Serum protein electrophoresis (SPEP) is widely considered the most important blood test for multiple myeloma. It detects abnormal proteins called M-proteins (or M-spikes) that myeloma cells produce, and it’s the first test doctors order when myeloma is suspected. But no single test can diagnose or manage myeloma alone. A panel of blood tests works together to detect the disease, confirm the diagnosis, assess organ damage, and track treatment response.
How SPEP Detects Myeloma
SPEP separates the proteins in your blood by electrical charge, creating a pattern that shows how much of each protein type is present. In a healthy person, the pattern is smooth and predictable. In someone with myeloma, a sharp, narrow spike appears, usually in the gamma region of the graph. This spike represents a flood of identical antibody proteins pumped out by cancerous plasma cells.
In most myeloma patients, the M-protein level is greater than 3 grams per deciliter. However, about one in five patients has a spike below 1 g/dL, and roughly 18% of myeloma patients show no spike on SPEP at all. That’s because some myeloma cells produce only fragments of antibodies (called light chains) that are too small or too sparse for SPEP to pick up reliably. This is why doctors never rely on SPEP alone.
Immunofixation: Identifying the Protein Type
When SPEP finds an M-spike, or when myeloma is still suspected despite a normal SPEP, immunofixation electrophoresis (IFE) is the next step. IFE identifies exactly which type of antibody the myeloma cells are making. This matters because different antibody types can behave differently and may require different monitoring strategies.
IFE is also more sensitive than SPEP. In one large comparison study, SPEP detected abnormal proteins in 288 samples while IFE caught them in 353 samples, picking up dozens of cases SPEP missed. IFE later becomes important during treatment too: a complete response to therapy requires that IFE come back negative on both blood and urine, meaning no detectable abnormal protein remains.
The Serum Free Light Chain Test
For the roughly 15 to 20% of myeloma patients whose disease doesn’t show up well on SPEP, the serum free light chain (FLC) assay fills the gap. This test measures two types of antibody fragments, kappa and lambda light chains, floating freely in your blood. Normal ranges are 3.3 to 19.4 mg/L for kappa, 5.71 to 26.3 mg/L for lambda, and a kappa-to-lambda ratio between 0.26 and 1.65.
A significantly skewed ratio suggests one type of plasma cell is overproducing, which is the hallmark of a plasma cell disorder like myeloma. The FLC test is especially critical for a subtype called light chain myeloma, where the cancer produces only these fragments rather than whole antibodies. It’s also how doctors measure treatment response in patients whose disease can’t be tracked by M-protein levels. A return to a normal ratio (0.26 to 1.65) is one of the criteria for complete response in these patients.
Blood Tests That Reveal Organ Damage
Myeloma doesn’t just show up as abnormal proteins. It damages organs in predictable ways, and doctors use the acronym CRAB to describe these effects: calcium elevation, renal (kidney) problems, anemia, and bone lesions. Several routine blood tests check for these.
- Calcium: Myeloma breaks down bone tissue, releasing calcium into the blood. A serum calcium level above 11 mg/dL, or more than 1 mg/dL above the normal upper limit, meets the diagnostic threshold for myeloma-related hypercalcemia.
- Creatinine: Abnormal proteins can clog and damage the kidneys. A serum creatinine above 2 mg/dL or a creatinine clearance below 40 mL per minute signals significant kidney impairment.
- Complete blood count (CBC): Myeloma cells crowd normal blood-forming cells out of the bone marrow. The most common finding is anemia, a low red blood cell count, which explains the fatigue many patients experience before diagnosis.
Any one of these CRAB findings, combined with evidence of abnormal plasma cells, is enough to confirm a diagnosis of active myeloma that needs treatment.
Blood Tests Used for Staging
Once myeloma is diagnosed, two additional blood tests help determine how advanced it is. The Revised International Staging System (R-ISS) relies heavily on beta-2 microglobulin and albumin levels to sort patients into three stages.
Stage I, with the best outlook, requires beta-2 microglobulin below 3.5 mg/L and albumin at or above 3.5 g/dL, along with normal LDH (a marker of cell turnover) and standard-risk genetics. Stage III, the most advanced, requires beta-2 microglobulin at 5.5 mg/L or higher combined with either high-risk genetic changes or elevated LDH. Everything in between falls into Stage II. Beta-2 microglobulin is particularly useful because it reflects both tumor burden and kidney function, two factors that directly influence prognosis.
Tracking Treatment Response
The same tests that diagnose myeloma become the tools for measuring whether treatment is working. Doctors repeat SPEP, IFE, and FLC tests at regular intervals and compare the numbers to your baseline.
A partial response means the M-protein in your blood has dropped by more than 50%. A very good partial response means M-protein is barely detectable. A complete response requires that immunofixation shows no abnormal protein in blood or urine and that bone marrow contains fewer than 5% plasma cells. For patients tracked by free light chains instead of M-protein, a complete response means the kappa-to-lambda ratio has returned to the normal range of 0.26 to 1.65.
These aren’t just clinical benchmarks. They directly shape decisions about whether to continue, change, or stop treatment. Rising M-protein or a worsening light chain ratio can signal relapse before symptoms appear, giving doctors a window to adjust course early.
Why No Single Test Is Enough
SPEP earns its reputation as the most important initial test because it catches the majority of myeloma cases and provides a measurable number that tracks the disease over time. But it misses roughly one in five patients entirely. Adding immunofixation and the free light chain assay brings detection rates close to 100%. Layer in CBC, calcium, creatinine, and beta-2 microglobulin, and you have a complete picture: detection, confirmation, organ damage assessment, staging, and a built-in system for monitoring treatment. Myeloma diagnosis and management depend on these tests working as a panel, not any one test in isolation.