5-Fluorouracil Sensitivity


Indications for Testing

  • Predict toxicity and responsiveness of tumor to 5-fluorouracil (5-FU) therapy

Laboratory Testing

  • 5-FU response testing
    • DPYD and TYMS mutations detected that may increase risk for 5-FU toxicity and predict responsiveness to 5-FU
      • Alternative chemotherapeutic agents, therapeutic drug monitoring, altered 5-FU doses, or increased surveillance for adverse drug reactions may be indicated

Clinical Background

5-fluorouracil (5-FU) is a fluoropyrimidine drug and the most frequently used chemotherapeutic in the treatment of colorectal cancer. Genetic mutations in the DPYD and TYMS genes may predict the risk of 5-FU toxicity, responsiveness to 5-FU therapy, and clinical outcome to aid in patient care.


  • Prevalence




    c.1679T>G (also known as DPYD*13, rs55886062)

    1% – French Caucasians

    c.1905+1G>A (also known as DPYD*2A, IVS14+1G>A, rs3918290)

    0.47-2.2% – Dutch, German, French, Turkish, Finnish

    Absent – Japanese, Korean, African-American

    c.2846A>T (also known as rs67376798)

    0% – French Caucasians


    3'-UTR 6 bp deletion (TTAAAG); also known as 1494 del; rs16430


    5% –  Caucasian




    5'-TSER 28bp VNTR (2R; 3R); also known as rs45445694

    • 5’-TSER 28bp VNTR (2R; 3R); also known as rs45445695
    • G>C SNP in 2nd repeat of 3R allele (3RC); also known as rs34743033


    41-48% – Caucasian, Hispanic, African-American

    19% – Singapore, Chinese

    5% – Japanese


    51% – Singapore, Chinese

    42.7% – Japanese

    26-37% – Caucasian, Hispanic, African-American


    39.9% – Japanese

    30% – Singapore, Chinese

    15-33% – Caucasian, Hispanic, African-American


  • Three DPYD (c.1679T>G, c.1905+1G>A, c.2846A>T) single nucleotide polymorphisms (SNPs) are associated with reduced enzymatic function and an increased risk of grade III-IV 5-FU toxicity
    • Homozygous or compound DPYD heterozygous mutations
      • Associated with dihydropyrimidine dehydrogenase (DPD) deficiency
      • Avoidance of fluoropyrimidine therapy is recommended – alternate drug should be selected
    • Heterozygous DPYD mutations
      • Associated with 30-70% of normal DPD activity
      • Fluoropyrimidine should be initiated with reduced dosing
        • ~50% of standard dose is recommended, followed by titration of dose based on patient tolerability and therapeutic drug monitoring
  • Commonly evaluated TYMS polymorphisms include
    • 6 base pair deletion of the 3'-UTR (DEL) versus the wild type insertion (INS)
    • Variable number (2 or 3) of tandem repeats (2R, 3R) of the 5'-promoter enhancer region (5'-TSER)
    • G>C SNP in the second repeat of the 3R allele (3RG, 3RC)
  • Presence of the TYMS 3'-UTR deletion
    • Decreases TYMS expression
    • Increases response to 5-FU
    • Increases risk of toxicity
  • TYMS 5'-TSER genotypes 3RG/3RG, 3RG/3RC and 2R/3RG are associated with           
    • Increased TYMS expression
    • Decreased 5-FU responsiveness
    • Poor prognosis
  •  TYMS 5'-TSER genotypes 2R/2R, 3RC/3RC, and 2R/3RC are associated with
    • Decreased TYMS expression
    • Increased 5-FU responsiveness
    • Increased risk of toxicity


  • Dihydropyrimidine dehydrogenase (DPD) enzyme – encoded by DPYD gene
    • Catabolizes approximately 80% of 5-FU into an inactive form that is eliminated in the urine
    • Reduced DPD activity can lead to the accumulation of active 5-FU metabolite, increasing the risk for 5-FU toxicity
  • Thymidylate synthase (TYMS) enzyme – encoded by TYMS gene
    • Primary target for 5-FU
      • Remaining 5-FU drug is metabolized by different enzymes into an active form that inhibits the synthesis of DNA and RNA by competitive inhibition of TYMS or by direct incorporation of cytotoxic metabolites into nucleic acids
    • TYMS gene mutations result in reduced expression of TYMS and may be associated with higher clinical responsiveness to 5-FU therapy and increased risk of toxicity

Clinical Presentation

  • Grade III-IV toxicity (occurs in ~16% of treated patients)
    • Mucositis
    • Neutropenia
    • Nausea
    • Diarrhea
    • Neurological symptoms

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
5-Fluorouracil (5-FU) Toxicity and Chemotherapeutic Response, 5 Mutations 2007228
Method: Polymerase Chain Reaction/Single Nucleotide Extensions/Fragment Analysis

Predict toxicity and responsiveness of tumor to 5-FU therapy

Clinical sensitivity – estimated at 31% for the DPYD variants analyzed

Analytical sensitivity/specificity – 99%

Only targeted mutations in the DPYD and TYMS genes are evaluated

Rare diagnostic errors may occur due to rare sequence variations

Genetic and/or non-genetic factors that are not detected by this assay may affect 5-FU drug metabolism, efficacy, and risk for toxicity

Genotyping does not replace the need for therapeutic drug monitoring or clinical observation

Lack of detection of the targeted DPYD and TYMS mutations does not rule out risk for 5-FU toxicity or predict degree of responsiveness to 5-FU