Most of the drugs in DoseMeRx do not require a drug-drug interaction field (excluding PPIs and azoles) because it's part of the data that DoseMeRx receives every time you enter a drug level.
In general, between 40-60% of the variability in response between patients to the same dose is describable by ‘knowns’ or measurable criteria - e.g. height, weight, serum creatinine and so forth. The remaining variability in how a patient responds to a medication is ‘unknown’, but still measurable! It’s measurable in the models in DoseMeRx by fitting the way that a patient absorbs, distributes, metabolises, and excretes a medication (pharmacokinetics); and for a drug like warfarin, it’s also measurable in the INR (or end effect of a warfarin dose). These ‘unknown’ factors include diet, exercise, genes, and drug-drug interactions.
In our experience for the large majority of drug-drug interactions, DoseMeRx can fit these without a problem, and build a model that accurately reflects a patient’s pharmacokinetics with the interaction occurring.
There are two limitations where additional clinical caution should be taken:
Extremely large effect of drug-drug interactions
For example, cyclosporin with diltiazem. Diltiazem is a very strong CYP3A4 and PGP inhibitor and is sometimes used to increase cyclosporin concentrations significantly.
In these cases, DoseMeRx will not fit the provided concentrations. We have additional features that we enable on a per-site and per-user basis to assist in these situations, and DoseMe support will be happy to help you out.
Introducing a new interacting drug
DoseMeRx uses the pharmacokinetics of a patient from yesterday to predict a concentration today. As a result, if a brand-new interacting drug has been introduced, clinical judgement may be needed to adjust a dose in the short term. DoseMeRx has simulated a range of patients on a strong drug-drug interaction with warfarin, and found that typically three INRs and one week was sufficient for DoseMeRx to ‘learn’ the new pharmacokinetics with the interacting drug being taken.
For warfarin, it is especially critical for clinical staff to consider whether an unusual INR result that does not fit the DoseMeRx model is due to a once-off event (e.g. increased alcohol intake on a national holiday), or a consistent change that is likely to continue.