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The CRAB criteria: how myeloma is diagnosed | CION Cancer Clinics
CRAB stands for high Calcium, Renal (kidney) trouble, a low haemoglobin and Bone damage. If a marrow test proves abnormal plasma cells and even one of these signs is caused by them, doctors call it active myeloma and start treatment. Without any CRAB sign, the result is usually MGUS or smouldering myeloma, which are watched. This page explains each letter, the thresholds, and what a single result cannot tell you. 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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On this page
- What are the CRAB criteria in myeloma?
- What does each letter of CRAB look for?
- Can myeloma be treated before any damage shows?
- Which tests will you be asked to do?
- If you do not meet CRAB, what is it called instead?
- What do people often misread about CRAB?
- Which words on the report go with CRAB?
- Common questions about the CRAB criteria
The short answer
What are the CRAB criteria in myeloma?
CRAB is a checklist of four kinds of damage that myeloma causes: high Calcium, Renal (kidney) trouble, a low haemoglobin, and Bone damage. If abnormal plasma cells are present and even one of these is caused by them, the disease counts as active myeloma that needs treatment.
Why doctors use a checklist at all
Many people carry a small abnormal protein in the blood for years without harm. Treating all of them would expose healthy people to strong medicines for nothing. The CRAB checklist separates the people whose plasma cells are already hurting the body from those who can safely be watched.
What has to be true before CRAB is even checked
First, your haematologist needs proof of the abnormal plasma cells. That usually means a bone marrow test showing that at least 10% of the marrow cells are clonal plasma cells, meaning copies of one abnormal cell. A biopsy of a single tumour of plasma cells, called a plasmacytoma, can do the same job.
The damage has to come from myeloma
A low haemoglobin from iron shortage, or kidney trouble from long-standing diabetes, does not count. Your team has to be satisfied that the plasma cells are the cause. That judgement is why the checklist is read by a specialist and not by a report alone.
Letter by letter
What does each letter of CRAB look for?
These are the thresholds in the international criteria. Reference ranges differ between laboratories, so your team reads each number against your own lab's range and your symptoms.
C · Calcium
Blood calcium above 11 (in mg/dL), or clearly above the lab's upper limit. Myeloma dissolves bone and releases calcium into the blood.
You may notice
- Great thirst and passing lots of urine
- Constipation and vomiting
- Confusion or unusual sleepiness
R · Renal (kidney)
Serum creatinine above 2 (in mg/dL), or a creatinine clearance (how well the kidneys filter) below 40 (in mL/min), caused by the myeloma.
You may notice
- Less urine, or frothy urine
- Swollen feet and tiredness
A · A low haemoglobin
Haemoglobin below 10 (in g/dL), or more than 2 points below the lower limit of normal, with no other cause found.
You may notice
- Tiredness that rest does not fix
- Breathlessness on stairs
B · Bone
One or more lytic lesions, meaning holes in the bone, seen on a low-dose CT, PET-CT or X-ray. A fracture from very little force also raises the question.
Thinning bones alone, without a hole, do not count.Not sure whether this applies to you?
Ask an oncologistBeyond CRAB
Can myeloma be treated before any damage shows?
Yes. Three more markers, known as SLiM, mean damage is very likely soon, so treatment starts even without a CRAB sign.
Marker on your report
What it means
S · Sixty: 60% or more plasma cells in the marrow
The marrow is heavily taken over, even if counts look fine today
Li · Light chains: an involved-to-uninvolved ratio of 100 or more
One light chain is far higher than the other, and the kidneys are at risk
M · MRI: more than one focal spot in the bones
Areas of myeloma in the marrow that have not yet made a hole
How each letter is checked
Which tests will you be asked to do?
Blood tests
A complete blood count for haemoglobin, plus calcium, albumin, creatinine and electrolytes. These cover three of the four letters and are usually repeated to confirm a trend.
Protein tests
Serum protein electrophoresis, immunofixation and a free light chain test show the abnormal protein, its type and the light chain ratio. A urine protein test may be added.
Bone marrow test
A sample from the hip bone, taken under local anaesthetic, shows the share of plasma cells. The cells also go for chromosome tests that guide the plan.
Whole-body imaging
A low-dose CT or PET-CT looks for holes in the bone. An MRI may be added when the CT is clear but suspicion stays high.
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Where you stand
If you do not meet CRAB, what is it called instead?
Without CRAB or SLiM signs, the result usually falls into one of two watched conditions. Neither needs myeloma treatment today, but both need regular blood tests.
MGUS
The name is long, but the idea is simple: a small abnormal protein with no harm done. The protein is below 3 (in g/dL) and marrow plasma cells are under 10%. Most people with MGUS never develop myeloma. Check-ups are spaced out once the level is steady.
Smouldering myeloma
The protein is 3 (in g/dL) or more, or marrow plasma cells are between 10% and 60%, but there is still no damage. The chance of moving to active myeloma is higher than with MGUS, so tests are closer together. Some people at higher risk are offered a clinical trial.
What this page cannot tell you
It cannot tell you which group you are in. A single result read alone can mislead, because calcium, creatinine and haemoglobin move with dehydration, infection and other illnesses. Your haematologist puts the marrow result, the scans and repeat tests together before deciding.
Commonly believed
What do people often misread about CRAB?
"His calcium is normal, so it cannot be myeloma."
Only one CRAB letter is needed. Many people with active myeloma have a normal calcium but a low haemoglobin or holes in the bone. A normal result on one letter does not clear the other three.
"The report shows an M-spike, so she has cancer."
An M-spike on its own may be MGUS, which is not cancer and often never becomes it. The marrow test and the CRAB check decide what the spike means.
"An X-ray was clear, so the bones are fine."
Plain X-rays miss a lot of early bone damage. A low-dose CT or PET-CT sees far more, which is why the criteria prefer them.
"Smouldering myeloma is being ignored."
Watching is an active plan with regular tests and a clear trigger to start treatment. Treating too early brings side effects without a proven gain for most people.
On your report
Which words on the report go with CRAB?
- Clonal plasma cells
- Plasma cells that are all copies of one abnormal cell, shown on the marrow report.
- Myeloma-defining event
- Any CRAB or SLiM sign. One is enough to call the myeloma active.
- Creatinine clearance or eGFR
- An estimate of how well the kidneys filter. Lower means weaker.
- Lytic lesion
- A punched-out hole in the bone where myeloma has dissolved it.
- Corrected calcium
- Calcium adjusted for your albumin level, which gives a truer reading.
Questions we are asked
Common questions about the CRAB criteria
Do all four CRAB signs have to be present?
No. One sign is enough, as long as the abnormal plasma cells are proven and your team believes they are causing it. Some people have only bone damage. Others have only kidney trouble. The number of letters you meet does not decide how serious your myeloma is.
My father's haemoglobin is low. Does that mean myeloma?
Not on its own. A low haemoglobin has many common causes, including iron or vitamin shortage, kidney disease and long-term illness. It becomes a CRAB sign only when myeloma is proven and no other cause explains it. Your doctor will usually check iron, B12 and kidney function alongside the protein tests.
Why is a bone marrow test needed if the blood tests are abnormal?
Blood tests show the protein, but not how many abnormal plasma cells sit in the marrow. That share helps separate MGUS, smouldering and active myeloma. The marrow cells are also tested for chromosome changes that shape the risk group and the treatment plan.
Can a person meet CRAB and still not have myeloma?
Yes. High calcium, kidney trouble, low haemoglobin and bone holes all have other causes, such as other cancers, thyroid problems or diabetes. That is why the criteria need proof of clonal plasma cells and a link between them and the damage.
How often are CRAB tests repeated if we are only watching?
It depends on your group and how steady the results are. Smouldering myeloma is usually checked more often than MGUS, and the gap widens if nothing changes. Your haematologist will give you a schedule. Do not wait for it if new bone pain, thirst or tiredness appears.
Is high calcium an emergency?
It can be. Severe thirst, confusion, vomiting or drowsiness in someone with myeloma or a suspected diagnosis needs the nearest emergency department the same day, or a call to 108. High calcium is treated with fluids through a drip and medicines given in hospital.
Do CRAB results decide my stage?
No. CRAB decides whether myeloma needs treatment. Staging uses different tests: beta-2 microglobulin, albumin, LDH and the chromosome results from the marrow. Two people who both meet CRAB can be at very different stages, so ask about both separately.
What should we bring for a second look at the diagnosis?
Bring the marrow report, every protein test, blood counts, calcium and creatinine results with dates, and the scan discs as well as the printed reports. A list of current medicines helps too. The dates matter, because the trend often says more than one number.
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Sources
- National Cancer Institute — Plasma Cell Neoplasms (Including Multiple Myeloma) Treatment (PDQ) - Health Professional Version
- American Cancer Society — Tests to Find Multiple Myeloma
- Cancer Research UK — Myeloma
- NICE — Myeloma: diagnosis and management (NG35)
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
Myeloma Explained: What It Does to Bones, Kidneys and BloodStart here if myeloma is new to youReading a Serum Electrophoresis ReportWhere the M-spike on your report comes fromHypercalcaemia in Myeloma: An EmergencyWhen the C in CRAB needs same-day careKidney Failure at Myeloma DiagnosisWhat happens when the R in CRAB is presentMyeloma Staging: ISS and R-ISSStaging uses different tests from CRAB
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Not sure whether your report meets CRAB?
Tell us what has been found so far. CION's haematology team will read the results with you and help you reach the right specialist. One helpline serves every CION centre.