What Does an Undetectable BCR::ABL1 PCR Result Mean?

Receiving an “undetectable” or “not detected” result on your BCR::ABL1 PCR (Polymerase Chain Reaction) test is a major milestone in your treatment for Philadelphia-positive CML. It means the laboratory test did not find any trace of the BCR::ABL1 RNA transcript—the genetic signature of your CML—in the specific blood sample drawn ^[1].

However, it is important to understand that “undetectable” does not mean you are completely cured or that every single leukemia cell is gone ^[2]. PCR tests measure a surrogate marker in a small blood sample; they do not examine every cell in your body. An undetectable result simply means the disease level has dropped below the test’s limit of detection ^[1]. Microscopic amounts of residual CML cells can still safely exist in the body without causing symptoms or showing up on routine tests ^[3][2].

Here is how to interpret this result, how tests measure your response, and what it means for your ongoing treatment.

Understanding Your PCR Report

When you look at your lab report, you may see specific terms used to describe how much, or how little, leukemia RNA was found.

1. The International Scale (IS) Ratios

Your doctor tracks your progress using the International Scale (IS). The percentages you see on your report (like 0.1% or 0.01%) are not the percentage of leukemia cells in your body. They are standardized ratios comparing the amount of the leukemia gene (BCR::ABL1) to a normal control gene in your blood sample.

• MMR (Major Molecular Response): An IS ratio of 0.1% or lower ^[4].
• MR4 (Deep Molecular Response): An IS ratio of 0.01% or lower ^[5].
• MR4.5 (Deeper Molecular Response): An IS ratio of 0.0032% or lower ^[6].

2. Not Detected vs. Detected Below Quantification

As your levels drop, your results may say one of two things:

• Detected but below the limit of quantification: The test found a tiny trace of the BCR::ABL1 transcript, but the amount is too small for the lab to assign an accurate number to it.
• Not detected / Undetectable: The test found absolutely zero BCR::ABL1 transcripts in that specific sample ^[1].

3. Control-Gene Transcripts (Sample Adequacy)

To trust an “undetectable” result, the laboratory must prove the blood sample had enough high-quality RNA to run a highly sensitive test. They do this by counting control-gene transcripts (often a normal gene called ABL1) ^[7].

An “undetectable” result only officially qualifies as a Deep Molecular Response (DMR) if the lab found enough control genes to confirm the test’s sensitivity. For example, standard guidelines require an assay to count at least 10,000 control-gene transcripts to confirm MR4, and at least 32,000 to confirm MR4.5 ^[7][8]. If your sample did not have enough control genes, an “undetectable” result means the test could not find the leukemia gene, but the sample was not sufficient to formally categorize your response depth.

Why Consistent Lab Testing Helps

Not all PCR tests are identical. Different laboratories use slightly different equipment, methods, and conversion factors to report results on the International Scale ^[9][10]. Because of this, a sample that reads as “undetectable” at one laboratory might show a tiny, quantifiable amount at another lab ^[11][12].

To get the most accurate picture of your progress over time, it is highly recommended to have your blood tested at the same laboratory ^[13]. If you must change laboratories, do not panic if your result fluctuates slightly; your doctor will evaluate the trend while accounting for the new lab’s specific methods and conversion factors.

Does This Mean I Can Stop Treatment?

Because an undetectable result does not mean the leukemia is 100% eradicated, you should never stop or change your Tyrosine Kinase Inhibitor (TKI) medication on your own ^[2]. If the medication is stopped prematurely or without supervision, the residual CML cells can multiply, leading to a loss of your molecular response ^[14].

However, patients who maintain a very deep, stable molecular response for a long period may eventually become candidates for Treatment-Free Remission (TFR) ^[15]. TFR is a clinical strategy where a patient stops their TKI under strict medical observation.

Eligibility recommendations vary by clinical guideline and your specific medical history, but common examples of TFR criteria include:

• Being in the chronic phase of CML with no history of accelerated or blast phase disease ^[15].
• Having taken TKI therapy for a minimum amount of time (often 3 to 5 years, depending on the guideline) ^[16].
• Maintaining a confirmed, stable Deep Molecular Response (such as MR4 or MR4.5) for at least 2 years ^[15][16].
• Having reliable access to highly sensitive PCR tests with quick turnaround times ^[15].

If TFR is attempted: Monitoring continues indefinitely. At first, blood tests are required very frequently (e.g., monthly). A “molecular relapse” is generally defined as losing your Major Molecular Response (an IS ratio rising above 0.1%) ^[5][14]. If this happens, your doctor will promptly restart your TKI therapy. Research shows that the vast majority of patients who need to restart therapy successfully regain their molecular response ^[17][18][19].

If you are interested in TFR, or if you are experiencing side effects from your medication, discuss it with your hematologist. Until a supervised plan is in place, continue taking your TKI exactly as prescribed to maintain your excellent response.