The mean adherence rate for the entire cohort was 80.7%, with older subjects achieving a mean adherence rate of 87.5% and younger subjects achieving a mean adherence rate of 78.3%, a difference that is statistically significant, F(1,146) = 6.3, P = 0.01. Subjects were then classified as either good adherers if they adhered to 95% or better of prescribed doses, or poor adherers if they failed to attain a 95% adherence rate. As can be seen in , 53% of older subjects were classified as good adherers, whereas only 26% of younger subjects were able to attain a 95% adherence rate, a difference that is statistically significant [χ2(148) = 9.5, P = 0.004]. Using a more liberal 90% cut-off point to define good adherence, 71% of older subjects were found to be adherent versus only 37% of younger subjects, a difference which again is statistically significant, [χ2(148) = 13.0, P = 0.001].
Medication adherence in younger and older HIV-infected adults
Logistic regression analysis was performed to determine the predictive relationship between older age, cognitive dysfunction, and medication adherence, with good medication adherence conservatively defined as at least 95% of prescribed doses taken. The results of logistic regression revealed that age (β = 1.12, P = 0.005) and global neuropsychological performance (β = 0.92, P = 0.02) were significantly associated with medication adherence [χ2(2,148) = 14.9, P = 0.0006]. Older individuals were three times more likely to be good adherers than younger subjects [odds ratio (OR) 3.1, 95% confidence interval (CI) 1.40–6.76]. Neuropsychologically compromised subjects were 2.5 times more likely to be poor adherers (OR 2.5, 95% CI 1.19–5.35).
In an effort to determine what differentiated older subjects who were able to adhere adequately to their medication regimen from those who could not, the next analysis examined the association between neuropsychological status and medication adherence rates. As depicted in , 65% of the older subjects who were classified as good adherers scored within the normal range on the composite measure of global cognitive status. In contrast, only 17% of older subjects classified as poor adherers were cognitively normal, with 83% meeting the study criteria for global cognitive impairment. This difference in global neuropsychological impairment as a function of the adherence group is statistically significant [χ2(38) = 7.4, P = 0.009].
Medication adherence in older HIV-infected adults as a function of cognitive impairment
A series of chi-squared analyses were then performed to identify which component cognitive processes might be driving this relationship. Poor adherence was associated with impairment in executive function [χ2 (38) = 10.6, P = 0.001], memory [χ2(38) = 3.7, P = 0.05], and psychomotor slowing [χ2(38) = 4.3, P = 0.04] (see ). Of those older subjects with executive dysfunction, 74% were classified as poor adherers. In contrast, only 21% of older subjects who performed normally on tests of executive function were found to be poor adherers. A similar relationship emerged between the psychomotor slowing factor and medication adherence. Among those patients classified as poor adherers, 78% performed within the impaired range on the neuropsychological measures of psychomotor speed. Finally, 67% of older subjects with memory impairment were classified as poor adherers. The remaining cognitive domains were not significantly associated with medication adherence.
Current drug abuse or dependence was also associated with poor medication adherence. As can be seen in , 13 of the 14 subjects (93%) who met DSM-IV diagnostic criteria for current drug abuse or dependence were classified as poor adherers, whereas only 65% of the non-drug abusing subjects were poor adherers, a difference that is statistically significant [χ2(1,144) = 4.6, P = 0.04]. Cocaine was the most common drug of abuse, with nine out of 14 (64%) meeting diagnostic criteria for cocaine abuse or dependence. Although 77% of subjects who met DSM-IV diagnostic criteria for alcohol abuse/dependence were poor adherers, current alcohol abusers were not statistically more likely to be poor adherers than were subjects who did not meet diagnostic criteria for current alcohol abuse/dependence [χ2(1,144) = 0.73, P = 0.58]. Because of a low base rate of substance abuse among older subjects in this sample, we were unable to determine whether the concomitant presence of both older age and drug/alcohol abuse resulted in disproportionate difficulties with medication adherence.
Medication adherence among HIV-infected adults who meet Diagnostic and Statistical Manual of Mental Disorders IV diagnostic criteria for current drug abuse/dependence or current alcohol abuse/dependence