Methadone serum levels are indicated when the clinical picture does not agree with expected/typical responses to a given dose of methadone and when this additional clinical information would be of use in making decisions regarding changes in the methadone dose.
The serum methadone level for a given dose will vary between individuals because of individual tolerance and the influence of other factors (e.g. other medications, pregnancy, individual variations in hepatic metabolism and renal clearance).
Research to date on the clinical application of serum methadone levels is inconsistent, generally indicating that the overall clinical picture must be the foundation for any decisions regarding adequacy of methadone dose. Serum levels may not provide conclusive information.
Consult with BOPAS OST prior to any serum level testing.
Clinical situations where serum levels may be useful are:
Serum level testing:
This procedure should be undertaken in liaison with the dispensing pharmacist and the BOPAS OST service:
The patient must consume their methadone at the pharmacy at approximately the same time for 4 days (usually Mon-Thurs) prior to the blood test/s (on Fri). This is to ensure they reach a steady state for a known dose consumed daily under observation.
On the day of the blood test the patient presents to the laboratory for their trough blood level to be taken before they consume their dose and at the same time they have been consuming for previous four days.
If clinically indicated (see split dosing) a peak blood level is taken 3 ½ to 4 hours after the patient has consumed their dose, and before any takeaway doses are dispensed.
Serum methadone levels can be measured using capillary blood samples when venous access is difficult. A 1ml SST tube is sufficient.
Serum level interpretation and guidelines:
Serum methadone trough level:
Level to provide 24-hour relief of withdrawal symptoms is 650-1950 ngm/ml. Where a client presents with a higher trough level then 1950, this must be discussed with the OST specialist service.
Serum methadone or buprenorphine/naloxone trough and peak levels:
Where the peak: trough ratio is 2 or 2.5:1 or greater then split dosing may be indicated in a stable patient. Consult with the BOPAS OST Service, prior to instituting split dosing. See Split methadone or buprenorphine/naloxone dosing.
*See Section 5.5 Measuring Methadone Serum Levels page 39 of " New Zealand Practice Guidelines for Opioid Substitution Treatment 2014"
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