Monoclonal gammopathy of undetermined significance

MGUS is common with ageing and is usually discovered during evaluation of neuropathy, anaemia, renal impairment, inflammation or high total protein. Before accepting the label, ask about bone pain, fracture, infection, weight loss, sweats, neuropathy, bleeding, headache, visual change, oedema, dyspnoea and autonomic symptoms. Examine nodes, spleen, skeletal tenderness, neuropathy, heart failure, macroglossia and purpura. The central question is not merely whether a monoclonal band exists, but whether the clone or its protein is already causing disease.

Characterise the clone with SPEP, immunofixation, quantitative immunoglobulins and serum free light chains. Check FBC, creatinine or eGFR, calcium, albumin and liver tests and use urinalysis and protein or albumin quantification. Interpret free-light-chain ratio with kidney function and the renal reference context; a polyclonal rise in both chains differs from a dominant involved chain. Review previous results to distinguish a long stable band from rapid emergence. A very small band can still produce AL amyloid or renal disease.

Non-IgM MGUS has M-protein below 30 g/L, marrow clonal plasma cells below 10% and no myeloma-defining event. IgM MGUS has IgM below 30 g/L, lymphoplasmacytic marrow involvement below 10% and no related symptoms such as anaemia, hyperviscosity, nodes or spleen. Light-chain MGUS requires abnormal ratio with increased involved chain, no heavy-chain immunofixation, clonal marrow below 10%, urinary monoclonal protein below 500 mg/24 hours and no attributable organ injury. If marrow is not clinically required in a low-risk phenotype, diagnosis uses the complete non-invasive context.

Escalate investigation when M-protein is larger or non-IgG, ratio markedly abnormal, unexplained anaemia or renal impairment exists, calcium is high, symptoms are present or results progress. Marrow aspirate and trephine with flow and FISH distinguish plasma-cell and lymphoplasmacytic clones. Whole-body MRI or low-dose CT assesses myeloma bone disease; CT assesses nodes and spleen in IgM disease. Tissue biopsy is essential for suspected amyloid, monoclonal renal lesion or neuropathic process. Do not attribute common CKD or anaemia to the clone without mechanism.

A simple risk model uses three factors: non-IgG isotype, M-protein at least 15 g/L and abnormal free-light-chain ratio. Absence of all identifies a low-risk group, while accumulating factors raises long-term progression probability. This model supports but does not replace isotype-specific assessment, dynamic change, immunoparesis and patient life expectancy. A rapidly changing lower absolute value can be more concerning than a stable larger value. Explain that average progression is approximately 1% per year but individual risk is not constant or identical.

Monitoring should be proportionate. Recheck the clinical and laboratory picture after initial diagnosis to confirm stability. Low-risk stable MGUS can often move to primary-care monitoring or discharge with BSH safety-netting; intermediate and high-risk or IgM and light-chain phenotypes require a defined interval, commonly at least annual after early stability. At review assess symptoms, FBC, renal function, calcium and the same clonal marker. Do not perform routine serial skeletal surveys or PET. Image new pain and repeat marrow when a biochemical or clinical change can alter classification.

Monoclonal gammopathy of clinical significance is the critical exception. MGRS includes light-chain deposition, proliferative glomerulonephritis and other clone-mediated kidney lesions confirmed by renal biopsy and expert typing. Neuropathy may reflect anti-MAG IgM, cryoglobulin, POEMS or amyloid. Skin, eye and complement disorders also occur. These patients may need clone-directed treatment despite not meeting myeloma or lymphoma tumour criteria, because the treatment target is the pathogenic protein and endangered organ rather than clone size.

Communicate risk without turning a precursor state into active cancer. Provide written symptoms that trigger earlier assessment and identify who owns follow-up. Avoid repeated testing more often than an action could reasonably follow, particularly in frail patients with stable low-risk disease. Address osteoporosis through fracture-risk guidance, vaccination and recurrent infection conventionally. Family screening is not indicated. A new cancer diagnosis or immunosuppressive treatment is an opportunity to review the paraprotein but not an automatic reason to treat it.