Understanding CML
What BCR-ABL, MMR, neutrophils and the rest of your results actually mean, explained by someone living with CML.
Darren, who built Onward · 7 min readWhen you are diagnosed with chronic myeloid leukaemia, nobody hands you a glossary. You come out of an appointment holding a printout covered in numbers and abbreviations, and the one thing you want is for somebody to tell you what they mean.
This page is that. It covers the numbers that turn up on a CML blood test, what each one is, and why your team keeps an eye on it.
I am not a clinician. I have CML myself, and I wrote this because I wanted it when I was starting out. It is general information rather than advice, and your care team knows your situation in a way a web page never can.
Last updated 22 September 2026
Carries oxygen around your body
Haemoglobin is the protein inside your red blood cells that carries oxygen from your lungs to the rest of your body. It is usually reported in grams per litre, so a result looks something like 136 g/L.
If it drops low, which is called anaemia, you can feel tired, breathless on stairs you used to manage fine, or look pale. At diagnosis CML can push it down because the bone marrow is busy making leukaemia cells instead of healthy ones. Treatment can lower it too. For most people it recovers as things settle, and your team watches the direction of travel rather than any single figure.
Your body's infection fighters
White blood cells are the part of your immune system that fights infection. The count is reported in billions per litre, written as ×10⁹/L.
CML often pushes this count very high at diagnosis, sometimes many times what it should be. For a lot of people that is exactly how CML gets found, through a routine blood test taken for something else entirely. It usually settles once you start treatment. At the other end, if it falls too low, infections can take hold more easily, so it is watched closely in both directions.
Your first line of defence
Neutrophils are the most common type of white blood cell and your body's first responders against infection. They are counted separately from the overall white cell count, because they matter in their own right.
TKI treatment can lower them, which is called neutropenia. If they drop too far, your team may pause your treatment or adjust the dose to let them recover.
That can feel alarming the first time it happens, as though something has gone wrong. It usually has not. A short stop to let your counts come back up is often the safest thing to do, and it is a normal part of finding the dose that suits you.
Help your blood to clot
Platelets are tiny cells that clump together to help your blood clot and stop bleeding. Like white cells, they are counted in ×10⁹/L.
CML can raise them at diagnosis, and treatment can lower them, which is called thrombocytopenia. If they go too low you might notice you bruise more easily, or that a nosebleed or a cut takes longer to stop. Worth mentioning to your team if you spot it.
A liver enzyme
ALT, or alanine aminotransferase, is an enzyme found mostly in your liver. It appears on your results in units per litre, U/L.
A rise can be a sign the liver is under some strain, sometimes from medication. It is checked so your team can keep your liver healthy while you are on treatment. Mild rises are common in the early months and often settle on their own.
A waste product the liver clears
Bilirubin is a yellow substance made when old red blood cells break down. A healthy liver clears it away. It is measured in micromoles per litre, µmol/L.
Higher levels can point to how the liver is coping, or simply to faster red-cell turnover. With some TKIs a mild rise is expected and means very little on its own. It is usually just kept an eye on.
A check on your kidneys
Creatinine is a waste product from your muscles that your kidneys filter out of your blood. Also reported in µmol/L.
It is a simple check on how well your kidneys are working while you are on treatment. It can shift a little with how hydrated you are and how much muscle you carry, which is another reason single readings matter less than the pattern.
The main measure of your response
BCR-ABL is the abnormal gene that drives CML. It forms when two chromosomes swap a piece of material between them, producing a protein that tells blood cells to keep multiplying when they should stop.
The blood test measures how much of it is present, shown as a percentage on something called the International Scale, or IS. The scale exists so that a result from one laboratory means the same thing as a result from another.
It is the key measure of how well treatment is controlling the leukaemia. Lower is better, and a trend heading downwards over time is the goal.
BCR-ABL is usually shown on a logarithmic scale, which is worth understanding because it makes the numbers behave in a way that feels wrong at first.
On a log scale, each step is a tenfold drop. Going from 10% to 1% is the same size of step as going from 1% to 0.1%, or from 0.1% to 0.01%. So progress that looks like it is slowing down on an ordinary chart is often nothing of the sort.
It also means small movements are normal. The test has a degree of built-in variability, and the same sample can give slightly different numbers. This is precisely why your team looks at the pattern across several results rather than reacting to any one of them. If something does concern them, they will usually repeat the test before anything else happens.
How deep your response is
MR is a shorthand for how far your BCR-ABL has fallen. Rather than saying "0.032%", people say "MR4.5". Each number is a tenfold step below the one before.
| Milestone | BCR-ABL (IS) |
|---|---|
| MR2 | 1% |
| MMR, also called MR3 | 0.1% |
| MR4 | 0.01% |
| MR4.5 | 0.0032% |
| MR5 | 0.001% |
Reaching and holding these milestones shows treatment is working well. MMR is the one you will hear talked about most often. Deeper, sustained responses can open up options your team may talk through with you in time.
Your CML tablet
A TKI, or tyrosine kinase inhibitor, is the type of targeted tablet used to treat CML. Imatinib, dasatinib, nilotinib and bosutinib are the ones you are most likely to come across, and there are others.
It blocks the faulty BCR-ABL protein so the leukaemia cannot grow. Taking it consistently is what keeps CML controlled, which is why keeping track of your doses is worth the small effort.
If you are struggling with side effects, that is worth raising rather than quietly putting up with. There is usually more than one option, and switching is common.
Staying well after stopping treatment
TFR means stopping your TKI while staying in a deep, sustained molecular response, with closer monitoring than usual. It is only ever done with your care team, and there are published criteria they work to.
For some people with a long, deep response, a carefully monitored stop can be possible. Many who stop stay in remission. If CML does return, it almost always responds again on restarting treatment, which is why the monitoring is close.
This is not something to act on from a web page, or from an app. It is a conversation to have with the people who know your history.
You will often see a "normal range" printed next to your results. Those ranges are not universal. They vary between laboratories, and they vary by age and sex. A result flagged as slightly outside the range at one hospital might sit comfortably inside it at another.
It is also why a single figure just outside a range is rarely the emergency it can feel like at eleven at night. Your team is looking at your numbers, your history and your treatment together.
For medically reviewed information about CML, including diagnosis, treatment and support, these are the UK organisations worth knowing:
Onward holds all of this in one calm, private place, with these explanations a tap away from every result. Free, and your data never leaves your device.
General information to help you understand your results, not medical advice. Your care team knows your situation best. Never change your treatment based on anything you read here.