Dosing Algorithm of Ryan et al (Coventry or Warwickhire Method) for Oral Anticoagulants

 

NOTE: The dose recommendations are only suggestions. Oral anticoagulant dosing can be difficult and serious complications may occur follow dosing errors. Adjustments in dosage should only be undertaken by an experienced clinician.

 

Step 1: Measure the INR (international normalized ratio).

 

Step 2: Compare the current INR with the target INR and determine if the dose needs to be adjusted.

 

Ratio (today's INR) / (target INR)	Dosage Adjustment
> 1.20	Method A
0.80 to 1.20	no change in dose
< 0.80	Method B

 

Step 3: Adjust dosage using Method A (for high current INR):

 

new dose in mg =

= (previous dose in mg) * (((1.2 * (target INR)) / (today's INR)) ^(0.333))

 

If the value of today's INR is greatly increased, then a dose should be stopped for 1, 2 or 3 days, depending on the degree of elevation.

 

where:

• Taking the expression to the power 0.333 is the cube root.

 

Step 3: Adjust dosage using Method B (for low current INR):

 

new dose in mg =

= (previous dose in mg) * (((0.8 * (target INR)) / (today's INR)) ^(0.333))

 

Step 4: Schedule for followup (based on recommendations of the British Society of Hematology at the time of publication)

 

Results	When	Recall Schedule
low	today	in 7 to 14 days
normal	today	in 28 days
	today + previous one	in 42 days
	today + previous 2 visits	in 56 days
	today + previous 3 visits	in 70 days
	today + previous 4 visits	in 84 days
high	today	in 7 to 14 days

 

where:

• I modified this in the spreadsheet for high INR's so that followup might be in 1-3 days.

 

Limitations:

• Concurrent disease, noncompliance or failure to control diet may cause wide variation in the level of anticoagulation achieved.

• The formulas do not seem to be alterable (James et al, 1989).

• If the person has a high INR at low daily dose, the adjustments do not significantly change the dose after rounding. This argument seems less important if fractional dosing is allowed without rounding, or if the dose is rounded down to the next interval rather than up (for example, a dose of 3.9 being rounded down to 3.5 rather than 4.0 if the INR is too high).