Clinical Vignette:
A primary care physician obtains annual labs on their patient and notices a total protein of 9.1 g/dL (normal: 6.0–8.3 g/dL) and an albumin of 3.0 g/dL (normal: 3.5–5.0 g/dL), giving a globulin gap of 6.1 g/dL. A globulin gap > 4 g/dL warrants further investigation, and an SPEP is sent — which returns with an M-protein.
The Case
Patient: | 72-year-old female |
Presentation: | Incidental finding on routine labs — no symptoms |
Referring finding: | Total protein 9.1 g/dL, Albumin 3.0 g/dL → Globulin gap of 6.1 g/dL |
SPEP result: | M-protein of 0.3 g on electrophoresis |
Final characterization: | IgG Kappa MGUS |
She wasn't fatigued. She wasn't having bone pain. She wasn't in the ER with a pathologic fracture. She was simply a patient whose numbers told a story before she had any symptoms — and that is exactly why these referrals matter.
Step 1: The SPEP — What Are We Actually Looking At?
Before anything else, I look at the serum protein electrophoresis itself. The SPEP separates proteins in the blood by their electrical charge and migration speed — and when a single clone of plasma cells produces one identical immunoglobulin, it shows up as a sharp, narrow spike in the gamma region. We call this an M-protein, and its presence is the first thing I am looking for. (For a deeper dive into how to read an SPEP, check out Let's Decode the SPEP.)
In this patient — yes, there is an M-spike. It measures 0.3 g. Small, but real.
💡 Key concept: The globulin gap (total protein minus albumin) is what often first tips off the referring provider. A globulin gap > 4 g/dL should prompt an SPEP. This patient's gap of 6.1 g/dL made the referral entirely appropriate.
Step 2: CRAB — Is There End-Organ Damage?
Once I confirm an M-protein exists, my next question is: has this protein caused any damage yet? I review whatever CBC, CMP, and imaging is already available — and if nothing has been sent, I order it. (For a full breakdown of CRAB criteria and the diagnostic framework for multiple myeloma, see Multiple Myeloma: The Basics.)
For this patient — no anemia, normal calcium, normal creatinine, no bone lesions. No CRAB criteria. No end-organ damage. This matters enormously, because it is what separates MGUS from smoldering myeloma and active multiple myeloma. But the absence of CRAB is not the end of the workup. Not even close.
Step 3: The Workup I Always Complete — And Why It Matters
Here is where I want to spend the most time, because this is where I see the most variation in practice.
Identifying an M-protein on SPEP is the beginning, not the conclusion. There are three tests I order on every single patient referred for a positive SPEP:
🔬 A. SPEP with Immunofixation Electrophoresis (IFE)
This is the most important test you can add, and I cannot emphasize it enough.
SPEP tells you that there is a spike and roughly how big it is. But it does not tell you what kind of immunoglobulin it is. Immunofixation electrophoresis (IFE) uses antibodies directed against specific immunoglobulin heavy chains (IgG, IgA, IgM) and light chains (kappa, lambda) to characterize the M-protein precisely.
In this patient: IgG Kappa. That is the complete monoclonal characterization. Without IFE, you have a spike without an identity.
⚠️ Do not skip the IFE. It is not redundant — it is essential. SPEP alone cannot tell you whether the protein is IgG, IgA, IgM, or something else entirely. The subtype matters for prognosis, risk stratification, and follow-up interval.
🔬 B. Quantitative Immunoglobulins
This gives me the actual serum levels of IgG, IgA, and IgM. I want to know:
How elevated is the involved immunoglobulin (in this case IgG)?
Are the uninvolved immunoglobulins suppressed?
Suppression of uninvolved immunoglobulins — for example, low IgA and IgM in a patient with IgG myeloma — is a sign of immune paresis and is associated with more advanced or aggressive disease.
🔬 C. Serum Free Light Chains (Kappa and Lambda) with Ratio
This one deserves its own moment. Light chain myeloma exists.
In some patients, plasma cells produce only the light chain portion of the immunoglobulin (kappa or lambda) — with no heavy chain attached. These free light chains are small enough to spill into the urine, which is why they were historically detected on urine protein electrophoresis (UPEP). The problem? They may not show up as a clear spike on SPEP, and they can easily be missed if you stop your workup after an SPEP that looks relatively unremarkable.
The serum free light chain assay (sFLC) detects these directly in the blood. The kappa/lambda ratio tells you whether there is a clonal excess. A normal ratio is approximately 0.26–1.65. When it is markedly elevated (or suppressed, in lambda disease), that is a signal of clonal light chain production — even in the absence of a visible M-spike.
🧠 The clinical pearl: A patient can have life-threatening light chain myeloma with a near-normal SPEP. The free light chain ratio is not optional — it is a required part of the monoclonal protein workup.
Putting It Together: What Does This Patient Have?
Criterion | Status |
|---|---|
M-protein < 3 g/dL | ✅ (0.3 g) |
Clonal bone marrow plasma cells < 10% | ✅ (consistent with presentation) |
No CRAB criteria | ✅ |
No amyloidosis or related disorder | ✅ |
Diagnosis: IgG Kappa MGUS
The rate of progression from MGUS to multiple myeloma or a related disorder is approximately 1% per year. Risk stratification uses the M-protein size, immunoglobulin subtype, and free light chain ratio to guide follow-up frequency.
For a low-risk MGUS like this one, I typically repeat labs (SPEP, CBC, CMP, free light chains) every 6 months.
The Takeaway
SPEP tells you a spike exists. IFE tells you what it is. Always order both.
Assess for CRAB criteria in every patient — it determines whether this is MGUS, smoldering, or active myeloma.
Free light chain ratio is not a bonus test. Light chain myeloma can be invisible on SPEP.
Quantitative immunoglobulins help characterize immune paresis and establish a baseline.
MGUS does not need treatment — but it needs a thoughtful, ongoing relationship with hematology.The incidental elevated total protein that started this case was not a false alarm. It was the first chapter of a story we now get to follow closely — and because we caught it early and worked it up completely, this patient has the best possible chance at staying one step ahead.
As always, reply with questions or cases you'd like to see covered. More clinical pearls coming soon.
— Dr. Astha Thakkar
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