MONDAY, Nov. 14 (HealthDay News) -- Most people taking the blood-thinner warfarin need blood tests every four weeks to make sure they're receiving the right dose of medication, but new research suggests that some people could safely have those tests done just once every 12 weeks.

Longer intervals between monitoring isn't an option for everyone on warfarin, just those who have received the same dose of medication for six months or longer, noted the study, published in the Nov. 15 issue of the Annals of Internal Medicine.

"Sometimes it's difficult to go every four weeks, and we found that for patients who are very stable who have been monitored every four weeks, going every 12 weeks was not different in dosing," said the study's lead author, Dr. Sam Schulman, a professor of medicine and director of the clinical thromboembolism program at McMaster University in Hamilton, Ontario.

"Since there are 2 million patients in North America on warfarin, it could add quite a bit of cost savings if they could go longer [between tests]," said Schulman.

Warfarin (brand names Coumadin, Jantoven) thins the blood, and it's prescribed to help prevent blood clots. Blood clots can cause heart attacks and strokes. If too little warfarin is given, the blood isn't thinned enough and blood clots can form. But too much warfarin can thin the blood excessively, making internal bleeding a risk.

Once the right dose is found, some people remain stable on that dose indefinitely. For others, constant changes are needed to their warfarin dose. Schulman said that about one-third of his patients have stable doses.

The test that's done every four weeks to monitor a patient's warfarin dose is called international normalized ratio (INR) monitoring.

The current study included 250 people who were receiving unchanged warfarin doses for at least six months. They were randomly assigned to one of two groups: one that received INR monitoring every four weeks or one that was monitored every 12 weeks.

The researchers found that people who were monitored every four weeks had an optimal dose of warfarin 74.1 percent of the time, compared with 71.6 percent for the group monitored every 12 weeks.

"Assessment of warfarin dose every 12 weeks seems to be safe and non-inferior to assessment every four weeks," wrote the study's authors.

"This study contributes a bit to answering the question of how often you have to test, but there was no information on [blood clots] and bleeding events," noted Dr. Jeffrey Berger, an assistant professor and director of cardiovascular thrombosis at New York University Langone Medical Center in New York City. But he added that complications such as blood clots and bleeding are rare events, and to properly assess the risk of these complications with longer monitoring, a much larger trial would be necessary.

He also said, "I think this is a very important study in the current horizon where we have an increasing number of potential alternatives to Coumadin. It's important to understand the tradeoffs between drugs."

The U.S. Food and Drug Administration has approved two alternatives to warfarin: dabigatran etexilate (Pradaxa) and rivaroxaban (Xarelto). And Berger said that another drug, apixaban (Eliquis), will likely be approved soon.

The newer medications don't require monitoring of the dosage, but they are significantly more expensive than warfarin. In Canada and the United States, warfarin costs about 10 cents per day, while Schulman said that a newer medication can be as much as $3.60 a day.

Still, Berger pointed out that without the cost of monthly monitoring, the newer drugs may end up being more cost-effective.

More information

Learn more about warfarin from the U.S. National Library of Medicine.