Medication Adherence (PDC)

Medication adherence is whether a patient takes a medicine the way it was prescribed, and the standard way to measure it is proportion of days covered, or PDC. PDC counts the days in a period on which the patient had the drug in hand, based on fill dates and days' supply, and divides by the days in the period. Fill a 30-day supply four times over six months and your PDC is 120 divided by 182, about 66 percent. The usual threshold for "adherent" is 80 percent, which Medicare uses in its star ratings for diabetes, blood pressure, and cholesterol drugs.

The reason employers and plans care is not sentimental. Half of patients with chronic conditions stop taking their medicine within a year, and the people who stop are the ones who end up in the hospital. A member who skips a $10-a-month blood pressure generic and has a stroke costs the plan $60,000 to $100,000 in the first year and the family far more than that. Studies across large plans put the cost of non-adherence in the United States between $100 billion and $300 billion a year. The reasons people stop are ordinary: the drug costs too much, they feel fine, the refill is a hassle, nobody explained the side effect. Every one of those has a fix cheaper than the hospital stay.

The takeaway: if a monthly drug is more than you can comfortably pay, say so to the pharmacist before you stop taking it. A generic in the same class, a 90-day fill, or a manufacturer program almost always exists, and the drug only works if it is taken.