How Is a Rising BCR::ABL1 in CML Checked for TKI Resistance?

Seeing a rising BCR::ABL1 level on your lab report is frightening, but a single increased result does not automatically mean your leukemia has become resistant to your tyrosine kinase inhibitor (TKI) ^[1]. Doctors evaluate a rising level through a careful, step-by-step process. First, they verify the laboratory context and look at the trend over time ^[2]. Next, they conduct a supportive review of your daily medication routine to check for missed doses or drug interactions ^[3]. Finally, they review your blood counts and, if a clinically important rise is confirmed, they consider specialized testing for biological resistance ^[4][5]. Importantly, whether a rising level is caused by practical adherence barriers or true biological resistance, it is not your fault.

Safety Warning: Never stop, skip, halve, or double your TKI doses based on a lab result without explicit instructions from your care team.

Understanding the Result: Laboratory Variation and Milestones

To understand your result, it helps to know how the test works. Molecular testing uses quantitative PCR (qPCR), which measures the level of BCR::ABL1 RNA transcripts in your blood relative to a control gene ^[6]. Results are reported as a percentage on the International Scale (IS) to allow for standardized comparisons ^[1].

Because qPCR is an incredibly sensitive test, it has inherent technical variability ^[1]. A single minor increase might just be normal lab fluctuation. Instead of panicking over one number, your doctor will interpret the result based on treatment milestones and time on therapy ^[2]. For example:

• At 3 months of treatment, an IS level of ≤10% is an early milestone; confirming a level >10% can indicate treatment failure ^[2].
• Major Molecular Response (MMR) corresponds to a level of ≤0.1% IS. A confirmed rise above 0.1% defines a loss of MMR ^[7][8].

If a result is unexpected, your doctor will usually recommend repeating the test to confirm the trend—ideally using the same standardized laboratory ^[1][6].

The Medication and Adherence Review

If the rise is confirmed, the next step is a nonjudgmental review of how you take your TKI ^[3]. Taking daily oral cancer therapy exactly as prescribed is very difficult, and resistance can occur even with perfect adherence. Missing doses does not mean you caused the rise, but disclosing missed doses is crucial for solving the problem.

Your care team will look at:

• Dose history and barriers: They will ask exactly how many doses were missed, delayed, or reduced. Be honest if you are skipping doses because of severe nausea, high out-of-pocket costs, or difficulty fitting the schedule into your life ^[9][10]. Finding a practical barrier is often easier to address than biological resistance.
• Pharmacy records: Your team may review refill histories to spot gaps caused by insurance delays ^[11][12].
• Side effect management: If side effects are the barrier, your team can prescribe symptom-management medications or consider a dose reduction ^[13].

Checking for Drug Interactions and Dosing Errors

Because TKIs are processed by the liver, other substances can act as inhibitors or inducers of liver enzymes (like CYP3A4), altering the amount of TKI in your blood ^[14]. Your pharmacist or doctor will review:

• Prescription interactions: Certain drugs, like the anti-seizure medication phenytoin, can severely lower the effectiveness of imatinib ^[15].
• Supplements and antacids: Some over-the-counter products alter how TKIs are absorbed. Never stop or start a medication or supplement without consulting your pharmacist or doctor.
• Food requirements: TKIs have different absorption rules. For example, bosutinib must be taken with food ^[14], while the nilotinib capsule requires strict fasting before and after the dose ^[16]. Taking the drug incorrectly can lower the amount of medication in your system.

Checking Blood Counts and Testing for TKI Resistance

Alongside the PCR test, your doctor will check a routine Complete Blood Count (CBC) ^[4]. The CBC evaluates your hematologic response (the normalization of white blood cells and platelets). A normal CBC does not rule out molecular resistance, but a declining CBC can signal toxicity or disease progression ^[4].

If adherence and interactions are ruled out and a loss of response is confirmed, your doctor may order specialized tests:

• BCR::ABL1 Kinase-Domain Mutation Testing: This blood test looks for new genetic mutations inside the leukemia cells that block the TKI from working ^[5][17]. Advanced methods like Next-Generation Sequencing (NGS) can detect low-level mutations, though very small amounts may have uncertain clinical meaning ^[18]. If a known resistance mutation is found, it helps your doctor select a different TKI most likely to work for you ^[19].
• Understanding Negative Results: A negative mutation test does not guarantee the TKI is still working. Kinase-domain mutations account for approximately half of acquired resistance cases, meaning doctors must also consider non-mutational resistance ^[20][5].
• When is a Bone Marrow Biopsy Needed? A bone marrow biopsy is not automatic for every rising PCR result. It is typically reserved for specific situations, such as unexplained drops in blood counts, loss of cytogenetic response, or if your doctor suspects the CML is progressing to an advanced phase ^[21]. The marrow allows them to evaluate the cells physically and perform cytogenetics (looking for new chromosomal abnormalities) ^[4].

When to Contact Your Team Promptly:
While waiting for a repeat PCR, call your hematology team immediately if you develop symptoms such as persistent fever, drenching night sweats, unexplained weight loss, unusual bleeding, extreme fatigue, bone pain, or a new feeling of fullness in your upper left abdomen. These do not prove progression, but they require prompt medical evaluation.