Foster score, a self-reported inhaled corticosteroid adherence score for patients with asthma, demonstrated poor agreement with objective measures of adherence, suggesting that the score alone is not an accurate indicator of adherence, according to Ulla Møller Weinreich, PhD, and colleagues.

For a paper published in the Journal of Asthma and Allergy, the team examined the test-retest reliability on adherence with inhaled corticosteroids in adults with asthma using the Foster score. “Adherence with controller medication is a major challenge in asthma management,” the study authors wrote. “Thus, a reliable method of measurement is mandatory to assess adherence.” The Foster score measurement, they explained, is obtained by asking the patient a single question: “How many days, out of 7, do you take your medicine as recommended?”

The study cohort included patients with asthma with more than one routine follow-up appointment at a university hospital outpatient clinic. The objective Medication Possession Ratio (MPR) was calculated based on pharmacy redemption data and physician-prescribed doses of inhaled corticosteroids. The difference between Foster score and MPR at the first and second visit was assessed using a Bland–Altman plot, with outcomes reported as limits of agreements and bias. Foster scores from both visits were used to calculate an intraclass correlation coefficient (ICC).

Clinicians Should Not Rely on Foster Scores Alone

The researchers found that self-reported adherence with asthma controller medication measured by Foster score was significantly higher than with the objective MPR. The Bland–Altman plot for MPR and Foster score at the first and second visit showed upper and lower limits of agreement of 83.5 – (− 1.6) and 80.9 – (− 6.9) and biases were 41.0 and 37.0, respectively. Of the included patients, 93.1% reported identical Foster scores between visits, resulting in an excellent ICC of 0.92. Absolute median difference between Foster scores and MPR at first and second visit was 8.7 percentage points.

When assessed by Foster score, the researchers observed a significant overestimation of adherence to ICS, compared with objectively assessed adherence calculated by MPR. Overall, MPR improved by 7.7% between the two visits, while Foster score remained unchanged. Less than one-fifth of the included patients were adherent, and only 10% were persistently adherent during the observational period.

“Our study suggests that clinicians should not rely on Foster scores alone to assess adherence,” the study authors wrote. “Furthermore, we observed extremely poor persistency in adherence over time. Therefore, our study underlines the importance of focusing on persistency when meeting the patient in a clinical setting.”