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EC: In selecting an equianalgesic calculator, are there any issues that clinicians should be aware of?
TQ: There are several equianalgesic tables available, and a few desktop or PDA calculators. There is no standard for determining equianalgesic values between opioids, so different tables and calculators may use different conversion factors. For this reason, it is very important that all clinicians caring for the patient (including those in the pharmacy) should use the same calculator. It is prudent to have a second clinician independently check calculations. It is a good idea to check the sourceóare references or some other evidence base cited? Just because a drug is included in the calculator, doesnít mean it is a good choice. Select the drug based on patient and drug characteristics, then go to the calculator to determine the safe starting dose of the new drug. Equianalgesic doses are mostly based on limited research. At best they are approximations. By its nature, a calculator cannot take into consideration individual patient characteristics, or common variations in pharmacokinetics. For all of these reasons the safe starting dose is the calculated equianalgesic dose minus some significant factor, usually defined as 25-50% of the calculated dose. Most tables and calculators grossly underestimate the potency of methadone. Other opioids in the calculator are assumed to have a potency in a fixed ratio with the other opioids. There is no fixed ratio between methadone and other opioids. The safest approach would be to remove methadone from the equianalgesic calculator and use a dedicated evidence-based conversion method.
In terms of characteristics of an electronic calculator, there are a number of factors to consider. Are most of your clinical tools on a desktop or handheld platform? Look for a calculator that fits your style. If you need both, try to get them from the same source, so that the reference tables are the same. If a trial version is available, get it. Youíll want to test drive to determine ease of use and real world clinical utility. If you are in a group practice or institution, try to get your colleagues to adopt the same product. Consistent use of the same tools facilitates both patient safety and clinical efficacy. Many patients are on more than one opioid, such as a fentanyl patch supplemented by oral morphine for breakthrough pain. Try to get a calculator that can simultaneously convert two or more opioids.
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EC: What do you think is the most common misconception about opioid rotation? Why do you think that is?
TQ: I think there are three common barriers/misconceptions: I donít think the term "opioid rotation" is widely recognized among non-specialists. It is not as descriptive as "opioid switching". I think most clinicians are unaware of the significant differences, especially potency differences, between opioids. There has been a well-documented deficit regarding pain management in the basic education of physicians, nurses, and pharmacists that is only now being addressed. Fortunately, there are a great many resources available to clinicians, but we are all limited by ďnot knowing what we donít know. Many clinicians looking at an opioid equianalgesic table or calculator will assume that this is an easy "cookbook" method for dose calculation. Doing the calculation is just one step in a complex process.