Medicine possession ratio as proxy for adherence to antiepileptic drugs: prevalence, associations, and cost implications

OBJECTIVE: To determine the adherence status to antiepileptic drugs (AEDs) among epilepsy patients; to observe the association between adherence status and age, sex, active ingredient prescribed, treatment period, and number of comorbidities; and to determine the effect of nonadherence on direct medicine treatment cost of AEDs. METHODS: A retrospective study analyzing medicine claims data obtained from a South African pharmaceutical benefit management company was performed. Patients of all ages (N=19,168), who received more than one prescription for an AED, were observed from 2008 to 2013. The modified medicine possession ratio (MPRm) was used as proxy to determine the adherence status to AED treatment. The MPRm was considered acceptable (adherent) if the calculated value was ≥80%, but ≤110%, whereas an MPRm of <80% (unacceptably low) or >110% (unacceptably high) was considered nonadherent. Direct medicine treatment cost was calculated by summing the medical scheme contribution and patient co-payment associated with each AED prescription. RESULTS: Only 55% of AEDs prescribed to 19,168 patients during the study period had an acceptable MPRm. MPRm categories depended on the treatment period (P>0.0001; Cramer’s V=0.208) but were independent of sex (P<0.182; Cramer’s V=0.009). Age group (P<0.0001; Cramer’s V=0.067), active ingredient (P<0.0001; Cramer’s V=0.071), and number of comor-bidities (P<0.0001; Cramer’s V=0.050) were statistically but not practically significantly associated with MPRm categories. AEDs with an unacceptably high MPRm contributed to 3.74% (US$736,376.23) of the total direct cost of all AEDs included in the study, whereas those with an unacceptably low MPRm amounted to US$3,227,894.85 (16.38%). CONCLUSION: Nonadherence to antiepileptic treatment is a major problem, encompassing ~20% of cost in our study. Adherence, however, is likely to improve with the treatment period. Further research is needed to determine the factors influencing epileptic patients’ prescription refill adherence.