What does a normal serum protein electrophoresis (SPEP) indicate, and should additional testing be performed if a plasma‑cell disorder is still suspected?

Normal SPEP Does Not Rule Out Plasma Cell Disorders

A normal serum protein electrophoresis (SPEP) does not exclude a plasma cell disorder, and additional testing with serum free light chain (FLC) assay and urine studies must be performed when clinical suspicion remains.

Why SPEP Alone Is Insufficient

SPEP has significant diagnostic limitations in detecting plasma cell disorders:

• Sensitivity is only 71% for plasma cell neoplasms, meaning nearly 3 in 10 cases will be missed if relying on SPEP alone 1
• Negative predictive value is 94%, which sounds reassuring but still leaves a 6% false-negative rate—unacceptable when missing a diagnosis has serious mortality implications 1
• Light chain-only myeloma (approximately 15-20% of multiple myeloma cases) produces no intact immunoglobulin, rendering SPEP normal or showing only hypogammaglobulinemia 2
• Oligosecretory and nonsecretory myeloma (2-3% of cases) produce minimal or no detectable monoclonal protein on SPEP 2

Mandatory Additional Testing When Suspicion Persists

Serum Free Light Chain (FLC) Assay

This test is essential and should always accompany SPEP in the initial screening panel 3, 4:

• The combination of SPEP plus FLC achieves 100% sensitivity for detecting plasma cell disorders, compared to only 97% when using SPEP plus urine protein electrophoresis (UPEP) 4
• FLC detects light chain-only disease that SPEP completely misses 3, 4
• An abnormal FLC ratio (normal 0.26-1.65) indicates clonal plasma cell expansion even when SPEP is normal 5
• FLC is particularly critical for detecting AL amyloidosis, where 25-30% of patients have normal SPEP 3

Urine Protein Electrophoresis with Immunofixation

24-hour urine collection with total protein, UPEP, and urine immunofixation is required for suspected light chain myeloma 2:

• Bence Jones proteinuria may be present even with normal SPEP 2
• Urine studies detect monoclonal light chains excreted renally 2
• However, serum FLC has largely replaced the need for routine urine testing except when AL amyloidosis is specifically suspected 3

Serum Immunofixation Electrophoresis (IFE)

Perform serum IFE when SPEP is normal but clinical suspicion remains high 3:

• IFE is more sensitive than SPEP for detecting small monoclonal proteins 6
• Can identify monoclonal bands in unusual locations (alpha or beta regions) that appear as normal polyclonal increases on SPEP 7, 8

Risk Stratification Guides Testing Intensity

High-Risk Clinical Features Requiring Comprehensive Workup

Proceed with bone marrow biopsy, imaging, and complete laboratory evaluation when any of the following are present 9:

• Unexplained anemia, renal insufficiency, or hypercalcemia
• Lytic bone lesions on imaging
• Serum monoclonal protein >15 g/L (even if detected)
• IgA or IgM monoclonal protein type
• Abnormal FLC ratio
• Clinical features suggesting AL amyloidosis (nephrotic syndrome, cardiomyopathy, neuropathy, hepatomegaly)

Bone Marrow Examination

A bone marrow aspirate and biopsy with flow cytometry and FISH studies is mandatory when 2, 9:

• SPEP is normal but FLC ratio is markedly abnormal (>100 or <0.01)
• Clinical CRAB criteria are present (hypercalcemia, renal failure, anemia, bone lesions)
• Intermediate or high-risk MGUS features exist
• Cytogenetics should include FISH for del(17p), t(4;14), t(14;16), t(14;20), gain 1q, and del 1p 10, 11

Common Pitfalls to Avoid

• Never rely on SPEP alone when plasma cell disorder is suspected—the 29% false-negative rate is clinically unacceptable 1
• Do not assume normal SPEP excludes myeloma in patients with lytic bone lesions—71% sensitivity means 3 in 10 cases are missed 1
• Always order FLC with initial SPEP—this combination achieves 100% sensitivity and would have prevented missed diagnoses in documented cases 4
• Recognize that renal impairment can cause mildly elevated FLC ratios (up to 3.1) without malignancy; interpret FLC in context of creatinine 5
• Remember that M-bands can appear in alpha or beta regions, not just gamma—these unusual locations may be overlooked as normal bands 7, 8

Recommended Diagnostic Algorithm

Initial screening panel (all patients with suspected plasma cell disorder) 3, 4:

• SPEP
• Serum FLC assay
• Complete blood count, calcium, creatinine

If SPEP normal but FLC abnormal or clinical suspicion high 9, 3:

• Serum immunofixation electrophoresis
• 24-hour urine protein with UPEP and urine immunofixation (especially if AL amyloidosis suspected)
• Bone marrow biopsy with flow cytometry and FISH
• Skeletal survey or whole-body MRI 2

If all serum and urine studies normal but suspicion remains 2:

• Bone marrow examination to assess for oligosecretory/nonsecretory myeloma
• MRI spine and pelvis to detect occult lesions
• Tissue biopsy if localized lesion present