Serum protein electrophoresis, often abbreviated SPEP or SPE, is a laboratory test that separates the proteins in your blood into groups based on their size and electrical charge, then displays the results as a pattern of peaks on a graph. Doctors order it most often when they suspect a blood cancer called multiple myeloma or a precancerous condition involving abnormal antibody production, but the test reveals far more than that. The shape and size of each peak can point toward liver disease, kidney problems, chronic infections, immune deficiencies, and inherited conditions that would otherwise require a different set of tests to catch.
How the Test Works
A small sample of your blood serum (the liquid left after blood cells and clotting proteins are removed) is placed on a gel or fed into a thin tube, and an electrical current is applied. Because each protein carries a slightly different charge and has a different molecular weight, the proteins migrate at different speeds through the medium. Smaller, more heavily charged proteins travel farther; larger or less charged ones lag behind. After a set time the migration is stopped, the gel is stained or the tube’s detector reads the protein absorbance, and the result is plotted as a tracing with distinct peaks.
1IntechOpen. Serum Protein Electrophoresis and Its Clinical ApplicationsTwo main technologies are in clinical use. Traditional agarose gel electrophoresis (AGE) has been the workhorse for decades: the serum is applied to a thin agarose slab, proteins migrate in the gel, and a dye is used to visualize them. The newer method, capillary zone electrophoresis (CZE), runs the sample through a narrow capillary tube and reads protein absorbance directly with ultraviolet light. A head-to-head comparison found that CZE had better reproducibility, with between-run precision ranging from about 1 to 5 percent coefficient of variance compared with roughly 4 to 8 percent for AGE. The two methods agreed on the classification of abnormal patterns about 96 percent of the time, and agreed completely when identifying the type of abnormal antibody present.
2PubMed. Comparison of serum protein electrophoresis by agarose gel and capillary zone electrophoresis in a clinical settingReading the Five Classic Bands
Regardless of technique, the result is a graph with five major zones, each representing a family of proteins. Reading them from left to right on the tracing gives you a snapshot of what your blood proteins are doing.
- Albumin: The tallest peak, sitting at the far left. Albumin is the most abundant protein in blood and does critical work transporting hormones, drugs, and fatty acids while keeping fluid from leaking out of blood vessels. A low albumin peak can signal liver disease (since the liver makes albumin), malnutrition, kidney disease that causes protein loss, or chronic inflammation.
- Alpha-1 globulins: A small peak just to the right of albumin. Its main component is alpha-1 antitrypsin, a protein that protects lung tissue. When this peak is unusually flat, it can be an early clue to alpha-1 antitrypsin deficiency, a genetic condition that raises the risk of lung and liver disease.
- Alpha-2 globulins: This peak rises during acute inflammation because it contains proteins like haptoglobin and alpha-2 macroglobulin that the body pumps out in response to infection or tissue damage. A tall alpha-2 peak alongside a low albumin peak is a hallmark of nephrotic syndrome, a kidney condition in which protein spills into the urine.
- Beta globulins: Contains transferrin (which carries iron) and components of the complement immune system. Changes here can reflect iron deficiency or certain liver problems.
- Gamma globulins: The rightmost peak, composed almost entirely of antibodies (immunoglobulins). This is the region doctors scrutinize most closely when looking for signs of myeloma or related conditions, because a cancer of antibody-producing cells usually shows up as a sharp, narrow spike here.
The pattern of which bands are elevated or depressed relative to each other matters at least as much as any single measurement. A trained eye can scan the tracing and form a working differential diagnosis before a single follow-up test is ordered.
The Test’s Most Common Job: Screening for Myeloma and Related Conditions
The primary reason SPEP gets ordered is to look for a monoclonal protein, sometimes called an M-protein or M-spike. Normal antibodies are “polyclonal,” meaning many different immune cells each produce their own slightly different version. When a single clone of plasma cells multiplies out of control, it churns out one identical antibody in enormous quantities. That flood of identical protein shows up on the SPEP tracing as a sharp, tall spike, usually in the gamma region, that stands out from the gentle curve of normal polyclonal antibodies.
This M-spike can indicate several things on a spectrum of severity. A small M-protein in someone with no symptoms often points to monoclonal gammopathy of undetermined significance (MGUS), a common precancerous state that requires monitoring but not treatment. A larger M-protein with certain other lab findings may indicate smoldering myeloma, an intermediate stage. And a large M-protein combined with organ damage, bone lesions, or other criteria leads to a diagnosis of active multiple myeloma. The International Myeloma Working Group recommends SPEP as one of three tests that should always be performed together when evaluating any of these conditions.
3International Myeloma Foundation. MGUS, Smoldering Myeloma & Multiple Myeloma: DifferencesIn a study at a single center that tested 150 suspected cases, roughly 11 percent showed a monoclonal gammopathy, with ten percent ultimately diagnosed as multiple myeloma.
4PubMed Central. The role of serum protein electrophoresis in the detection of multiple myeloma: an experience of a corporate hospitalAnother study of 131 patients found that about 18 percent had M-bands, while nearly half had a more general increase in gamma globulins, and about 12 percent showed decreased albumin. In patients with an M-band, IgG concentrations were markedly elevated, with smaller rises in IgA and IgM.
5PubMed Central. Significance of Serum Protein Electrophoresis in the Detection of Multiple Myeloma: A Diagnostic Interpretation of Patients with Varied ImmunoglobulinsSPEP is often run on older adults as a screening tool, partly because MGUS becomes increasingly common with age. One underappreciated side benefit is that the test simultaneously reports albumin levels. In elderly patients, clinicians tend to focus on the gamma region and may overlook a low albumin reading, missing an opportunity to flag malnutrition or chronic illness early.
6American Journal of Clinical Pathology. 77 Leveraging Serum Protein Electrophoresis for Early Detection of Hypoalbuminemia in the Elderly: A Call for Comprehensive Interpretation and InterventionDiagnostic Patterns Beyond Myeloma
While myeloma screening gets the most attention, the shape of the SPEP tracing carries diagnostic clues for a surprisingly wide range of conditions. Experienced clinicians learn to recognize a handful of classic patterns.
Kidney Disease
In nephrotic syndrome and other protein-losing kidney conditions, the SPEP tracing shows a characteristic combination: the albumin peak drops because albumin leaks through damaged kidneys into the urine, the alpha-2 peak rises (the liver compensates by making more alpha-2 macroglobulin, which is too large to leak), and the gamma region falls because smaller immunoglobulins are also lost.
7Journal of Urology and Renal Diseases. The Role of Protein Electrophoresis in Differential Diagnosis of Renal DisordersAlpha-1 Antitrypsin Deficiency
A flattened or absent alpha-1 peak on SPEP can be the first hint of alpha-1 antitrypsin deficiency, a genetic disorder that affects the lungs and liver. In one screening study, researchers flagged samples where the alpha-1 globulin band fell below 3 percent, then measured the actual alpha-1 antitrypsin concentration. Of the subjects who went on to genetic testing, about 87 percent carried mutations in the SERPINA1 gene responsible for the disorder.
8PubMed. Alpha-1 Antitrypsin Deficiency Screening Using Serum Protein ElectrophoresisA similar study that started from the SPEP tracing found mutations in over 90 percent of patients with low alpha-1 antitrypsin concentrations, including some rare mutations not previously described.
9PubMed Central. Utility of the Serum Protein Electrophoresis in the Opportunistic Screening for the Deficiency of Alpha-1 AntitrypsinThis matters because alpha-1 antitrypsin deficiency is widely underdiagnosed. Many people carry the mutations for years without knowing, developing emphysema or liver disease that gets attributed to other causes. Because SPEP is already being run for other reasons, reading the alpha-1 band carefully offers an essentially free screening opportunity.
Immune Deficiency
A flat or near-absent gamma region signals hypogammaglobulinemia, meaning the body is not producing enough antibodies. This is the hallmark finding in common variable immunodeficiency (CVID), a condition where patients suffer repeated infections because their immune system cannot generate a proper antibody response. The starting point for diagnosing CVID is often noticing decreased gamma globulins on SPEP.
10PubMed Central. Are we forgetting to carry out serum protein electrophoresis as part of diagnosis workup?Chronic Inflammation and Infection
Chronic infections and autoimmune diseases tend to produce a broad, rounded elevation of the gamma region rather than a sharp spike. This polyclonal pattern reflects many different immune cell clones ramping up antibody production simultaneously, and it looks completely different from the narrow M-spike of myeloma. Distinguishing a polyclonal bump from a monoclonal spike is one of the most practically important calls a lab can make on an SPEP tracing.
Follow-Up Tests That Work Alongside SPEP
SPEP rarely tells the full story by itself. When an abnormality appears on the tracing, additional tests narrow down what it means.
Immunofixation electrophoresis (IFE) is the standard next step when an M-spike is found. It uses antibodies directed against each type of heavy chain (IgG, IgA, IgM) and each type of light chain (kappa and lambda) to identify exactly which class of immunoglobulin the monoclonal protein belongs to.
11Medical Laboratory Journal. Utility of immunofixation in complementing and empowering serum protein electrophoresis in the diagnosis of paraproteinemia: Experience at a tertiary care centerThis classification matters because different immunoglobulin subtypes behave differently, affect prognosis, and sometimes require different treatment strategies.
The serum free light chain (sFLC) test has become indispensable over the past two decades. Some myelomas produce only the light-chain portion of an antibody, without the heavy chain. These “light chain only” myelomas can be invisible or nearly invisible on a standard SPEP tracing. The sFLC test catches them by measuring the ratio of kappa to lambda free light chains in the blood. In one clinical trial comparing methods, all 25 light-chain myeloma patients had measurable disease by the sFLC test and by urine methods, but among intact-immunoglobulin myeloma patients, the sFLC test detected measurable disease in about 62 percent compared with only 35 percent by urine electrophoresis, demonstrating the sFLC test’s greater sensitivity for monitoring.
12PubMed Central. Comparison of serum free light chain and urine electrophoresis for the detection of the light chain component of monoclonal immunoglobulins in light chain and intact immunoglobulin multiple myelomaIn practice, labs that run both SPEP and sFLC catch cases that either test alone would miss. One study at a South African academic center reported that four patients with completely normal SPEP tracings had abnormal free light chain ratios, and those patients turned out to have light-chain myeloma or a related plasma cell tumor.
13African Journal of Laboratory Medicine. Serum-free light chain test utilisation at a South African academic laboratory and comparison with serum protein electrophoresis resultsUrine protein electrophoresis (UPEP) has traditionally been used alongside SPEP, especially to look for Bence Jones proteins, which are light chains that spill into the urine. While UPEP still has a role, recent work suggests that urine free light chain measurements may complement or even partly replace the traditional 24-hour urine collection, which patients find inconvenient.
14The Journal of Applied Laboratory Medicine. Value of Urinary Free Light Chain Testing for Monitoring of Bence Jones ProteinuriaThings That Can Interfere with the Results
Like any lab test, SPEP has its pitfalls. A number of substances in the blood can create false peaks, mask real ones, or shift the tracing in misleading ways. Interferences include substances naturally present in the blood, like hemoglobin from broken red blood cells (hemolysis) or fibrinogen from an incompletely clotted sample, as well as external compounds like radiocontrast dyes, certain antibiotics, and monoclonal antibody therapies used to treat cancer.
15PubMed. Recognition and management of common, rare, and novel serum protein electrophoresis and immunofixation interferencesThe monoclonal antibody therapy issue deserves extra attention because it has become increasingly relevant. Drugs like daratumumab, used to treat myeloma itself, are monoclonal antibodies that can show up as their own small spike on SPEP, potentially being mistaken for residual disease. Labs need to know which medications a patient is taking to avoid this trap.
Lipemia, meaning fat particles in the blood from a recent meal or from conditions that raise blood lipid levels, is another common interference. In capillary electrophoresis, severely lipemic samples from patients with naturally high lipid levels caused a significant positive shift in the albumin fraction and a negative shift in the gamma fraction. Interestingly, the effect depended on the source of the lipids: intravenous lipid emulsions added to normal samples did not cause the same interference.
16PubMed. Effects of lipemia on capillary serum protein electrophoresis in native ultra-lipemic material and intravenous lipid emulsion added seraHemolysis is problematic in a different way. When red blood cells break open, the released hemoglobin migrates to the beta-2 region of the tracing, artificially inflating the beta fraction and suppressing the apparent proportion of other fractions.
17PubMed. Effects of haemolysis, lipaemia, bilirubinaemia and fibrinogen on protein electropherogram of canine samples analysed by capillary zone electrophoresisThese interferences highlight why interpretation of SPEP is not purely automated. A technologist or pathologist reviewing the tracing needs to look at the sample itself, check for hemolysis or lipemia, review the patient’s medication list, and factor all of that in before signing out a result.
What to Expect If Your Doctor Orders One
From your perspective as a patient, the test is simple. A standard blood draw is all that is needed. No fasting is strictly required for most labs, though a heavily lipemic sample from a recent fatty meal can cause the interferences mentioned above, so some physicians prefer a fasting sample. Results typically come back within a few days, reported as a percentage and an absolute concentration for each of the five zones, along with the visual tracing itself.
If the tracing looks normal, that is reassuring but not the final word. A very small M-protein can hide within the normal-looking background, especially if it migrates in the beta region where transferrin already creates a peak. That is why the free light chain test and immunofixation exist as more sensitive companions. On the other hand, a clearly abnormal SPEP does not always mean cancer. Chronic infections, autoimmune diseases, and even some medications can alter the tracing. The pattern matters, and it needs to be read in the context of your symptoms and other lab work.
SPEP in Veterinary Medicine
Serum protein electrophoresis is not exclusively a human test. Veterinarians use it in dogs, cats, horses, and even exotic species like reptiles and birds. The basic principle is the same, though the normal reference ranges and the number of distinct protein bands vary between species. In veterinary medicine, SPEP is considered especially useful for distinguishing between infectious disease and cancer. A polyclonal increase in gamma globulins points toward infection, while a sharp monoclonal spike suggests a plasma cell tumor, just as it does in humans.
18Journal of Veterinary Research. Application of native agarose gel electrophoresis of serum proteins in veterinary diagnosticsDespite its usefulness, SPEP remains underused in veterinary practice compared with human medicine. Reviews of the veterinary literature describe it as “a relatively little-used diagnostic tool” even though it can provide information about inflammation, immune status, and liver function in a single inexpensive test.
19Veterinárnà medicÃna. Serum proteins and their diagnostic utility in veterinary medicine: a reviewIn exotic animal medicine, SPEP has found a niche because many exotic species mask signs of illness until they are severely sick. A shift in the protein fractions on electrophoresis can be one of the earliest laboratory clues that something is wrong.
20PubMed Central. Applications of serum protein electrophoresis in exotic pet medicineA Brief History of the Technique
The roots of protein electrophoresis go back to the work of Arne Tiselius, who developed moving boundary electrophoresis in the 1930s and won the Nobel Prize in Chemistry for it in 1948. His method involved watching proteins migrate in free solution, which was groundbreaking but cumbersome and impractical for routine clinical labs. By the early 1950s, zonal electrophoresis on supporting media like filter paper and starch was beginning to replace Tiselius’s approach.
21PubMed. How It All Began: A Personal History of Gel ElectrophoresisThe shift to agarose gels came later and made the technique fast and reliable enough for everyday clinical use. Capillary electrophoresis arrived in the 1990s and added automation and improved precision. Despite these technological upgrades, the fundamental concept has not changed: apply a current, let proteins sort themselves, and read the pattern. The five-zone tracing that clinicians interpret today would have been recognizable to a laboratory scientist working half a century ago, even if the equipment producing it looks completely different.