Examining adherence with medications used in treating diabetic peripheral neuropathic pain
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The present study is a retrospective cohort analysis which sought to examine adherence to medications used in managing painful diabetic peripheral neuropathy (PDPN) and to determine their association with oral antidiabetic (OAD) medication adherence using the Texas Medicaid prescription claims database. The study objectives were to: 1) provide a description of PDPN and OAD medication use among the study subjects; 2) determine if PDPN medication adherence differs among individual PDPN agents (i.e., tricyclic antidepressants, gabapentin, pregabalin and duloxetine); 3) determine if pre-index OAD and post-index OAD medication adherence differs among mono, dual, and triple OAD therapies; and 4) determine if PDPN medication adherence is related to post-index OAD medication adherence while controlling for covariates. Study participants were adult (≥18 years) Medicaid beneficiaries prescribed OAD and PDPN medications. The index date was the first PDPN prescription. Data were extracted from June 1, 2003 to October 31, 2009 and prescription claims were analyzed over an 18-month study period (i.e., 6 months pre-index and 12 months post index period). Medication possession ratio (MPR) was used as a proxy measure of medication adherence. An MPR less than 80 percent was regarded as being non-adherent to prescribed medication, while an MPR greater than or equal to 80 percent was regarded as being adherent to prescribed medication. Objective 1 was addressed using descriptive statistics (i.e., mean, standard deviation, frequency). Univariate analysis (ANOVA) was employed to address Objectives 2 and 3. Multivariate analyses (i.e., multiple linear regression and logistic regression) were conducted to address Objective 4. For the logistic regression MPR was dichotomized at the cut-off value of 80 percent. A total of 4,277 patients met the study’s inclusion criteria. The overall mean MPR (±SD) for PDPN medications was 75.4 percent (±23.9). Mean MPR (±SD) was highest for duloxetine (85.6% ±18.2) and was lowest for pregabalin (69.4% ±24.9). Mean MPR differed significantly among individual PDPN medications (p<0.0001). The overall mean MPR (±SD) for OAD medications in the pre and post-index period was 73.0 percent (±24.3) and 64.5 percent (±25.6) respectively. In both pre and post-index periods, mean MPR differed significantly among mono, dual, and triple OAD therapies (p<0.0001). In the pre-index period, mean MPR (±SD) was highest for monotherapy users (75.4% ±24.7) and was lowest for triple therapy users (63.9% ±22.9). Similarly, mean MPR (±SD) was highest for monotherapy users (69.0% ±26.1) and was lowest for triple therapy users (52.9% ±21.8) in the post-index period. After controlling for the covariates, PDPN adherence (i.e., MPR) was statistically significant (p<0.0001) and positively related to post-index OAD adherence (i.e., MPR). PDPN patients who were non-adherent (i.e., MPR<80%) to their PDPN medications (or neuropathic pain medications), compared to those who were adherent (MPR≥80%), were significantly less likely to be adherent to their OAD medications [Odds Ratio (OR) = 0.626, 95% CI=0.545-0.719]. In addition, post-index OAD adherence (i.e., MPR) did not differ significantly (p>0.05) when pregabalin, duloxetine and gabapentin users were individually compared to tricyclic antidepressants users. In conclusion, PDPN patients who were adherent (i.e., MPR≥80%) to their PDPN medications, compared to those who were not adherent (i.e., MPR<80%), were more adherent to their OAD medications. Also, adherence to OAD medications was independent of the type of PDPN medication used. PDPN patients need to be educated regularly that neuropathic pain medications only relieve the pain associated with the neuropathy but achieving adequate glycemic control remains the only established approach for slowing down the progression of the neuropathy and other complications associated with the diabetes.
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