Van Leeuwen et al evaluated patients undergoing a change in therapy from one vitamin K antagonist to another. They identified coefficients which can help to determine the new maintenance dose required. The authors are from Leiden University Medical Center in The Netherlands.
Patient selection: receiving a maintenance dose of a vitamin K antagonist
Parameter:
(1) maintenance dose of vitamin K antagonist in mg
(2) new vitamin K antagonist
maintenance dose for new vitamin K antagonist =
= (conversion factor) * (maintenance dose for previous vitamin K antagonist)
|
From (Previous)
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To (New)
|
Conversion Factor
|
|
warfarin
|
phenprocoumon
|
0.41
|
|
warfarin
|
aceoumarol
|
0.53
|
|
phenprocoumon
|
warfarin
|
2.36
|
|
phenoprocoumon
|
acenocuomarol
|
1.15
|
|
acenocoumarol
|
phenoprocoumon
|
0.84
|
|
acenocoumarol
|
warfarin
|
1.85
|
where:
• The conversion factor for warfarin nad phenprocoumon given in the abstract may be misinterpreted relative to this table.
• The conversion factors do not exactly match the inverse when going in the reverse. For example warfarin to phenprocoumon has a coefficient of 0.41. The inversion of 0.41 is 2.44, not 2.36.
• Tale 3 shows that the conversion factor may vary slightly with age.
Interpretation:
• Changing between vitamin K antagonists may be associated with risk of miscalculation.
• If in doubt check with hematologist or pharmacist.
• The patient's INR should be closely monitored after any change in vitamin K antagonist or dose.
