Warfarin dosage

[johnep]

My wife has just returned home from hospital with a new hip. Because of a family history of clotting, it was decided to give her warfarin rather than heparin. The anticoagulant nurse called yesterday and decided her dosage needed to increase from 3mg to 3.5mg. No problem says I, we have some 1mg tablets, we will just cut one in half. Oh no she says, that won't be accurate enough, she must have the 0.5mg tab. Drs closed for afternoon when I called round, but managed to get in surgery at 5pm. Oh!, says receptionist, she hasn't had these before and will need to see the Dr. I explained that not considered advisable for my wife to get into and out of a car and I had the 'yellow book'. Fortunately, the next pt was late or did not arrive, and I was able to explain to a Dr and get the script.

I just wonder why half a 1mg was not good enough. Sure 0.4mg or 0.6 mg would not make much difference. Any anticoagulant pharmacists on the forum?
johnep

[Pharmanaut]

My wife has just returned home from hospital with a new hip. Because of a family history of clotting, it was decided to give her warfarin rather than heparin. The anticoagulant nurse called yesterday and decided her dosage needed to increase from 3mg to 3.5mg. No problem says I, we have some 1mg tablets, we will just cut one in half. Oh no she says, that won't be accurate enough, she must have the 0.5mg tab. Drs closed for afternoon when I called round, but managed to get in surgery at 5pm. Oh!, says receptionist, she hasn't had these before and will need to see the Dr. I explained that not considered advisable for my wife to get into and out of a car and I had the 'yellow book'. Fortunately, the next pt was late or did not arrive, and I was able to explain to a Dr and get the script.

I just wonder why half a 1mg was not good enough. Sure 0.4mg or 0.6 mg would not make much difference. Any anticoagulant pharmacists on the forum?
johnep

Not an anticoag pharmacist myself, though would you say that half of a 1mg tablet for one dose to give breathing space to get 0.5mg script would be significant?

The receptionists are probably doing what we all have to do these days 'covering themselves' in case of complaint or the dreaded litigation.

[paul2008]

Warfarin has a long half life so common practice in many places is to give 3mg and 4mg on alternate days as the weekly dose wil be correct. Some patients need the equivalent of 2.75 mg daily so can have higher or lower doses two days a week for example on the weekend. In my last job we never used the half miligram tablets due to the risk of confusion.

In my opion its like a lot of pharmacy and medicine... its just the way it is done in your area, impact on patient safety zero, following local protocol 100%

[Zoggite]

"Won't be accurate enough" ?!?!?!

What a load of tosh!
What about the the variations in bioavailability of ingested warfarin due to diet, then? Is your wife limited to how much broccoli she can eat in a day, lest the vitamin K content affects her anticoagulant therapy ?!?!
Out of curiosity, how "inaccurate" is half a 1mg-warfarin tablet? has anyone ever studied the question?

[Shwampa]

Warfarin has a long half life so common practice in many places is to give 3mg and 4mg on alternate days as the weekly dose wil be correct...

Not to put you down, but alternating days will give you different weekly doses from one week to the next because of the odd number of days in a week. Maybe tell them to take the 3mg for 4 days a week and 4mg for 3 days if that's 24mg you want as weekly dose.

[paul2008]

Not to put you down, but alternating days will give you different weekly doses from one week to the next because of the odd number of days in a week. Maybe tell them to take the 3mg for 4 days a week and 4mg for 3 days if that's 24mg you want as weekly dose.

Well actually alternating daily doses gives you the correct dose over two days so no need to worry about the actual weekly dose in this case(or can assume it to be approximately equal to the average daily dose).

The principle is that as long as the dose is correct over the week the INR should be pretty stable and in range, dosing warfarin, like anything else is a steady state system so every 7 days you don't automatically start from scratch, I guess also thinking in terms of weekly dose is more of a mental short cut than a pharmacological rule.

In addition the weekly dose rule is used when dose adjustment is required as a 10 to 50% adjustment is standard practice so if the patient is on 3mg, (21mg a week) increasing the dose by 10% can be achieved by an extra 2mg per week rather than 0.3mg per day which is not measurable.

One of the most important principles in warfarin dosing especially in elderly patients is that the dosing regimen should be easy to follow so while 1mg daily and 2mg on Wednesday as and Saturdays may be as close to an ideal dose as we can get, telling the patient to take the higher dose on the weekends is easier to remember and safer (in terms of a risk of overdose or forgetting) and the effects will not be too different.

Some people think having a lot of strengths will confuse patients (I certainly think a patient with poor eye sight should get on one strength only even if it means taking a number of tablets). Others think its the schedules thats the most important for example dosing the same thing every day and avoiding more complex dosing schedules. Last month I would have told you that one of these approaches was certainly the best, at this point I don't know.

[Mensfuff]

The half life of most clotting factors is around the 72 hour mark. This means that if you were to take 4mg one day, then 3mg the next etc. then the actual fluctuation in free-drug-blood-level would be very small if any. This would give you a reasonably consistent therapeutic dosage range and ultimately the effect of anti-coagulation would remain roughly constant.

I would advise that you get you wife to see the doctor/warfarin clinic when you can, however in the meantime taking half a tablet a day or taking an extra milligram on alternate days would have roughly the same effect (especially as the aim is long term increase of prothrombin time, and not to make sure you are EXACTLY on an INR of 2.5).

Hope this helps

[johnep]

Anticoag nurse coming today or tomorrow. However they seem to work to strict protocols and dose/day has to be exactly as stated.
johnep