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Blood Test Tracker

M-Protein (Monoclonal Spike), Serum

The size of a monoclonal protein (M-spike), a single antibody made by one group of plasma cells. Most people have none.

AI-verified against cited sources. Awaiting expert reviewAlso called M-spike, M-proteinLOINC 33358-3

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What it is

An M-spike is a narrow band on serum protein electrophoresis (SPEP) that is made by one group of identical plasma cells. Plasma cells normally make a wide mix of antibodies, so a single tall band stands out. The lab measures the band and reports it in g/dL (or g/L). If there is no band, many labs print 'not detected' instead of a number. This test shows how much monoclonal protein is present. A separate test, immunofixation, names its type, such as IgG kappa.

Why it matters

The size of the M-spike is one of the main numbers clinicians use to follow a monoclonal protein. Together with the protein type, a free light chain test and your symptoms, it helps tell MGUS, a common condition that usually causes no symptoms, from smoldering or active multiple myeloma and related conditions. Once a monoclonal protein is found, the same number is tracked over time. Small ups and downs between tests are common because the measurement is not exact.

If it’s high

Any measurable M-spike means one group of plasma cells is making extra antibody. The most common reason is MGUS, which usually causes no symptoms. A larger spike is more likely to need closer follow-up, but the size alone does not tell what is going on. Your clinician reads it together with the protein type, a free light chain test, calcium, kidney function, a blood count and sometimes a bone marrow test or imaging.

Common causes

  • Monoclonal gammopathy of undetermined significance (MGUS), the most common reason
  • Monoclonal antibody medicines, which can mimic a spike
  • Smoldering multiple myeloma
  • Multiple myeloma
  • Waldenström macroglobulinemia (an IgM spike)
  • AL amyloidosis and other related plasma cell or B-cell conditions

What can help

  • Ask which type of monoclonal protein it is and whether a free light chain test is also needed
  • Ask how often to repeat the test, and use the same lab and method each time so the numbers can be compared
  • Tell your clinician about any monoclonal antibody medicine you take
  • Ask what changes in this number or in your other results would lead to more testing

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: up to 0 g/dL. Source: ARUP Laboratories, Mayo Clinic Laboratories.

IMWG size cutoff between MGUS and smoldering myeloma (serum M-protein)

  • Under 3 g/dL (the size range IMWG uses for MGUS)below 3 g/dL
  • 3 g/dL or more (IMWG size cutoff for smoldering myeloma; bone marrow and other findings decide the diagnosis)3 and above g/dL

MGUS risk model (Mayo, adopted by IMWG): M-protein size

  • Under 1.5 g/dL (this risk factor absent)below 1.5 g/dL
  • 1.5 g/dL or more (one of three MGUS risk factors)1.5 and above g/dL

What can affect your result

  • Where the band sits: spikes in the beta region (often IgA) are the hardest to measure because other proteins travel there
  • The method: gel electrophoresis, capillary electrophoresis and mass spectrometry can give slightly different numbers for the same sample
  • A broad rise in normal antibodies, which can make a spike harder to measure
  • Monoclonal antibody medicines such as daratumumab, which can mimic a spike
  • Sample quality: hemolysis and fibrinogen can create false peaks, and a serum sample is required
  • Age: monoclonal proteins become more common with age
  • Monoclonal antibody medicines such as daratumumab, elotuzumab and isatuximab can appear as a band that looks like a patient's own M-protein, and they can make it harder to judge treatment response.
  • A broad rise in normal antibodies (a polyclonal background) can hide or distort a small spike.
  • A spike that travels with other proteins in the beta region, which often happens with IgA, is harder to measure.
  • Hemolysis, fibrinogen (from a plasma sample), iodinated contrast dye and some antibiotics can create false peaks on electrophoresis. ARUP rejects plasma and uses serum.
  • Results from different methods (gel or capillary electrophoresis, mass spectrometry) and different labs may not be comparable, so follow-up tests are best done at the same lab.
  • Some labs ask you to fast before a serum protein electrophoresis; MedlinePlus mentions up to 12 hours. Follow your lab's instructions.

Changes build over weeks to months, so follow-up is spaced out rather than repeated week to week.

When to talk to a clinician

A monoclonal protein of any size is worth a follow-up conversation with the clinician who ordered the test. Ask about next steps soon if the M-spike is 1.5 g/dL or more (one of three Mayo MGUS risk factors, along with a type other than IgG and a free light chain ratio outside the usual range), if it is 3 g/dL or more (the IMWG size cutoff between MGUS and smoldering myeloma), or if it keeps rising on repeat tests. Talk with your clinician soon about bone pain, a bone broken by a minor injury, tiredness or anemia with no clear cause, kidney problems, high calcium, or numbness and tingling with no other cause. Call your local emergency number for chest pain, trouble breathing, confusion or fainting.

Questions you could ask

  • Could any medicine I take affect this result?
  • Which type of monoclonal protein is it (IgG, IgA, IgM or light chain only)?
  • Should I also have a free light chain test or a urine test?
  • How often should we repeat this, and should I use the same lab each time?
  • What changes in this number or in my other results would lead us to do more testing?

Questions people ask

What is a normal M-spike?

The usual result is no M-spike at all. ARUP prints its reference as <=0.00 g/dL, and many labs print 'not detected.' A small M-spike is found in about 3% to 6% of adults over 50 and is often harmless, but it is worth discussing with your clinician.

Does a small change from one test to the next matter?

Not always. The measurement is not exact, so small differences can come from the test itself. Mayo Clinic Laboratories, citing IMWG guidance, treats a rise of less than 0.5 g/dL as stable. It helps to compare results from the same lab and method, and your clinician looks at the trend over time.

What do the 1.5 and 3 g/dL numbers mean?

They are cutoffs clinicians use, not diagnoses. An M-protein of 1.5 g/dL or more is one of three risk factors in the Mayo MGUS model. A level of 3 g/dL or more is the size IMWG uses to separate MGUS from smoldering myeloma. Bone marrow findings, free light chains and symptoms are also needed, so ask your clinician how your numbers fit together.

Show the science and sources

Units

  • 1 g/dL = 1 g/dL
  • 1 g/dL = 10 g/L

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For clinicians and researchers

M-protein is quantified on serum protein electrophoresis (now usually capillary zone electrophoresis) by integrating the monoclonal peak against the polyclonal background, using a perpendicular drop or tangential skimming; this stays semi-quantitative. The limit of quantification varies from about 1 g/L to over 25 g/L depending on the peak position and the background, and beta-region peaks are the hardest because of co-migrating proteins such as transferrin and C3 (J Clin Med 2025). Polymerized IgA or IgM can show as several adjacent peaks that are summed as one. The limit of detection of SPEP is about 0.5–2 g/L (Ann Lab Med 2026), and small bands are better seen by immunofixation. ARUP reports the monoclonal protein in g/dL with a reference of <=0.00. Mayo's new MASS-FIX quantitative study (MALDI-TOF mass spectrometry, effective November 2025) calculates the M-protein from the percent peak area times the nephelometric IgG, IgA or IgM value; it cannot quantify IgD, IgE or free light chains alone, which are followed with serum free light chains or IgD/IgE assays. Thresholds in common use (all from IMWG-based or Mayo sources): MGUS is a serum M-protein under 3 g/dL with under 10% clonal bone marrow plasma cells and no end-organ damage; smoldering myeloma is 3 g/dL or more and/or 10% or more clonal plasma cells without myeloma-defining events; IMWG measurable disease is a serum M-protein of 1 g/dL (10 g/L) or more; the Mayo MGUS model counts an M-protein of 1.5 g/dL or more, a non-IgG isotype and a free light chain ratio outside the usual range (58% 20-year progression with all three versus 5% with none); the Mayo 20/2/20 smoldering model uses an M-spike over 2 g/dL. Mayo suggests checking serum viscosity when an IgM, IgA or IgG spike is above 4, 5 or 6 g/dL, and a 24-hour urine monoclonal protein study when a first serum spike is above 1.5 g/dL. Mayo (citing IMWG guidance) calls an M-protein stable when it rises by less than 0.5 g/dL. The size alone cannot exclude myeloma: in a series cited by AFP (2005), up to a fifth of people with myeloma had a spike under 1 g/dL. Results from different methods may not be comparable.

LOINC 33358-3 (primary), 104837-0, 51435-6, 35559-4. Specimen: serum.

Sources

  1. Protein Electrophoresis, Serum (test 0050640), ARUP Laboratories
  2. Protein Electrophoresis with Reflex to Immunofixation, Serum (test 2002109), ARUP Laboratories
  3. Monoclonal Protein Study, Expanded Panel, Serum (test 2002715), ARUP Laboratories
  4. QMPSS - Overview: Monoclonal Protein Study, Quantitative, Serum, Mayo Clinic Laboratories (2025)
  5. Current risk stratification and staging of multiple myeloma and related clonal plasma cell disorders, Leukemia (2025)
  6. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma, The Lancet Oncology (2014)
  7. Multiple Myeloma Laboratory Diagnostics Made Simple: Practical Insights and Key Recommendations, Journal of Clinical Medicine (2025)
  8. Paradigm Shift in Monoclonal Protein Detection: From Electrophoresis-based to Mass Spectrometry-based Methods, Annals of Laboratory Medicine (2026)
  9. Understanding and Interpreting Serum Protein Electrophoresis, American Family Physician (2005)
  10. Protein electrophoresis - serum, MedlinePlus (National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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