemia, compared to 16 percent of matched controls. That gap is too wide to dismiss, and untangling why it exists matters for anyone living with this condition.

Why the Diagnosis Itself Says “No Anemia”

MGUS stands for monoclonal gammopathy of undetermined significance. It describes a situation where abnormal plasma cells in the bone marrow produce a single type of immunoglobulin protein, called a monoclonal protein or M-protein, but the level of abnormal cells is low and no organ damage has occurred. The “undetermined significance” part is the key phrase: the condition is present, but it has not crossed the threshold into causing problems. Hematologists use a set of criteria often remembered by the acronym CRAB (calcium elevation, renal insufficiency, anemia, bone lesions) to decide whether a plasma cell disorder has become symptomatic. If any of those features are present and clearly attributable to the abnormal plasma cells, the diagnosis typically shifts from MGUS to multiple myeloma or another malignancy requiring treatment.

One research group described MGUS as a condition “without evidence of end-organ damage such as hypercalcemia, renal insufficiency, anemia, or bone lesions,” which neatly captures the diagnostic logic.¹ So in the strictest clinical sense, if MGUS were directly causing your anemia, your doctor would probably reconsider whether your diagnosis is still just MGUS. This creates a paradox: anemia should be absent in MGUS by definition, yet it frequently shows up in practice.

Anemia Is Strikingly Common in MGUS Patients

A study examining multimorbidity in MGUS patients found that 43 percent of people with MGUS had anemia, compared with just 16 percent of age- and sex-matched controls without MGUS. Chronic kidney disease was also significantly more common in the MGUS group. The researchers noted that “the higher prevalence among cases of anemia and CKD, which may be involved in the pathogenesis of, or surveillance for, MGUS, warrants additional investigation.”² In other words, even the study authors were unsure whether anemia was somehow linked to MGUS itself, whether it was a side effect of the diseases that travel alongside MGUS, or whether the anemia was simply the reason blood tests were drawn in the first place, leading to the MGUS diagnosis as an incidental finding.

That last possibility, called detection bias, is worth understanding. MGUS rarely produces symptoms on its own. It is usually discovered because a doctor ordered a blood test for some other reason. If you went to your doctor because you felt fatigued and your blood work showed anemia, the same protein electrophoresis panel might also pick up an M-protein. In this scenario, MGUS did not cause the anemia; the anemia just happened to lead you to the blood draw that uncovered the MGUS. Sorting out which situation applies to any individual patient takes careful investigation.

When the Monoclonal Protein Itself Destroys Red Blood Cells

There is at least one well-established situation where a monoclonal protein directly causes anemia, and it involves a specific subtype of MGUS. In cold agglutinin disease, a monoclonal IgM antibody produced by clonal bone marrow cells attacks the person’s own red blood cells. The antibody binds to red blood cells at cooler temperatures, such as in the fingers, toes, and nose, triggering complement activation that marks those cells for destruction. In most cases the red blood cells are cleared by the liver in a process called extravascular hemolysis. In more severe episodes, the complement cascade goes further, assembling a membrane attack complex that punches holes directly in red blood cells, causing intravascular hemolysis that can become life-threatening.³

Cold agglutinin disease is classified under a newer umbrella term, “monoclonal gammopathy of clinical significance” (MGCS), which recognizes that some monoclonal proteins cause real harm through the protein itself rather than through tumor burden or bone marrow infiltration. This distinction matters because the treatment approach differs: rather than targeting the plasma cell clone the way you would in myeloma, doctors focus on stopping the antibody-mediated red blood cell destruction. The existence of cold agglutinin disease makes it clear that a monoclonal protein can absolutely cause severe anemia, even when the underlying clonal population in the bone marrow is small enough that it would otherwise qualify as MGUS.

IgM MGUS and the Different Paths It Takes

Not all MGUS is the same. The type of immunoglobulin the abnormal cells produce shapes the risks going forward. IgG and IgA forms of MGUS, the most common types, tend to progress toward multiple myeloma when they progress at all. IgM MGUS follows a different trajectory. When IgM MGUS transforms into a malignancy, it most often becomes Waldenström macroglobulinemia or another lymphoproliferative disorder rather than myeloma.⁴

This distinction has practical implications for anemia. The IgM class of antibodies is particularly prone to acting as an autoantibody. Cold agglutinin disease, discussed above, is overwhelmingly an IgM phenomenon. Other IgM-related conditions that can cause or contribute to anemia include cryoglobulinemia, where the protein precipitates in cold temperatures and can damage small blood vessels, and certain neuropathies and kidney diseases driven by IgM deposition. So if you have IgM MGUS specifically, the chance that the monoclonal protein is doing something clinically meaningful, including contributing to anemia, is higher than for other MGUS subtypes. Racial distribution also differs: IgM MGUS makes up a smaller proportion of all MGUS in African and African American populations compared to white populations, while non-IgM MGUS shows the opposite pattern.⁴

Inflammation, Cytokines, and the Bone Marrow Environment

Even when the monoclonal protein is not directly attacking red blood cells, the biological environment created by MGUS may contribute to anemia through subtler mechanisms. Research using genetic analysis methods has identified inflammatory cytokines, specifically CXCL10 and IL-6, as causal risk factors for developing MGUS.¹ IL-6 in particular is well known to suppress red blood cell production and promote what hematologists call anemia of chronic inflammation. In this type of anemia, the body has enough iron stored away, but inflammatory signals interfere with iron metabolism and the production of new red blood cells. The result is a mild to moderate anemia that does not respond to iron supplements.

Separately, studies of the bone marrow environment in MGUS patients have found that the endothelial cells lining blood vessels within the marrow behave differently than normal. These cells express receptors for erythropoietin, the hormone that drives red blood cell production, and they respond to erythropoietin by proliferating and promoting new blood vessel formation. In MGUS patients, these responses are particularly active.⁵ The clinical significance of this is still being worked out, but one possibility is that erythropoietin is being “used up” supporting abnormal blood vessel growth in the marrow rather than doing its normal job of stimulating red blood cell production. This is speculative, but it illustrates how the MGUS microenvironment might indirectly contribute to anemia without the condition meeting the formal CRAB criteria for myeloma.

When MGUS and Myelodysplastic Syndrome Overlap

One of the more sobering findings in recent years is that MGUS and myelodysplastic syndrome (MDS) co-occur at rates higher than chance would predict. MDS is a separate group of bone marrow disorders characterized by ineffective blood cell production, and anemia is its hallmark symptom. A retrospective study of 14 patients with both MDS and monoclonal gammopathy found that these cases represented about 10 percent of all MDS patients seen at the institution over 14 years. The rate of MGUS in MDS patients was significantly higher than in age-matched controls, and the fact that most patients had both conditions diagnosed simultaneously, without prior chemotherapy to explain the overlap, suggested a genuine biological co-occurrence rather than one causing the other.⁶

A further analysis of 14 patients with MGUS or myeloma coexisting with a higher-risk form of MDS found that almost 86 percent had severe anemia or drops in all blood cell lines, and nearly half eventually progressed to acute myeloid leukemia.⁷ These are small patient series, so the exact numbers should be held loosely. But the message is clear: if you have MGUS and develop significant anemia, your doctor should consider the possibility that a second bone marrow disorder is present alongside the MGUS. The anemia in these cases comes from the MDS, not from the MGUS itself, but the two conditions share bone marrow real estate and may share underlying biological drivers related to aging and genomic instability.

The Diagnostic Challenge of Anemia in Someone With MGUS

For a clinician, finding anemia in a patient with known MGUS sets off a branching decision tree. The first question is whether the MGUS has progressed. If the M-protein level has risen, bone marrow plasma cells have increased, or other CRAB features are developing, the patient may be transitioning from MGUS to smoldering or active myeloma, and the anemia is a red flag for that transition. The second question is whether the monoclonal protein itself is causing trouble, as in cold agglutinin disease or other MGCS-related conditions. The third question is whether an entirely unrelated cause of anemia is present: iron deficiency from GI bleeding, vitamin B12 or folate deficiency, chronic kidney disease, MDS, or simply the anemia of aging.

One published case report illustrated the complexity nicely. A patient with MGUS was found to have iron deficiency anemia, but endoscopic examination of the upper GI tract and colon found no pathological lesions, and testing for Helicobacter pylori was negative.⁸ In that case, the iron deficiency anemia coexisted with MGUS but had no clear cause attributable to the monoclonal protein. In another case, a kidney transplant patient developed persistent anemia that led to a bone marrow biopsy revealing MGUS, with pathological plasma cells making up 4.6 percent of bone marrow cells. The anemia improved with an erythropoiesis-stimulating agent, and the MGUS itself did not progress.⁹ These case reports show that anemia in someone with MGUS often has an explanation that is separate from the monoclonal protein, but reaching that conclusion takes a careful workup.

Practical Steps If You Have MGUS and Are Anemic

If you have been told you have MGUS and your blood counts show anemia, the most important thing is not to assume the two are unrelated and equally not to panic that you have myeloma. Your doctor will likely check a few things in sequence. A complete blood count and reticulocyte count help determine whether the bone marrow is producing enough new red blood cells or whether they are being destroyed too quickly. Iron studies, vitamin B12, and folate levels rule out nutritional deficiencies. A serum protein electrophoresis and free light chain assay check whether the M-protein has increased since the last measurement. If the anemia is hemolytic, meaning red blood cells are being destroyed, a direct antiglobulin test and cold agglutinin titer can determine whether the monoclonal protein is acting as an autoantibody.

Depending on results, a repeat bone marrow biopsy may be warranted. This can assess whether the plasma cell percentage has crept upward, whether there are features suggestive of MDS, and whether the overall marrow architecture is normal. The transplant case mentioned above is instructive: bone marrow aspiration showed a low percentage of abnormal plasma cells, confirming MGUS rather than myeloma, and the anemia was manageable with an erythropoiesis-stimulating agent.⁹ Not every anemic MGUS patient needs a bone marrow biopsy, but if the anemia is unexplained after standard workup, or if other concerning changes are present, it becomes a reasonable step.

The Growing Recognition of MGUS-Related Conditions

The traditional view of MGUS as a harmless precursor that either stays quiet or eventually progresses to myeloma has expanded considerably. The concept of monoclonal gammopathy of clinical significance (MGCS) now encompasses a range of disorders where the monoclonal protein causes organ damage through mechanisms other than tumor growth. Cold agglutinin disease is one example. Others include AL amyloidosis (where light chains deposit in organs), monoclonal gammopathy-associated kidney disease, and certain peripheral neuropathies driven by the M-protein binding to nerve tissue.³

This shift in thinking is good news for patients. It means that when a monoclonal protein is causing a problem like anemia, there is a growing framework for recognizing it and directing treatment at the right target. In cold agglutinin disease, for instance, complement inhibitors and treatments that deplete the clonal B cells producing the offending antibody can dramatically improve hemoglobin levels. In MGUS-related kidney disease, early identification and treatment can preserve kidney function. The old binary of “watch and wait or treat myeloma” is giving way to a more nuanced understanding that acknowledges the many ways a small clonal population can punch above its weight.

Chronic Kidney Disease and the Anemia Connection

The multimorbidity study that found 43 percent anemia prevalence in MGUS patients also reported that 36 percent had chronic kidney disease, compared to 18 percent of controls.² This is worth dwelling on because chronic kidney disease is itself a major cause of anemia. Damaged kidneys produce less erythropoietin, so the signal that tells the bone marrow to make red blood cells weakens. The kidneys can be affected by the monoclonal protein directly, through light chain deposition or other mechanisms, or they may be damaged by unrelated causes like diabetes and hypertension that happen to be common in the same age group affected by MGUS. A latent class analysis within that study identified a subgroup making up about 31 percent of MGUS patients characterized by the combination of high blood pressure, abnormal cholesterol, anemia, and chronic kidney disease all clustering together. This suggests a patient profile in which the anemia is multiply determined, with kidney disease, chronic inflammation, and possibly direct effects of the monoclonal protein all contributing at once.

For someone in this situation, treating any single cause in isolation may not fully correct the anemia. Erythropoiesis-stimulating agents can help when kidney-related erythropoietin deficiency is part of the picture. Correcting iron deficiency, when present, remains essential. And ongoing monitoring to ensure the MGUS is not progressing to a state where marrow infiltration itself becomes the dominant cause of low blood counts is a necessary part of long-term care. The tangle of contributing factors is what makes anemia in MGUS genuinely complicated and why a simple yes-or-no answer to whether MGUS causes anemia undersells the reality patients and clinicians face.

1
Wiley Online Library (Mediators of Inflammation). Targeting the Inflammation–Metabolism Axis in MGUS: Causal Roles of CXCL10 Mediated by Blood Metabolites
2
Europe PMC / International Journal of Cancer. Multimorbidity in patients with monoclonal gammopathy of undetermined significance
3
Haematologica. IgM monoclonal gammopathies of clinical significance: diagnosis and management
4
PubMed Central. Prevalence, clinical aspects, and natural history of IgM MGUS
5
Europe PMC / Oncotarget. Role of erythropoietin in the angiogenic activity of bone marrow endothelial cells of MGUS and multiple myeloma patients
6
PubMed. Co-occurrence of monoclonal gammopathy and myelodysplasia: a retrospective study of fourteen cases
7
PubMed. Coexistence of plasma cell neoplasia and myelodysplastic syndrome with excess blasts: case reports and literature review
8
International Journal of Case Reports and Images. Iron deficiency anemia in monoclonal gammopathy of undetermined significance
9
PubMed Central. Monoclonal Gammopathy of Undetermined Significance Diagnosed by Persistent Anemia Following Living Kidney Transplantation: A Case Report