Two recent studies that were released this week question the value of the usage of a genetic test to guide long-term use of the blood thinner Plavix.

AstraZeneca PLC has been benefited from these results and so has its blood thinner Brilinta. An earlier study showed Brilinta to be superior to Plavix and it is awaiting the decision of the FDA, which is expected next month, to market the drug.

They even underscore the difficulties facing doctors and drug companies finding out how to use growing troves of genetic data to tailor treatments to individual patients.

Plavix prevents blood platelets from clotting together and is also used to prevent clots that can lead to heart attacks and strokes in patients who have an unstable heart condition called acute coronary syndrome.

It is prescribed a lot when such patients are treated with stents, which prop open diseased coronary arteries to prevent clots from forming in the devices.

The FDA added a black-box warning to the label for Plavix indicating that doctors can raise the dose or change to a different treatment for patients who do not respond well to the drug.