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Diagnosis and tests

PCR

The related article (“Molecular tests: PCR and NGS”) explains part of the blood count. This topic looks closely at the polymerase chain reaction (PCR): how it works and why it matters so much in leukaemia.

Last updated: ~1 min read
Branching copies of DNA.
Figure 1.Illustration: Copies double

01Short answer

PCR is a method that copies a very small piece of DNA millions of times. The sample is heated and cooled in turns: heating separates the two DNA strands; on cooling, short pieces called “primers” attach to the target, and an enzyme builds a new strand. Each cycle doubles the number of copies. In leukaemia, PCR has two main jobs: (1) to show whether a specific change is present, such as the BCR::ABL1 or PML::RARA fusion, or an NPM1 or FLT3 mutation; and (2) quantitative PCR (qPCR) measures how much of it there is. qPCR is very sensitive and can find one leukaemia cell among 100,000 normal cells. That is why it is the key method in treating CML and measuring MRD. For fusion genes, a DNA copy is first made from RNA (RT-PCR).

02Simple explanation

Copying: millions of times.

Present or not, and how much.

Very sensitive: used for MRD.

03Scientific explanation

International Scale (IS): in CML, BCR::ABL1 results are given as an IS percentage so labs can be compared.

RT-PCR: fusion genes are measured at RNA level, so the sample must be processed quickly.

04What happens in the body?

Doubling

Each cycle doubles the copies of the target DNA.

A thermal cycler

The machine heats and cools the sample precisely.

A row of tubes in a machine.
Figure 2.PCR tubes in a thermal cycler.

A growth curve

The more target there is, the earlier the curve rises.

An S-shaped curve on a screen.
Figure 3.A PCR amplification curve.

05A PCR cycle

Four steps.

Template — A DNA sample.1TemplateA DNA sample.
Heating — Strands separate.2HeatingStrands separate.
Primers — Bind to the target.3PrimersBind to the target.
Copies — Doubling.4CopiesDoubling.

06How PCR is used in leukaemia

PurposeExample
DiagnosisPML::RARA (APL), BCR::ABL1
MutationNPM1, FLT3 (AML)
AmountBCR::ABL1 IS (CML)
MRDA fusion or a patient-specific target
Detecting relapseThe amount rising again

07Key terms

PCR
Polymerase chain reaction.
Primer
A short piece of DNA that recognises the target.
qPCR
Quantitative PCR.
RT-PCR
PCR starting from RNA.
IS
International Scale.

08Key points

  • PCR copies DNA.
  • It shows whether a change is present.
  • qPCR measures how much.
  • It is very sensitive.
  • It is central in CML and MRD.
  • Follow the trend with your doctor.

09Frequently asked questions

Does “not detected” mean the leukaemia is completely gone?

No; it means the level is below what the test can detect. Your doctor will explain what it means.

Is a small rise worrying?

The trend matters more than one result; your doctor will decide.

Why must the sample arrive quickly?

RNA breaks down quickly.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Does “not detected” mean the leukaemia is completely gone?

  2. Is a small rise worrying?

  3. Why must the sample arrive quickly?

My questions

12Sources

  1. National Cancer Institute (NCI). Definition: polymerase chain reaction
  2. MedlinePlus Genetics. What is a gene variant and how do variants occur?
  3. National Cancer Institute (NCI). CML treatment (PDQ), patient version
  4. American Cancer Society. Treatment of Acute Promyelocytic Leukemia (APL)
  5. Cleveland Clinic. Differentiation Syndrome: Symptoms, Causes & Treatment
  6. National Cancer Institute (NCI). Chronic Myeloid Leukemia Treatment (PDQ®) — Health Professional Version
  7. World Health Organization. Childhood cancer — Fact sheet
  8. National Cancer Institute (NCI). Cancer in Children and Adolescents — Fact Sheet
  9. Blood (American Society of Hematology), 2021. 2021 Update on MRD in acute myeloid leukemia: a consensus document from the European LeukemiaNet MRD Working Party
  10. Blood (American Society of Hematology), 2022. International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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