How is M (monoclonal) spike measured in an adult patient with suspected multiple myeloma or plasma cell disorder?

How to Measure M Spike

M spike is measured using serum protein electrophoresis (SPEP), which identifies the monoclonal protein as a homogeneous peak on a densitometer tracing, with its concentration calculated based on the size of the peak relative to total serum protein. 1

Primary Measurement Method: Serum Protein Electrophoresis (SPEP)

• SPEP is the fundamental test for detecting and quantifying M-protein in blood, appearing as a distinct spike (M-spike) typically in the gamma globulin region, though it can occasionally appear in the beta region. 2, 3

• The concentration is calculated by measuring the area under the M-spike peak on the densitometer tracing and expressing it as grams per deciliter (g/dL). 1

• SPEP must be performed on every patient with suspected plasma cell disorder as part of the initial diagnostic workup. 1

Essential Complementary Testing

You cannot rely on SPEP alone - the following tests must be performed simultaneously to ensure accurate diagnosis and monitoring:

Serum Immunofixation Electrophoresis (SIFE)

• SIFE is mandatory to confirm the presence and identify the specific type of M-protein (IgG, IgA, IgM) and light chain (kappa or lambda). 1, 4
• SIFE is more sensitive than SPEP and can detect M-proteins that SPEP misses, particularly small monoclonal proteins. 2

Quantitative Immunoglobulins

• Nephelometric quantification of IgG, IgA, and IgM levels must be obtained to measure total concentration of each immunoglobulin class and detect immune paresis (suppression of uninvolved immunoglobulins). 1, 4

Serum Free Light Chain (FLC) Assay

• The serum FLC assay with kappa/lambda ratio is required as part of initial workup, providing high sensitivity when combined with SPEP and SIFE. 1
• The combination of SPEP plus serum FLC achieves 100% sensitivity for detecting plasma cell disorders. 2

Urine M-Spike Measurement

For complete evaluation, 24-hour urine collection is mandatory:

• Urine protein electrophoresis (UPEP) on a concentrated 24-hour specimen identifies urinary M-protein (Bence Jones protein) as a homogeneous peak, with concentration calculated based on peak size and total 24-hour protein excretion. 1

• Urine immunofixation electrophoresis (UIFE) must be performed even if no peak is visible on UPEP to confirm presence and type of light chains. 1, 5

• Random urine samples cannot replace 24-hour collection, even when corrected for creatinine concentration. 1, 5

• Approximately 20% of multiple myeloma patients have measurable urinary M-proteins, making this an essential component of the workup. 1

Critical Pitfalls to Avoid

Serial Monitoring Requirements

• Once M-protein is quantified, the same test method must be used for all serial measurements to ensure accurate comparison and track disease progression. 1, 5

• Serial measurements should be performed at least every 3 months in patients with active myeloma. 4

Special Populations

• Approximately 15-20% of myeloma cases produce only light chains (not complete antibodies), which may not create a visible spike on standard SPEP and require urine testing or serum FLC assays for detection. 2

• About 3% of patients have truly non-secretory myeloma with no detectable M-protein on either SPEP or UPEP, requiring alternative monitoring methods such as bone marrow assessment. 1, 2

• Serum FLC assay cannot replace 24-hour urine protein electrophoresis for monitoring patients with measurable urinary M-proteins. 1, 5

Technical Considerations

• Nephelometric quantitation may overestimate M-protein concentration when values are high, so be aware of this limitation when interpreting results. 1

• Urine samples must be adequately concentrated to improve detection sensitivity and avoid false-negative results. 1, 5

• In rare cases, a single M-protein can appear as two bands on SPEP, simulating biclonal gammopathy - immunofixation is essential to differentiate true biclonality from this artifact. 3, 6

Biological Variation in Stable Disease

• The biological coefficient of variation for serum M-spike is approximately 7.8%, meaning changes less than this may represent normal fluctuation rather than true disease progression. 7

• Urine M-spike has much higher biological variation (35.5%) compared to serum M-spike, making it less reliable for detecting small changes in disease burden. 7

• Serum FLC has intermediate biological variation (27.8%), similar to urine M-spike but with the advantage of being more frequently measurable. 7