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M-protein, free light chains and what they track | CION Cancer Clinics
M-protein is the single abnormal antibody made by myeloma cells. Free light chains are loose kappa or lambda fragments of antibody in the blood. When one kind rises far above the other, it points to one abnormal clone. Together these numbers show how active myeloma is and how it responds to treatment. This page explains what each line on the report tracks, and what it cannot tell you on its own. At CION Cancer Clinics, our haematology team plans myeloma and lymphoma care with you, discussed at a tumour board and explained in plain words.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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The short answer
What do M-protein and free light chains actually measure?
M-protein is the single abnormal antibody made by myeloma cells, and free light chains are loose fragments of antibody floating in the blood. Together they are the main way your haematologist tracks how much myeloma is active, without repeating a bone marrow test every few months.
How an antibody is built
A normal antibody is made of two heavy chains and two light chains joined together. The heavy chain gives the antibody its type: IgG, IgA, IgM and so on. The light chain is one of two kinds, kappa or lambda. Healthy plasma cells make a little more light chain than they need, so a small amount always floats free.
What changes in myeloma
Myeloma cells are copies of one plasma cell, so they all make the same antibody. That one antibody builds up as the M-protein, for example "IgG kappa". The same cells often release extra free light chains of their own kind, so either kappa or lambda rises and the balance between the two tips over.
Why doctors use both
Most people make a whole M-protein and extra light chains. Some make only light chains, which do not show well on the standard protein test. A small group make almost nothing measurable. Using both tests means fewer people are missed, and each one covers the other's blind spot.
On your report
What do the lines on the report mean?
- M-spike or paraprotein
- The measured amount of the abnormal antibody, usually printed in g/dL or g/L. Lower is better during treatment.
- Immunofixation
- A test that names the type of the M-protein, such as IgG kappa or IgA lambda. It says positive or negative, not how much.
- Kappa and lambda free light chains
- The two kinds of loose fragment, each with its own number and a normal range printed beside it.
- Kappa/lambda ratio
- One number divided by the other. An abnormal ratio suggests one clone of cells is making too much of one kind.
- Involved and uninvolved
- The involved light chain is the one the myeloma makes. The uninvolved one is the other kind.
- dFLC
- The difference between the involved and uninvolved light chain, sometimes used to follow light-chain disease.
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Which number tracks what?
No single line tells the whole story. Your team reads them together and against your own earlier results.
M-protein
The steadiest guide for most people with IgG or IgA myeloma. It falls slowly, over weeks, because whole antibodies stay in the blood a long time.
Most useful for
- Following response cycle by cycle
- Spotting a slow rise during maintenance
Free light chains
These clear from the blood within hours, so they change quickly. They give an early sense of whether treatment is biting.
Most useful for
- Light-chain-only myeloma
- Early response when the kidneys are affected
The ratio
A normal ratio after treatment is one of the signs of a deep response. On its own, a slightly abnormal ratio is common and does not prove myeloma.
Weak kidneys raise both light chains and nudge the ratio without any myeloma.Normal antibody levels
IgG, IgA and IgM totals show whether your healthy antibodies are recovering. Low levels explain frequent infections and may shape infection protection.
Over the months
How are these numbers used through treatment?
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At diagnosis: the baseline
The first M-protein and light chain results are the starting point every later result is compared with. Keep a copy of these reports safe.
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During treatment: before each cycle
Results are usually checked every cycle or two. A steady fall is what your team hopes to see. The size of the fall is described as a partial, very good partial or complete response.
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When the spike disappears
If the M-protein is too small to measure, immunofixation is used to look for any trace. A negative immunofixation, with a marrow check, is needed before a response is called complete.
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Maintenance or watching
Tests move to longer gaps. What matters now is a rising trend across two or more results, not one small change.
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A confirmed rise
A clear, repeated rise can be the first sign that myeloma is returning, often months before symptoms. It prompts a review, not panic.
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Commonly believed
What do families often misread on these reports?
"The number went up a little, so the treatment has failed."
Small ups and downs happen between labs, machines and days. Your team looks for a confirmed rise across repeat tests before calling it a change. Getting tests done at the same lab helps.
"No M-spike means no myeloma."
Light-chain-only myeloma often shows no spike on the standard test. That is why the free light chain test is done alongside it at diagnosis.
"An abnormal ratio means the cancer is back."
A mildly abnormal ratio is common with weak kidneys, infections and other conditions. It is read with the M-protein, the involved light chain level and your symptoms.
"Once immunofixation is negative, we can stop checking."
A negative result is good news, but tests continue. Regular checks are how a return is caught early, while there are still good options.
Being straight with you
What can these numbers not tell you?
They cannot tell you how long treatment will keep working, or what your outlook is. A low M-protein is encouraging, but chromosome results, kidney function and how you tolerate treatment also shape the picture.
Reference ranges differ between laboratories
Each lab prints its own normal range for kappa, lambda and the ratio. A result that is abnormal at one lab can sit inside the range at another. Compare a result only with earlier results from the same lab, and let your haematologist read it with your symptoms and repeat tests.
Some medicines can confuse the test
Certain antibody medicines used in myeloma, such as daratumumab, can show up as a faint band on immunofixation. Your team knows to allow for this, and special tests can tell the two apart if needed.
Who these tests do not suit well
A few people have non-secretory myeloma, which makes almost no measurable protein. For them, marrow tests and scans do the tracking instead.
What to ask at your next visit
Ask which number your team is following for you, what it was at the start, and what it is now. Ask what change would make them act, and when the next test is due. Write the answers beside the reports so the whole family reads the same story.
Questions we are asked
Common questions about M-protein and light chains
My report says "IgG kappa". What does that mean?
It names the abnormal antibody. IgG is the heavy chain type and kappa is the light chain type. It does not tell you how serious the disease is. It tells your team which numbers to follow, and it stays the same through treatment in most people.
Is a raised free light chain always myeloma?
No. Kidney disease, infections, autoimmune conditions and inflammation can raise both kappa and lambda. What points towards a plasma cell problem is one kind rising far more than the other, giving a clearly abnormal ratio. Your haematologist decides whether more tests are needed.
Why is the M-protein falling so slowly?
Whole antibodies last a long time in the blood, so the M-protein can take weeks to fall even when treatment is working well. Free light chains fall faster. A slow but steady fall is usually what is expected. Ask your team how your fall compares with what they hoped for.
Should we get the tests done at the same lab each time?
Yes, where you can. Different labs use different machines and ranges, and a switch can look like a change that is not real. If you must change labs, tell your haematologist and bring the old reports so the trend can be read properly.
What is a 24-hour urine test for?
It collects every drop of urine over a full day and a night to measure light chains passed into the urine, sometimes called Bence Jones protein. It helps judge kidney risk and response in some people. Follow the collection instructions exactly, because a missed sample makes the result unreliable.
Does a zero M-protein mean the myeloma is gone?
It means the protein is too low to measure on that test. Immunofixation, a marrow check and sometimes a very sensitive test for leftover cells are needed to describe how deep the response is. Even then, regular tests continue because myeloma can return.
Can diet or supplements lower these numbers?
No food or supplement is known to lower M-protein or light chains. Some herbal products can harm the kidneys or interact with myeloma medicines. Tell your team about anything you take, and do not stop or change a prescribed medicine on your own.
Who should read these results with us?
Your treating haematologist, with every earlier report laid out by date. A single result read at home late at night often causes needless worry. If you want a second look, bring the baseline reports, the latest ones and the treatment dates so the trend is clear.
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Dr. Basudev Pokhrel reviews blood counts, transfusion needs and blood disorders, and works with the CION tumour board on blood cancers.
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Sources
- American Cancer Society — Tests to Find Multiple Myeloma
- Cancer Research UK — Myeloma
- National Cancer Institute — Plasma Cell Neoplasms (Including Multiple Myeloma) Treatment (PDQ) - Patient Version
- Leukemia & Lymphoma Society — Myeloma
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Reading a Serum Electrophoresis ReportWhere the M-spike number comes fromThe CRAB Criteria: How Myeloma Is DiagnosedWhen a light chain ratio alone means treatmentKidney Failure at Myeloma DiagnosisWhy light chains matter for the kidneysRelapsed Myeloma: Sequencing the Next LineWhat happens when the numbers rise again
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