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SPEP, UPEP, Immunofixation, and Free Light Chains: The Myeloma Workup NPs Need to Know

 

SPEP, UPEP, Immunofixation, and Free Light Chains: The Myeloma Workup NPs Need to Know

The dipstick misses light chains. SPEP finds the spike. UPEP catches what spills into the urine. And free light chains changed everything.

This post complements the immunoglobulin levels post and the SPEP section from the autoimmune serologic review. When you suspect a monoclonal gammopathy—whether it's MGUS, multiple myeloma, Waldenström's, or amyloidosis—you need to understand the full diagnostic panel: SPEP, UPEP, immunofixation, and serum free light chains.

The Diagnostic Panel

TestWhat It DoesWhen to Order
SPEP (Serum Protein Electrophoresis)Separates serum proteins by charge. Detects and quantifies an M-spike (monoclonal protein) in the gamma region.Suspected myeloma, MGUS screening, unexplained elevated total protein, unexplained proteinuria, polyclonal gammopathy evaluation
Immunofixation (IFE)Identifies the heavy chain (IgG, IgA, IgM) and light chain (kappa or lambda) type of the monoclonal protein.Confirm and characterize any M-spike seen on SPEP. More sensitive than SPEP for detecting small monoclonal proteins.
Serum Free Light Chains (sFLC)Measures kappa and lambda free light chains and calculates the kappa/lambda ratio.Essential for light chain myeloma, AL amyloidosis, and non-secretory myeloma where SPEP may be negative. Also used for MGUS risk stratification and monitoring.
UPEP (Urine Protein Electrophoresis)Detects monoclonal protein (Bence Jones protein = free light chains) in urine. Requires 24-hour urine collection.Myeloma workup, AL amyloidosis, monitoring light chain excretion post-treatment. Detects the light chains the dipstick misses.
Urine ImmunofixationIdentifies the type of monoclonal protein in urine (kappa vs. lambda).Confirm any monoclonal band on UPEP

Why the Dipstick Misses Myeloma

Critical Connection to Urinalysis Post

As discussed in the urinalysis post, the urine dipstick detects albumin only. It does NOT detect free light chains (Bence Jones protein). A patient with light chain myeloma can have massive proteinuria on a 24-hour collection with a completely negative dipstick. This is why UPEP or urine immunofixation is essential in the myeloma workup. If you suspect myeloma, never rely on the dipstick to assess proteinuria.

The Screening Panel for Suspected Myeloma

Per current IMWG (International Myeloma Working Group) guidelines, the recommended initial workup when you suspect a plasma cell neoplasm is:

  1. SPEP + serum immunofixation
  2. Serum free light chains (kappa and lambda with ratio)
  3. 24-hour UPEP + urine immunofixation

Together, these three tests detect >99% of myeloma cases. Using SPEP alone misses ~15–20% of cases (light chain only and non-secretory myeloma).

When to Suspect Myeloma in Primary Care

Red Flags That Should Trigger the Workup
  • Unexplained anemia (especially normocytic) + elevated total protein or globulin gap
  • Bone pain (especially back, ribs) without clear mechanical cause + anemia
  • Unexplained renal insufficiency (especially with bland urine sediment and proteinuria)
  • Hypercalcemia without obvious cause
  • Markedly elevated ESR (>100 mm/hr) with rouleaux on peripheral smear
  • Recurrent bacterial infections with low immunoglobulins
  • Peripheral neuropathy without other explanation (think AL amyloidosis)
  • Incidental M-spike on SPEP ordered for another reason
  • Pathologic fractures or lytic bone lesions on imaging

MGUS: The Precursor State

Monoclonal Gammopathy of Undetermined Significance (MGUS) is a pre-malignant condition found in ~3% of adults >50. It progresses to myeloma at ~1% per year. MGUS criteria: M-protein <3 g/dL, bone marrow plasma cells <10%, no end-organ damage (CRAB: Calcium elevation, Renal insufficiency, Anemia, Bone lesions). Risk stratification uses: M-protein size, non-IgG isotype, and abnormal free light chain ratio. Low-risk MGUS can be monitored in primary care with annual SPEP, CBC, creatinine, and calcium.

Free Light Chains: The Game-Changer

Serum free light chains revolutionized myeloma diagnostics. Normal kappa/lambda ratio is approximately 0.26–1.65. An abnormal ratio indicates clonal light chain production. FLC is the most sensitive test for detecting light chain myeloma, AL amyloidosis, and non-secretory myeloma—conditions that SPEP and UPEP may miss. The "involved/uninvolved" FLC ratio is also used for monitoring treatment response and detecting early relapse.

The Pitfalls

  • FLC ratio is affected by renal function: In CKD, both kappa and lambda accumulate, and the ratio may be mildly abnormal (kappa/lambda up to 3.1 in renal failure) without a clonal process. Use renal-adjusted reference ranges.
  • SPEP can miss small M-proteins: Immunofixation is more sensitive. If SPEP is negative but clinical suspicion is high, the immunofixation and FLC may still be positive.
  • Don't confuse polyclonal gammopathy with myeloma: A broad-based elevation in the gamma region = polyclonal (autoimmune disease, chronic infection). A narrow spike = monoclonal (MGUS, myeloma, Waldenström's). Immunofixation distinguishes them definitively.
  • Urine collection errors: UPEP requires a complete 24-hour collection. Incomplete collections underestimate proteinuria. Some centers now accept a random urine immunofixation for screening, but 24-hour quantification is still needed for monitoring.
  • IgA myeloma is harder to quantify on SPEP: IgA M-proteins migrate in the beta region and may be missed or under-quantified. Quantitative IgA and FLC are important adjuncts for monitoring IgA myeloma.

Bottom Line

The myeloma workup requires three tests: SPEP with immunofixation, serum free light chains, and 24-hour UPEP with urine immunofixation. Using any one alone will miss cases. The urine dipstick is blind to light chains. Free light chains are the most sensitive test for light chain disease. And a monoclonal spike on SPEP in a patient over 50 is MGUS until proven otherwise—but it requires monitoring because 1% per year will progress to myeloma.

Stay sharp out there.

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