Exploring the Agreement Between Self-Reported Medication Adherence and Pharmacy Refill-Based Measures in Patients with Kidney Disease

Figures & data

Table 1 Baseline Characteristics of the Study Sample Comparing Non-Dialysis CKD Patients and KF Patients

	Overall Sample	CKD Patients	KF on Dialysis Patients	p-value*
No. of patients	113	60	53
Age (years)	65.1 ± 12.2	67.4 ± 10.4	62.4 ± 13.5	0.027
Gender (M/F)	64.6 / 35.4	61.7/ 38.3	67.9 / 32.1	0.488
Indigenous population (%)	4.8%	3.6%	6.1%	0.540
Education
<12 years	46.0%	51.7%	39.6%	0.508
12–13 years	14.2%	15.0%	13.2%
13–15 years	11.5%	10.0%	13.2%
≥16 years	28.3%	23.3%	34.0%
Carer status (Carer available)	25.5%	8.9%	44.2%	<0.001
Gross annual income
<$25,000	54.0%	43.3%	66.0%	0.141
>$25,000–50,000	31.9%	41.7%	20.8%
>$50,000–75,000	7.1%	6.7%	7.6%
>$75,000	7.1%	8.3%	5.7%
Smoking status
Current smoker	9.4%	7.1%	12.0%	0.694
Former smoker	55.7%	57.1%	54.0%
Non-smoker	34.9%	35.7%	34.0%
Cause of renal disease
Diabetic nephropathy	32.1.%	33.3%	30.8%	0.073
Hypertension/Vascular disease	20.5%	20.0%	21.2%
Glomerular disease	12.5%	8.3%	17.3%
Polycystic kidney disease	5.4%	3.3%	7.7%
Others/Unknown aetiology	29.5%	35.0%	23.1%
Comorbidity burden
Diabetes	41.0%	43.6%	38.0%	0.557
Hypertension	90.5%	89.1%	92.0%	0.612

Notes: *p-value indicates the significance of difference between CKD and KF patients. p values <0.05 are printed in bold.

Abbreviations: CKD, Chronic kidney disease; KF, Kidney failure; M, Male; F, Female.

Table 2 Results of Measures of Self-Reported and Pharmacy Refill-Based Medication Adherence for the Overall Sample and CKD, KF Sub-Groups

Results of Test Instruments	Overall Population	CKD	KF	p-value*
	N= 113	N=60	N=53
Self-reported Morisky medication adherence score	6.5 ± 1.4	6.8 ± 1.2	6.1 ± 1.5	0.026
Category of adherence based on Morisky medication adherence score
High adherence (score = 8)	29.2%	35.0%	22.6%	0.229
Medium adherence (score 6 to <8)	39.8%	40.0%	39.6%
Low adherence (score <6)	31.0%	25.0%	37.7%
MPR for antihypertensive medications	0.89 ± 0.15	0.91 ± 0.15	0.85 ± 0.15	0.008
Lower adherence as a category based on MPR of antihypertensive medications (<0.8)	26.0%	18.2%	35.6%	0.049
PDC for antihypertensive medications	0.85 ± 0.14	0.88 ± 0.14	0.82 ± 0.13	0.004
Lower adherence as a category based on PDC of antihypertensive medications (<0.8)	32.0%	18.2%	48.9%	0.001
MPR for cardiometabolic medications including statins	0.90 + 0.17	0.92 ± 0.15	0.88 ± 0.19	0.175
Lower adherence as a category based on MPR of cardiometabolic medications (<0.8)	18.5%	14.0%	23.7%	0.261
PDC for cardiometabolic medications including statins	0.87 ± 0.15	0.89 ± 0.14	0.85 ± 0.16	0.289
Lower adherence as a category based on PDC of cardiometabolic medications (<0.8)	18.5%	14.0%	23.7%	0.261

Notes: *p-value indicates the significance of difference between CKD and KF patients calculated by rank-sum test for continuous variables and chi-square test for categorical variables. p values <0.05 are printed in bold.

Abbreviations: CKD, Chronic kidney disease; KF, Kidney Failure; MPR, Medication possession ratio; PDC, Proportion of days covered.

Table 3 The Trend of Pharmacy Refill-Based Markers of Medication Adherence Among MMAS Categories of Self-Reported Adherence

Measure of Adherence	High Adherence (MMAS score=8)	Medium Adherence (MMAS Score=6-<8)	Low Adherence (MMAS score <6)	p-value* for Trend	p-value** ANCOVA
MPR for antihypertensive medications	0.97 ± 0.10	0.89 ± 0.15	0.80 ± 0.16	<0.001	<0.001
PDC for antihypertensive medications	0.93 ± 0.07	0.86 ± 0.14	0.78 ± 0.15	<0.001	0.001
MPR for cardiometabolic medications	0.98 + 0.11	0.91 ± 0.14	0.80 ± 0.20	<0.001	0.003
PDC for cardiometabolic medications	0.94 ± 0.06	0.89 ± 0.14	0.79 ± 0.19	0.004	0.014

Notes: *p-value for trend calculated using non-parametrical tests. **p-value for analysis of covariance evaluates association between MPR/ PDC and MMAS category after adjusting for age, patient group (whether KF or CKD), education level and income. p values <0.05 are printed in bold.

Abbreviations: MMAS, Morisky medication adherence scale; CKD, Chronic kidney disease; KF, Kidney Failure; MPR, Medication possession ratio; PDC, Proportion of days covered.

Table 4 MMAS Scores in Patients with High vs Low Adherence Category Based on MPR and PDC for Antihypertensives and Other Cardiometabolic Medications

MMAS Score	High Adherence (MPR >0.8)	Low Adherence (MPR <0.8)	p-value* ANCOVA	High Adherence (PDC >0.8)	Low Adherence (PDC <0.8)	p-value* ANCOVA
	Medication Possession Ratio			Proportion of days covered
MMAS score (vs antihypertensives)	6.8 ± 1.2	5.5 ± 1.4	<0.001	6.9 ± 1.1	5.5 ± 1.4	<0.001
MMAS score (vs cardiometabolic medication)	6.8 ± 1.1	5.2 ± 1.0	<0.001	6.8 ±.1	5.2 ±.0	<0.001

Notes: *p-value for analysis of covariance evaluated association between MMAS score and MPR/PDC category (<0.8 vs >0.8) after adjusting for age, patient group (whether KF or CKD), education level and income. p values <0.05 are printed in bold.

Abbreviations: MMAS, Morisky medication adherence scale; CKD, Chronic kidney disease; KF, Kidney Failure; MPR, Medication possession ratio; PDC, Proportion of days covered.

Table 5 Cohen’s Kappa Exploring Agreement Between Low Adherence MMAS Category (MMAS Score <0.6) and Low Adherence Based on Pharmacy Refill-Based Measures for MPR & PDC (<0.8)

	Agreement (%)	Cohen’s Kappa (k)
MPR for antihypertensive medications	70.0%	0.26*
PDC for antihypertensive medications	72.0%	0.35**
MPR for cardiometabolic medications	75.3%	0.35**
PDC for cardiometabolic medications	75.3%	0.35**

Notes: *Indicates the parameter estimate has a p value <0.01.** indicates p value <0.001.

Abbreviations: MMAS, Morisky medication adherence scale; MPR, Medication possession ratio; PDC, Proportion of days covered.

Table 6 Bias and Limits of Agreement for the Parameter Estimate as per Bland Altman Plots Evaluating Agreement Between MMAS Score and Pharmacy Refill-Based Measures

Adherence Measure	Bias	Limits of Agreement
MPR for antihypertensive medications (3 cases/100 values outside agreement)	−0.08	−0.40–0.24
PDC for antihypertensive medications (4 cases/100 values outside agreement)	−0.05	−0.36–0.27
MPR for cardiometabolic medications (3 cases/81 values outside agreement)	−0.09	−0.40–0.23

Abbreviations: MMAS, Morisky medication adherence scale; MPR, Medication possession ratio; PDC, Proportion of days covered.

Figure 1 Bland Altman plots exploring agreement between MMAS and MPR of antihypertensives.

Figure 2 Bland Altman plots exploring agreement between MMAS and PDC of antihypertensives.

Figure 3 Bland Altman plots exploring agreement between MMAS and MPR of cardiometabolic medications.

Figure 4 Bland Altman plots exploring agreement between MMAS and PDC of cardiometabolic medications.