David Clotterfield and Clara Clearvessel take warfarin and make various comments about their lives. At the end of each scene, test your knowledge; Are their comments true or false?
Clara Clearvessel had her day-off planned to the minute. It would start with a lie in till 7.30, an hour on the exercise bike, a shower then off to have her INR blood test at the local blood collecting lab. After that, she was looking forward to breakfast at the newest café in town: In fact, she’d even booked for 9.30 to be sure of getting a table. She hadn’t expected the traffic to be so bad though and was concerned when she didn’t get to the lab until 9.20. She spotted an elderly gentleman entering just ahead of her but was sure she could beat him to the reception counter.
He moved faster than she expected though, so they both put their blood request forms on the counter at the same time. The receptionist looked up, “Clara Clearvessel and David Clotterfield. Who wants to go first?”
Clara turned on the charm as she spoke to the old man. “Would you mind if it’s me? You see I’m in a rush and starving hungry. I haven’t had breakfast so that they can get a more accurate INR result and I’ve had to wait two weeks for this test anyway. I’m sure I’d get much better control if they’d only test me every day.”
What do think about Clara’s comments? Are they true or false?
You don’t need to be starved to get an accurate INR test. The time of day also makes no difference to the result. It doesn’t matter how long ago you took your tablet either, so long as it wasn’t more than 48 hours ago. There are only a few investigations such as cholesterol level or fasting glucose, where you need to miss breakfast before your test.
Usually people do well if they have INR tests just once or twice a month, so long as they’re stable on warfarin, and don’t have much variation in INR results or warfarin dose.
Some people need to have tests more often because they’re less stable, and may have tests once (or occasionally twice) a week.
People who are self-testing can choose to test as often as they like. One study showed the best possible control was obtained with two tests a week. This is not practical for most people and probably not necessary.
Testing every day, as Clara Clearvessel suggested, could potentially be dangerous as a change in warfarin dose take several days to have an effect. For example, suppose your INR was high on Monday and you decreased your warfarin dose; if you tested again on Tuesday, the INR might still be high (as the warfarin dose change hadn’t yet taken effect) so you might be tempted to reduce the dose again. The cumulative effect of the two dose reductions could then cause a major drop in INR putting you at risk of having a clot.