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Health care organizations may be able to use pharmacy data to identify patients with schizophrenia and poor antipsychotic adherence.To determine whether a pharmacy-based measure of outpatient adherence, the medication possession ratio (MPR), is associated with adverse outcomes among patients with schizophrenia, as evidenced by increased psychiatric admission.Cohort study linking pharmacy and utilization data for veterans with schizophrenia. MPRs were calculated by dividing the number of days’ supply of antipsychotic medication the veteran had received by the number of days’ supply they needed to receive to take their antipsychotic continuously. Using multivariate regression, the relationship between MPRs and psychiatric admission was examined.Sixty-seven thousand seventy-nine veterans who received a diagnosis of schizophrenia and had outpatient antipsychotic medication fills between October 1, 1998 and September 30, 1999.Patients with MPRs close to 1.0 had the lowest rates of admission. As patients secured progressively smaller proportions of required antipsychotic medication (and had smaller MPRs), rates of admission climbed. Among patients on one antipsychotic (n = 49,003), patients with poor adherence (MPRs < 0.8) were 2.4 times as likely to be admitted as patients with good adherence (MPRs from 0.8–1.1). 23% of poorly adherent patients but only 10% of adherent patients were admitted. Once admitted, poorly adherent patients had more hospital days. Patients who received excess medication also had higher admission rates.Many health care systems may be able to use pharmacy data to identify poorly adherent patients with schizophrenia. These patients are at-risk for admission and may benefit from intervention.