Measuring Adherence With Accuracy

Nov 11, 2012
Since medications are taken outside the view of a pharmacist, how can we accurately assess adherence? Testing the Medication Possession Ratio (MPR) on asthma.

Medication nonadherence is a major problem for the healthcare consumer and for the country as a whole, as we’ve discussed on this site. But because medications are often taken (or not taken) outside the view of a physician or pharmacist, how can we assess a patient’s adherence with any sort of accuracy?

One method of measuring adherence is the Medication Possession Ratio (MPR), which is calculated using prescription-claims data. By tracking when and how frequently patients refill their medications, and ruling out other possibilities, such as the patient switching to another medication, Express Scripts can estimate how adherent the patient is to his or her medication.

When a patient takes all of the available medication but procrastinates getting a new prescription or a refill, the patient no longer possesses any medication on hand. MPR is the ratio of the number of days for which a patient has medication on hand divided by the total number of days a patient was observed. Therefore, a patient with perfect adherence would have an MPR of 1.0 or 100%. Adherence is typically measured over six to 12 months for most conditions.

Importance of Adherence in Asthma

One condition where there is a need to improve adherence to chronic medications is asthma. Asthma affects about 24.6 million Americans, and it is responsible for one-half million hospitalizations in any given year for those whose condition is poorly controlled. And yet, 56% of asthma patients are nonadherent to medication therapy.

A recent Express Scripts study published in the American Journal of Pharmacy Benefits shows that home-delivery pharmacy is an important stand-alone intervention to help asthma patients reach optimal adherence to their controller medication. We drew these conclusions by studying disparities in MPR, but is that an appropriate metric in this case?

Potential Shortcomings of MPR as a Measure of Asthma Adherence

The challenge with measuring adherence for asthma-controller medication is that it is difficult to estimate how long it takes a patient to use all the medication in a single aerosol inhaler. The exact days’ supply for medication in multidose containers (liquids, drops) can vary. And for devices such as an inhaler, how much medication is used each time depends on the patient’s technique for using the administration device.

Checking Our Work

To verify the accuracy of our findings, Express Scripts repeated the study with a group of asthma medications that are supplied in fixed doses and therefore, less susceptible to the potential variances discussed above. The repeated study demonstrated that the inaccuracies in measuring the days of supply are not significant enough to contradict or reverse the earlier study’s results.

MPR is indeed a useful metric in measuring adherence, and home-delivery pharmacies clearly improve MPR.

As evidenced by this example of double-checking, rigor and due diligence are important to us. Our clients rely on our data to design their pharmacy benefit, and patients rely on our data to improve health outcomes. And Express Scripts takes these responsibilities seriously.

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Lab Staff
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