Conversion of other opioids to methadone: a retrospective comparison of two methods.

CONTEXT: A rapid method of methadone conversion known as the Perth Protocol is commonly used in Australian palliative care units. There has been no follow-up or validation of this method and no comparison between different methods of conversion.

OBJECTIVES: The primary objective of this study was to test the hypothesis that the achieved doses of methadone are independent of the conversion method (rapid vs slower). The secondary objectives included examining the relationship between calculated target doses, actual achieved doses and duration of conversions.

METHODS: This is a retrospective chart audit conducted at two hospital sites in the Brisbane metropolitan area of Australia which used different methadone conversion methods.

RESULTS: Methadone conversion ratios depended on previous opioid exposure and on the method of conversion used. The method most commonly used in Australia for calculating target doses for methadone when converting from strong opioids is a poor predictor of actual dose achieved. More appropriate conversion ratios are suggested.

CONCLUSION: Further research is needed to refine the ratios used in practice when converting patients from strong opioids to methadone. Caution and clinical expertise are required. A palliative methadone registry may provide useful insights.