Skip to content
How It Works
Explore
Ask a Question Resources Pricing
Who It's For
Patients
Decode results, prep for appointments, weigh options.
Caregivers & advocates
Carry the research load for someone you love.
Providers & clinicians
Resources for patients in your practice.
Log In Ask a Question
PubMed This is a summary of 17 peer-reviewed journal articles Updated
Hematology

What Do MR4.0 and MR4.5 Mean on My CML Lab Report?

At a Glance

On a CML lab report, MR4.0 means BCR::ABL1 is 0.01% or lower on the International Scale, while MR4.5 means 0.0032% or lower. Both show a deep molecular response, but only sustained results may support a carefully monitored treatment-free remission attempt.

Seeing “MR4.0” or “MR4.5” on your chronic myeloid leukemia (CML) lab report is an encouraging sign. These numbers indicate you have achieved a Deep Molecular Response (DMR) [1][2].

To understand these numbers, it helps to know what the test is measuring. The PCR test does not directly count leukemia cells. Instead, it measures the level of the BCR::ABL1 fusion transcript—a molecular signal produced by the leukemia cells—in your blood [3][4].

The Numbers and the International Scale (IS)

When you see percentages like 0.01% or 0.0032%, these are reported on the International Scale (IS) [1]. The IS compares your current BCR::ABL1 level to a universal standardized baseline (defined as 100%), not your personal level at diagnosis [3].

The “MR” stands for Molecular Response, and the numbers represent “log reductions” (factors of 10) from that universal baseline:

Response Level IS Percentage What it Means (Compared to Baseline)
MMR (Major Molecular Response) ≤ 0.1% 3-log reduction (1,000 times lower) [1][5]
MR4.0 ≤ 0.01% 4-log reduction (10,000 times lower) [1][6]
MR4.5 ≤ 0.0032% 4.5-log reduction (about 32,000 times lower) [1][5]

Note: Sometimes a lab report might say “undetectable.” This simply means the BCR::ABL1 level is lower than what that specific laboratory’s test can detect; it does not necessarily mean there is zero leukemia in your body [7][4]. Furthermore, small fluctuations between MR4.0 and MR4.5 are normal and can reflect variations in the test itself rather than a change in your health [8]. Doctors look at the trend over time rather than a single number.

The Path to Treatment-Free Remission (TFR)

Achieving a sustained DMR is significant because it is a prerequisite for attempting Treatment-Free Remission (TFR) [9][10]. TFR is a supervised trial where carefully selected patients stop taking their daily targeted therapy (TKI) pills [10].

TFR is not a cure or a guaranteed success. Studies show that roughly half of patients (about 50-55%) who attempt TFR remain in remission long-term [11][12]. The others experience a molecular relapse—usually within the first 6 months—and need to restart their medication [13]. Fortunately, when treatment is restarted promptly, patients almost always regain their deep response [13][14].

Eligibility for a TFR attempt generally requires:

  • Sustained Response: Maintaining MR4.0 or MR4.5 for a continuous period (often 2 or more years) [10][15].
  • Time on Therapy: Being on TKI treatment for a minimum duration (often 3 to 5+ years) [10][15].
  • Disease History: Having CML in the first chronic phase, with no history of the disease progressing to accelerated or blast phases [10].
  • Test Compatibility: Having a typical BCR::ABL1 transcript type that can be reliably measured by a high-quality PCR lab [10][16].

(Note: These criteria are a general overview; specific guidelines vary among clinics and countries.)

What Monitoring Looks Like During TFR

If you and your doctor decide to attempt TFR, you will need frequent monitoring. A typical schedule requires PCR blood tests every month for the first 6 months, every 2-3 months for the rest of the first year, and every 3 months indefinitely after that [10][17].

During this time, doctors watch for a loss of Major Molecular Response (if the level rises above 0.1% IS) [8][13]. If this happens, treatment is generally restarted [14]. It is also important to note that some patients experience a TKI withdrawal syndrome after stopping their pills, which can cause new musculoskeletal or joint pain that should be reported to your doctor [12].

Never stop taking your medication without your hematologist’s direct supervision. If you have reached MR4.0 or MR4.5, celebrate this excellent milestone and discuss with your doctor what it means for your long-term treatment plan.

Common questions in this guide

What does MR4.0 mean on a CML blood test?
MR4.0 means the BCR::ABL1 level is 0.01% or lower on the International Scale. That represents a 4-log reduction, or about 10,000 times lower than the standardized baseline, and is considered a deep molecular response. Your clinician will interpret it alongside your earlier results and the overall trend.
Is MR4.5 better than MR4.0 in CML?
Yes. MR4.5 corresponds to a BCR::ABL1 level of 0.0032% or lower on the International Scale, which is a deeper measured response than MR4.0 at 0.01% or lower. Both are deep molecular responses, and small fluctuations between them can reflect test variation rather than a change in your health.
What does the International Scale mean on a CML PCR report?
The International Scale is a standardized way to report BCR::ABL1 results so that tests from different laboratories can be compared. Its 100% reference is a universal baseline, not necessarily your personal leukemia level when you were diagnosed.
Does an undetectable CML PCR result mean the leukemia is gone?
An undetectable result means the BCR::ABL1 signal was below that laboratory's detection limit. It does not prove that no leukemia cells remain, so regular PCR monitoring is still important.
Can I stop my CML medicine after reaching MR4.0 or MR4.5?
A sustained deep molecular response can be a prerequisite for considering treatment-free remission, but reaching MR4.0 or MR4.5 once does not mean it is safe to stop treatment. Eligibility also depends on factors such as time on TKI therapy, disease history, response duration, and reliable testing, and any attempt must be supervised by a hematologist.
How often are PCR tests needed during treatment-free remission?
A common monitoring schedule is a PCR blood test every month for the first six months, every two to three months for the rest of the first year, and every three months indefinitely afterward. Your hematologist will set the exact schedule. A rise above 0.1% on the International Scale, indicating loss of major molecular response, generally leads to restarting treatment.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Is my current PCR result explicitly measured on the International Scale (IS), and what is the sensitivity limit of the lab's test?
  2. 2.Looking at my past several results, have I maintained a stable deep molecular response (MR4.0 or MR4.5)?
  3. 3.Given my specific disease history and transcript type, am I a candidate for a Treatment-Free Remission (TFR) attempt in the future?
  4. 4.If we attempt TFR, what specific BCR::ABL1 level (e.g., loss of Major Molecular Response) would trigger the decision to restart my medication?
  5. 5.What is the exact blood testing schedule I would need to commit to if I stop taking my TKI?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

Can You Get Pregnant or Father a Child While on CML TKIs?Did the WHO Remove CML's Accelerated Phase in 2022?How Is the ELTS Risk Score Calculated for CML at Diagnosis?How Often Are PCR Tests Needed After Stopping a CML TKI?Is a Bone Marrow Transplant Needed for CML Treatment?Is Asciminib (Scemblix) FDA-Approved for First-Line CML?What Do BCR-ABL1 Fluctuations Mean During CML Treatment?What Happens If You Miss a TKI Dose for CML? Next StepsHow Is CML Different From Acute Leukemias? Key FactsWhat Is the T315I Mutation in CML and How Is It Treated?What Is TKI Withdrawal Syndrome in CML? Key Facts to Know

References

References (17)
  1. 1

    Analytical performance evaluation and method comparison of the 1copy BCR::ABL1 qPCR assay for monitoring chronic myeloid leukemia.

    Kim H, Kim DY, Kim H

    Practical laboratory medicine 2026; (50()):e00538 doi:10.1016/j.plabm.2026.e00538.

    PMID: 42179678
  2. 2

    Long-term treatment-free remission of chronic myeloid leukemia with falling levels of residual leukemic cells.

    Ross DM, Pagani IS, Shanmuganathan N, et al.

    Leukemia 2018; (32(12)):2572-2579 doi:10.1038/s41375-018-0264-0.

    PMID: 30315232
  3. 3

    Analytical Validation of a Highly Sensitive, Multiplexed Chronic Myeloid Leukemia Monitoring System Targeting BCR-ABL1 RNA.

    Brown JT, Beldorth IJ, Laosinchai-Wolf W, et al.

    The Journal of molecular diagnostics : JMD 2019; (21(4)):718-733 doi:10.1016/j.jmoldx.2019.03.002.

    PMID: 31026597
  4. 4

    Standardization of molecular monitoring of CML: results and recommendations from the European treatment and outcome study.

    White HE, Salmon M, Albano F, et al.

    Leukemia 2022; (36(7)):1834-1842 doi:10.1038/s41375-022-01607-z.

    PMID: 35614319
  5. 5

    Nilotinib vs. imatinib in Japanese patients with newly diagnosed chronic myeloid leukemia in chronic phase: long-term follow-up of the Japanese subgroup of the randomized ENESTnd trial.

    Nakamae H, Fukuda T, Nakaseko C, et al.

    International journal of hematology 2018; (107(3)):327-336 doi:10.1007/s12185-017-2353-7.

    PMID: 29076005
  6. 6

    Impact of additional genetic abnormalities at diagnosis of chronic myeloid leukemia for first-line imatinib-treated patients receiving proactive treatment intervention.

    Shanmuganathan N, Wadham C, Shahrin N, et al.

    Haematologica 2023; (108(9)):2380-2395 doi:10.3324/haematol.2022.282184.

    PMID: 36951160
  7. 7

    Rates of deep molecular response by digital and conventional PCR with frontline nilotinib in newly diagnosed chronic myeloid leukemia: a landmark analysis.

    Berdeja JG, Heinrich MC, Dakhil SR, et al.

    Leukemia & lymphoma 2019; (60(10)):2384-2393 doi:10.1080/10428194.2019.1590569.

    PMID: 30912699
  8. 8

    Molecular Monitoring of Chronic Myeloid Leukemia.

    Dominy K, Mokretar K, Reid AG, Khorashad JS

    Methods in molecular biology (Clifton, N.J.) 2020; (2065()):153-173 doi:10.1007/978-1-4939-9833-3_12.

    PMID: 31578694
  9. 9

    Durable treatment-free remission in patients with chronic myeloid leukemia in chronic phase following frontline nilotinib: 96-week update of the ENESTfreedom study.

    Ross DM, Masszi T, Gómez Casares MT, et al.

    Journal of cancer research and clinical oncology 2018; (144(5)):945-954 doi:10.1007/s00432-018-2604-x.

    PMID: 29468438
  10. 10

    Treatment-free remission in patients with chronic myeloid leukemia: recommendations of the LALNET expert panel.

    Pavlovsky C, Abello Polo V, Pagnano K, et al.

    Blood advances 2021; (5(23)):4855-4863 doi:10.1182/bloodadvances.2020003235.

    PMID: 34438444
  11. 11

    Effect of study-level factors on treatment-free remission rate in patients with chronic myeloid leukemia: a systematic review and meta-analysis.

    Kim J, Park J, Moon Y, et al.

    International journal of hematology 2019; (110(6)):683-689 doi:10.1007/s12185-019-02744-5.

    PMID: 31560117
  12. 12

    Treatment-free remission following frontline nilotinib in patients with chronic phase chronic myeloid leukemia: 5-year update of the ENESTfreedom trial.

    Radich JP, Hochhaus A, Masszi T, et al.

    Leukemia 2021; (35(5)):1344-1355 doi:10.1038/s41375-021-01205-5.

    PMID: 33707652
  13. 13

    Discontinuation of Tyrosine Kinase Inhibitors in Chronic Myeloid Leukemia With Losing Major Molecular Response as a Definition for Molecular Relapse: A Systematic Review and Meta-Analysis.

    Chen KK, Du TF, Xiong PS, et al.

    Frontiers in oncology 2019; (9()):372 doi:10.3389/fonc.2019.00372.

    PMID: 31139566
  14. 14

    Treatment-Free Remission After Second-Line Nilotinib Treatment in Patients With Chronic Myeloid Leukemia in Chronic Phase: Results From a Single-Group, Phase 2, Open-Label Study.

    Mahon FX, Boquimpani C, Kim DW, et al.

    Annals of internal medicine 2018; (168(7)):461-470 doi:10.7326/M17-1094.

    PMID: 29459949
  15. 15

    Discontinuation of tyrosine kinase inhibitors in chronic myeloid leukemia: when and for whom?

    Atallah E, Schiffer CA

    Haematologica 2020; (105(12)):2738-2745 doi:10.3324/haematol.2019.242891.

    PMID: 33054106
  16. 16

    When to Stop Tyrosine Kinase Inhibitors for the Treatment of Chronic Myeloid Leukemia.

    Laneuville P

    Current treatment options in oncology 2018; (19(3)):15 doi:10.1007/s11864-018-0532-2.

    PMID: 29520446
  17. 17

    Treatment-free remission in patients with chronic myeloid leukemia following the discontinuation of tyrosine kinase inhibitors.

    Haddad FG, Sasaki K, Issa GC, et al.

    American journal of hematology 2022; (97(7)):856-864 doi:10.1002/ajh.26550.

    PMID: 35357036

This page is for informational purposes only and does not constitute medical advice. Do not stop CML medication or consider treatment-free remission without your hematologist's direct supervision.

Get notified when new evidence is published on Chronic myeloid leukemia.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.