Serum protein electrophoresis, or SPEP, separates blood proteins into bands according to their electrical behavior. Clinicians use the pattern to investigate an unexplained high total protein, anemia, kidney dysfunction, bone pain, neuropathy, recurrent infection, suspected amyloidosis, or a possible plasma-cell or B-cell disorder. The best-known finding is a narrow monoclonal spike, often called an M protein, M spike, paraprotein, or monoclonal protein. SPEP can estimate the amount of a visible spike, but it usually cannot determine the immunoglobulin type and may miss small, light-chain-only, or beta-migrating proteins. Immunofixation or mass spectrometry, serum free light chains, quantitative immunoglobulins, urine studies, blood counts, chemistry tests, imaging, and sometimes bone marrow examination complete the assessment. A monoclonal result is not synonymous with multiple myeloma: many people have monoclonal gammopathy of undetermined significance, while some organ-damaging disorders produce only a tiny protein. This article explains how the tracing is organized, what broad and narrow changes mean, and how clinicians move from an SPEP result to an accurate diagnosis.
- SPEP displays a distribution of serum proteins; it does not directly count plasma cells or diagnose cancer.
- A narrow restricted peak suggests monoclonal production, while a broad gamma-region rise usually reflects polyclonal immune activation.
- Small monoclonal proteins, free light chains, and some IgA proteins may be missed or underestimated by SPEP.
- Immunofixation identifies heavy- and light-chain type, while serum free light chains improve detection of light-chain disorders.
- Clinical urgency depends on organ effects, laboratory trends, and the underlying clone—not simply the height of an M spike.
Table of Contents
- Reading the electrophoresis landscape
- Monoclonal spikes versus polyclonal patterns
- Why SPEP can miss important proteins
- Tests that identify and contextualize the band
- From MGUS to smoldering and active disease
- Nonmalignant and technical patterns
- Using SPEP for monitoring
- What to do with an abnormal result
Reading the electrophoresis landscape
SPEP begins with serum, the liquid portion of blood after clotting. The laboratory places serum on a gel or into a capillary system and applies an electric field. Proteins migrate according to charge and other physical properties. The instrument converts the separation into a tracing with several regions.
Albumin forms the largest and tallest peak. The alpha-1 region contains proteins such as alpha-1 antitrypsin. Alpha-2 includes haptoglobin, alpha-2 macroglobulin, and ceruloplasmin. The beta region contains transferrin, complement C3, beta lipoproteins, and sometimes immunoglobulins. Most immunoglobulins migrate in the gamma region, although IgA and selected monoclonal proteins can appear in beta or alpha regions.
The report may provide concentrations or percentages for albumin, alpha-1, alpha-2, beta-1, beta-2, and gamma fractions. These fractions are method dependent. They should not be treated as direct measurements of one protein because each region contains a mixture.
A pathologist or laboratory scientist examines the shape as well as the numbers. Important questions include:
- Is there a narrow, discrete restriction?
- Is the gamma region broadly elevated or reduced?
- Is there beta-gamma bridging?
- Is albumin low?
- Are alpha fractions increased in an acute-phase pattern?
- Does a suspected spike sit on a polyclonal background?
- Has the pattern changed from earlier studies?
A visible M spike is usually quantified by drawing boundaries around the peak and calculating its area. The result is often reported in grams per deciliter or grams per liter. Quantification becomes less precise when the band is small, overlaps transferrin or complement, or lies within a broad polyclonal background.
Fasting is usually not required, but specimen quality matters. Plasma should not be substituted for serum because fibrinogen can create a band that resembles a monoclonal protein. Hemolysis, high lipid levels, recent contrast agents, and specimen contamination can alter the tracing. The laboratory may request a repeat serum sample when the pattern is technically uncertain.
SPEP is often ordered after a chemistry panel shows a widened “globulin gap,” calculated from total protein minus albumin. That gap can prompt investigation, but it is neither sensitive nor specific for monoclonal disease. Dehydration can raise total protein, while small monoclonal proteins may leave total protein within range. Similarly, the albumin-to-globulin ratio is a broad screening clue rather than a replacement for electrophoresis. The ordering reason should be documented because an incidental narrow band in an asymptomatic older adult has a different pretest probability from a band found during evaluation of nephrotic proteinuria, cardiomyopathy, or lytic bone lesions. Before interpreting the tracing, clinicians also compare prior total protein, albumin, kidney function, blood counts, and inflammatory markers. This prevents a visually striking but clinically irrelevant fraction change from overshadowing a more important trend elsewhere in the record.
Monoclonal spikes versus polyclonal patterns
A monoclonal protein is produced by one expanded clone of plasma cells or B lymphocytes. Because the molecules are highly similar, they migrate together and form a narrow band or sharp peak. The protein may be an intact immunoglobulin such as IgG kappa, IgA lambda, or IgM kappa, or it may consist primarily of free light chains.
SPEP can suggest monoclonality but usually cannot assign isotype. Immunofixation or a validated mass-spectrometry method identifies the heavy chain and light chain. A restricted band can occur in monoclonal gammopathy of undetermined significance, multiple myeloma, smoldering myeloma, Waldenström macroglobulinemia, lymphoma, AL amyloidosis, light-chain deposition disease, and other monoclonal gammopathies of clinical significance.
A polyclonal increase is broad because many immune-cell clones are producing different immunoglobulins. Common causes include:
- chronic infection;
- autoimmune or inflammatory disease;
- chronic liver disease;
- selected lung and gastrointestinal disorders;
- immune reconstitution; and
- some malignancies that stimulate broad immune activation.
Polyclonal hypergammaglobulinemia is not an M protein. It may be substantial, but the wide-based shape and companion tests point toward multi-clonal production. Quantitative IgG, IgA, and IgM can show which classes contribute.
A mixed pattern is possible. A small monoclonal band may sit on top of broad inflammation, making it difficult to see or quantify. Conversely, oligoclonal bands can occur after stem-cell transplantation, infection, immune therapy, or recovery from profound immunosuppression. Several small restrictions are not automatically evidence of several malignant clones.
Low gamma globulins suggest hypogammaglobulinemia but do not identify the cause. Antibody deficiency, B-cell-depleting treatment, chronic lymphocytic leukemia, protein loss, nephrotic syndrome, or immunosuppressive therapy may reduce the region. Quantitative immunoglobulins and infection history are needed.
The size of a peak does not reliably predict organ injury. A large stable IgG MGUS protein may cause no immediate damage, while a very small clone may produce nephrotoxic or amyloid-forming light chains. The clinical question is therefore both “How much protein is present?” and “What is this protein doing?”
Why SPEP can miss important proteins
SPEP has a detection limit. Very small monoclonal proteins may blend into the background and be visible only by immunofixation or mass spectrometry. This is particularly important in AL amyloidosis, monoclonal gammopathy of renal significance, neuropathy-associated gammopathy, and early plasma-cell disease.
Light-chain-only disorders are another limitation. Free kappa and lambda chains are small, clear rapidly through the kidneys, and may not create a distinct serum spike. Serum free light-chain testing and urine monoclonal-protein studies increase sensitivity.
IgA monoclonal proteins often migrate in the beta region, where transferrin and complement already produce peaks. The M protein can be underestimated because the integration boundaries include normal proteins or cannot cleanly separate the band. Quantitative IgA and isotype-specific methods may provide a better treatment marker.
A very high immunoglobulin concentration can occasionally produce nonlinear measurement effects, requiring dilution. Cryoglobulins may precipitate if the specimen cools before processing and disappear from the measured serum. When cryoglobulinemia is suspected, the collection and transport tube must remain warm until serum is separated.
A negative SPEP does not exclude nonsecretory or oligosecretory myeloma. Some clones release little measurable protein, and disease is detected through marrow findings, imaging, free light chains, or tissue biopsy. Likewise, a normal total protein does not rule out a small monoclonal component.
Urine and serum answer partly different questions. Filtered free light chains can appear in urine, but reduced kidney filtration, variable collection, and tubular reabsorption affect the amount. Current screening commonly combines serum SPEP, serum immunofixation or mass spectrometry, and serum free light chains, with urine studies added for selected disorders and baseline characterization.
Analytical sensitivity should match the clinical setting. Routine screening for an incidental high globulin level differs from evaluation of unexplained nephrotic proteinuria, restrictive cardiomyopathy, macroglossia, autonomic neuropathy, or a bleeding disorder suggestive of amyloidosis. A highly suspicious phenotype warrants a complete monoclonal-protein screen even when SPEP is unrevealing.
Tests that identify and contextualize the band
Once SPEP shows a restriction, the next task is to define it. Serum immunofixation applies antibodies against IgG, IgA, IgM, kappa, and lambda to identify the matched heavy- and light-chain type. Additional IgD or IgE testing may be needed when a light chain is seen without a common heavy chain.
Serum free light-chain testing measures unbound kappa and lambda and calculates a ratio. A substantially skewed ratio supports clonal light-chain production, but kidney impairment can raise both chains and alter the expected interval. Results should be interpreted with estimated glomerular filtration rate and the assay-specific reference range.
Quantitative immunoglobulins measure total IgG, IgA, and IgM. They help assess the involved class and suppression of uninvolved immunoglobulins, sometimes called immunoparesis. The total involved class includes both monoclonal and normal polyclonal molecules, so it is not identical to the M-spike quantity.
Baseline evaluation often includes:
| Test | Purpose |
|---|---|
| Complete blood count | Looks for anemia, leukopenia, or thrombocytopenia |
| Creatinine and estimated GFR | Assesses kidney function and helps interpret free light chains |
| Calcium and albumin | Identifies hypercalcemia and provides metabolic context |
| Immunofixation or mass spectrometry | Types and confirms the monoclonal protein |
| Serum free light chains | Detects and quantifies light-chain imbalance |
| Urine protein and monoclonal studies | Evaluates renal protein loss and urinary light chains |
Imaging is selected according to symptoms and risk. Low-dose whole-body CT, PET/CT, or MRI can identify lytic lesions, focal marrow lesions, fractures, or extramedullary disease. A plain skeletal survey is less sensitive than modern cross-sectional imaging.
Bone marrow aspiration and biopsy measure clonal plasma cells, describe morphology, and provide flow-cytometric, cytogenetic, and molecular risk information. Not every low-risk incidental MGUS finding requires immediate marrow biopsy; the decision depends on protein type, amount, free light-chain ratio, symptoms, and other test abnormalities.
Tissue biopsy is required when a monoclonal protein may be damaging an organ but standard myeloma criteria are absent. Kidney biopsy can define monoclonal immunoglobulin deposition, and Congo red staining with accurate amyloid typing can establish amyloidosis. The presence of an M protein alone does not prove that it caused the organ disorder.
From MGUS to smoldering and active disease
MGUS is a laboratory and clinical diagnosis, not merely another name for an M spike. Non-IgM MGUS generally includes a serum monoclonal protein below 3 g/dL, fewer than 10 percent clonal plasma cells in marrow when assessed, and no myeloma-defining event or organ damage attributable to the clone. IgM MGUS and light-chain MGUS use related but distinct definitions.
MGUS is common with increasing age and often remains stable. It is a precursor state, not active cancer. Risk of progression is not identical for everyone. Important factors include M-protein size, non-IgG isotype, abnormal free light-chain ratio, immunoparesis, and evolving laboratory values.
Smoldering multiple myeloma has a larger clonal burden than MGUS but no myeloma-defining event. Criteria can include a higher serum M protein, a greater marrow plasma-cell percentage, or both. Its progression risk is higher and varies substantially with tumor burden, imaging, cytogenetics, and biomarkers.
Active multiple myeloma requires clonal plasma cells or a plasmacytoma plus a myeloma-defining event. Classic organ-damage features include hypercalcemia, renal impairment, anemia, and bone lesions when attributable to the plasma-cell disorder. Biomarker-defined events also include at least 60 percent clonal marrow plasma cells, an involved-to-uninvolved free light-chain ratio of at least 100 with the involved chain at least 100 mg/L, or more than one focal MRI lesion of sufficient size.
Those thresholds are applied strictly. A high free light-chain ratio caused by kidney disease or assay error is not a myeloma diagnosis. Anemia must not be attributed to the clone without considering bleeding, iron deficiency, inflammation, kidney disease, or another marrow disorder.
Other clonal diseases follow different pathways. An IgM protein may arise from lymphoplasmacytic lymphoma and can cause hyperviscosity, neuropathy, cryoglobulinemia, or cold agglutinin disease. AL amyloidosis and monoclonal gammopathy of renal significance may require treatment despite a small marrow clone because the secreted protein is toxic.
The classification therefore depends on the clone, protein, marrow, organs, and symptoms. SPEP contributes one important measurement but cannot make these distinctions alone.
Nonmalignant and technical patterns
Acute inflammation often lowers albumin and raises alpha-1 and alpha-2 fractions. Chronic inflammation may add a broad gamma increase. Nephrotic syndrome can produce low albumin, a prominent alpha-2 region from retained alpha-2 macroglobulin, and reduced gamma globulins from urinary loss.
Cirrhosis can create beta-gamma bridging, partly from polyclonal IgA. This broad connection between regions differs from a narrow beta-migrating monoclonal protein, although immunofixation may be needed when the shape is ambiguous.
Iron deficiency can raise transferrin and produce a beta-region increase. Hemolysis releases hemoglobin-haptoglobin complexes and may alter the alpha-2 or beta regions. Fibrinogen from a plasma-contaminated or incompletely clotted specimen can appear as a discrete band between beta and gamma.
Recent intravenous immunoglobulin can create broad or transient restricted bands derived from pooled donors. Oligoclonal patterns can appear during immune reconstitution after stem-cell transplantation or successful myeloma treatment. These findings should be correlated with treatment dates and prior isotype.
Therapeutic monoclonal antibodies are an increasingly important source of interference. Daratumumab and isatuximab are themselves monoclonal immunoglobulins and may appear on SPEP or immunofixation. Other antibody drugs can also create restrictions. The laboratory should receive the exact drug name and last dose.
Specialized displacement immunofixation or mass spectrometry can distinguish a therapeutic antibody from the patient’s clone. Without that information, a drug band may be mistaken for persistent disease, preventing a correct complete-response designation.
Radiographic contrast and some antibiotics have been reported to create transient artifacts on certain systems. Repeat sampling after clearance may resolve an unexpected band. A result that does not fit the history should be reviewed with the laboratory rather than automatically labeled monoclonal gammopathy.
Bisalbuminemia creates a split albumin peak and is usually benign. It may be inherited or acquired. Its location and immunofixation pattern distinguish it from an M protein.
Using SPEP for monitoring
For a quantifiable intact immunoglobulin M protein, serial SPEP is a central monitoring tool. The same laboratory method is preferred because peak integration and migration can differ among systems.
A trend is more informative than one small change. Hydration, albumin concentration, inflammation, and analytic variability can alter the reported value. Clinicians compare the M spike with free light chains, quantitative immunoglobulins, blood counts, kidney function, calcium, symptoms, and imaging.
During treatment, declining M protein generally indicates response. However, the serum half-life of the immunoglobulin affects how quickly the level falls. IgG persists longer than free light chains, so light-chain measurements may change sooner. Organ recovery may lag behind hematologic response.
A previously measurable peak can become too small for reliable SPEP quantification while remaining detectable by immunofixation. Response criteria distinguish partial, very good partial, complete, and stringent complete responses using combinations of SPEP, urine studies, immunofixation, marrow, and free light chains.
Relapse may present as a rising original M protein, a free-light-chain increase, new symptoms, or imaging progression. “Light-chain escape” describes renewed light-chain production without a parallel rise in the original intact immunoglobulin. Monitoring must therefore reflect the biology of the individual disease.
MGUS follow-up intervals depend on risk. Many patients have a repeat evaluation after the initial diagnosis to establish stability. Lower-risk disease may then need less frequent monitoring, while an evolving M protein, abnormal free light-chain ratio, non-IgG isotype, or symptoms prompts closer review.
The goal is not to normalize every minor tracing irregularity. It is to identify meaningful clonal change before irreversible organ damage while avoiding unnecessary invasive testing for stable low-risk findings.
What to do with an abnormal result
Begin with the exact wording. “Restricted band,” “possible monoclonal protein,” “M spike,” “polyclonal increase,” and “hypogammaglobulinemia” describe different patterns. Ask whether immunofixation was performed automatically and whether the spike was quantifiable.
Review symptoms that change urgency: persistent focal bone pain, unexplained fracture, progressive fatigue, weight loss, recurrent infection, foamy urine, edema, reduced urine output, neuropathy, orthostatic symptoms, bruising, shortness of breath, or features of hyperviscosity such as headache and visual change.
A newly detected M protein usually leads to blood count, creatinine, calcium, albumin, quantitative immunoglobulins, serum free light chains, and confirmation of isotype. Urine studies, imaging, marrow examination, or tissue biopsy are added according to risk and organ findings.
Hematology referral is appropriate for a confirmed monoclonal protein with concerning symptoms, abnormal blood counts, kidney dysfunction, hypercalcemia, a markedly abnormal free light-chain ratio, substantial or rising M protein, IgM-related symptoms, or suspected amyloidosis. Low-risk MGUS may sometimes be monitored in primary care under a defined protocol.
Emergency assessment is warranted for confusion, severe weakness, acute kidney injury, symptomatic hypercalcemia, spinal cord compression symptoms, rapidly progressive shortness of breath, or hyperviscosity manifestations. Treatment should not wait for routine outpatient follow-up in those settings.
A normal SPEP should not end an evaluation when clinical suspicion remains high. Complete monoclonal-protein testing and organ-directed investigation may identify a small or light-chain process that the tracing cannot show.
The most accurate interpretation treats SPEP as a map. It locates and estimates protein abnormalities, but immunochemical typing, functional consequences, and clinical context determine what the map means.
Patients can improve follow-up by using the same laboratory when practical and keeping a record of the original isotype, M-spike value, free light-chain ratio, kidney function, and treatment dates. Comparing like with like reduces false alarms from method changes and makes a genuinely evolving clone easier to recognize.
References
- Overview: Monoclonal Protein Isotype, Quantitative, Serum. Mayo Clinic Laboratories. Accessed 2026.
- Diagnosis and Management of Monoclonal Gammopathy of Undetermined Significance: A Review. 2025.
- Multiple myeloma: 2024 update on diagnosis, risk-stratification, and management. 2024.
- IMWG Criteria for the Diagnosis of Multiple Myeloma. International Myeloma Foundation. Accessed 2026.
- Interference of therapeutic monoclonal antibodies with serum protein electrophoresis and immunofixation. 2025.
- Applying Mayo Clinic MASSFIX to Urine Samples. Mayo Clinic Laboratories Insights. 2023.
Disclaimer
This article is for general educational purposes and does not diagnose monoclonal gammopathy, multiple myeloma, amyloidosis, or another blood disorder. SPEP methods and reference interpretations vary, and abnormal patterns require review with clinical findings and companion testing. Seek urgent medical care for severe weakness, confusion, acute kidney symptoms, spinal cord compression signs, or rapidly worsening illness.
