Pathology Handbook

6TGN (6-thioguanine nucleotide)

Synonyms 

Clinical Indications

Thiopurines are used as immunosuppressants.  The therapeutic efficacy of thiopurine is achieved by the enzyme HPRT converting the drug to active, cytotoxic metabolites, including 6-thioguanine nucleotides (6-TGNs) which are incorporated into DNA.

Accumulation of high levels of 6-TGNs are responsible for the side-effects of thiopurine drugs and leucopenia.

Therapeutic drug monitoring of 6-TGN levels can be useful for:

1. Treating patients with low TPMT activity

2. Non-compliance or treatment with a sub-optimal dose

3. Failure to respond to standard doses of thiopurine drugs

It is strongly recommended that a patient's TPMT status is checked before starting treatment with thiopurines

Request Form

Request on ICE

Drug dose and date of commencing treatment are required and the reason for the request must be given on the request form

Availability

Analysed by referral laboratory if specific criteria met.

Specific Criteria

Investigation or monitoring of CAH

Patient preparation 

No patient preparation is required but steady-state concentrations are reached at 2-4 weeks after a dose change.  It is recommended samples are timed at 4 weeks from the start of treatment or a change in dose

Turnaround Time

Weekly

Specimen

Plasma in lemon top (Biochemistry EDTA) tube

Volume

2 ml

Container

 Vacutainer lemon top

Collection

See under patient preparation

Drug dose and date of commencing treatment are required and the reason for the request must be given on the request form

Causes for Rejection

Incorrect sample

Unlabelled or inadequately labelled sample

Reference Range & interpretation

<235 pmol 6-TGN x 108cells sub-optimal dose / non-compliance
235 - 450 pmol 6-TGN x 108 cells maximum drug efficiency and clinical remission of inflammatory bowel disease
>450 pmol 6-TGN x 108cells Linked to increased risk of myelotoxicity