Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

TPMT
80291
FFTHO
90480
Thiopurine Methyltransferase (TPMT), Erythrocytes
Clinical Information: Thiopurine methyltransferase (TPMT) deficiency is a condition in which
patients treated with standard doses of the immunosuppressant azathioprine (Imuran) or the
antineoplastic drug 6-mercaptopurine (6-MP, Purinethol) may develop life-threatening
myelosuppression or severe hematopoietic toxicity. The metabolic conversion of azathioprine or 6-MP
to purine nucleotides and the subsequent incorporation of these nucleotides into DNA plays an
important role in both the therapeutic efficacy and the toxicity of these drugs. A competitive catabolic
route for the metabolism of thiopurines is catalyzed by the TPMT enzyme, which inactivates them by
thiomethylation. A balance must be established between these competing metabolic pathways so that: 1)
sufficient amounts of drug are converted to the nucleotide to act as an antimetabolite and 2) the
antimetabolite levels do not become so high as to cause potentially lethal bone marrow suppression.
TPMT deficiency is an autosomal recessive condition with an incidence of approximately 1 in 300
individuals homozygous for deleterious mutations in the TPMT gene; about 10% of the population is
heterozygous carriers of TPMT mutations. Adverse effects of azathioprine or 6-MP administration can
be observed in individuals who are either homozygous or heterozygous for TPMT deficiency. As such,
knowing a patient's TPMT status prior to treatment with azathioprine or 6-MP is important for purposes
of calculating drug dosages.
Useful For: Detection of individuals with low thiopurine methyltransferase activity who are at risk
for excessive myelosuppression or severe hematopoietic toxicity when taking azathioprine (Imuran) or
6-MP (Purinethol)
Interpretation: Expected values for individuals in the carrier range for thiopurine methyltransferase
(TPMT) deficiency are between 6.0 and 10.0 U/mL RBC. Expected values for individuals homozygous
for deleterious mutations in the TPMT gene (deficient TPMT) are < or =5.9 U/mL RBC. Results
between 10.1 and 14.9 U/mL RBC represents probable low normal.
Reference Values:
> or =15.0 U/mL RBC (normal)
10.1-14.9 U/mL RBC (low normal)
6.0-10.0 U/mL RBC (carrier)
0.0-5.9 U/mL RBC (deficient)
Reference values apply to all ages.
Clinical References: 1. Sandborn WJ:Pharmacogenomics and IBD: TPMT and thiopurines.
Inflamm Bowel Dis 2004;10 Suppl 1:S35-S37 2. Schedel J, Godde A, Schutz E, et al: Impact of
thiopurine methyltransferase activity and 6-thioguanine nucleotide concentrations in patients with
chronic inflammatory diseases. Ann N Y Acad Sci 2006;1069:477-491 3. Zhou S: Clinical
pharmacogenomics of thiopurine S-methyltransferase. Curr Clin Pharmacol 2006;1:119-128
Thiosulfate, Urine
Reference Values:
Reporting limit determined each analysis
Creatinine (mg/L)
U.S. Population (10th – 90th percentiles, median)
All participants:
335 - 2370 mg/L, median: 1180 (n=22,245)
Males:
495 - 2540 mg/L, median: 1370 (n=10,610)
Females:
273 - 2170 mg/L, median 994 (n=11,635)
Thiosulfate (mcg/mL)
Normal range: approximately 2.9+/-2.5 mcg/mL (based on an average creatinine concentration of 1
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