ORIGINAL ARTICLE

Adherence to phosphate binders in hemodialysis patients:prevalence and determinants

Yoleen P. M. Van Camp • Bernard Vrijens •

Ivo Abraham • Bart Van Rompaey •

Monique M. Elseviers

Received: 13 August 2013 / Accepted: 9 December 2013

� Italian Society of Nephrology 2014

Abstract

Background Phosphate control is a crucial treatment goal

in end-stage renal disease, but poor patient adherence to

phosphate binder therapy remains a challenge. This study

aimed to estimate the extent of phosphate binder adherence

in hemodialysis patients and to identify potential

determinants.

Methods Phosphate binder adherence was measured

blindly in 135 hemodialysis patients for 2 months using the

medication event monitoring system. Patient data, gathered

at inclusion through medical records, ad hoc questionnaires

and the short form (SF)-36 health survey, included: (1)

demographics, (2) perceived side-effects, belief in benefit,

self-reported adherence to the therapy, (3) knowledge

about phosphate binder therapy, (4) social support, and (5)

quality of life (SF-36). Phosphatemia data was collected

from charts. ‘Being adherent’ was defined as missing \1

total daily dose/week and ‘being totally adherent’ as

missing \1 total daily dose/week, every week.

Results Mean age of patients was 67 years and 64 % of

the sample was male. Over the 2 months, 78 % of the

prescribed doses were taken. Every week, about half of

patients were adherent. Over the entire 8-week period,

22 % of patients were totally adherent. Mean phosphatemia

levels were 0.55 mg/dl lower in adherent than nonadherent

patients (4.76 vs. 5.31 mg/dl). Determinants for being

totally adherent were living with a partner, higher social

support (both were interrelated) and higher physical quality

of life. Experiencing intake-related inconvenience nega-

tively affected adherence. The social support and quality of

life physical score explained 26 % of the variance in

adherence.

Conclusions Phosphate binder nonadherence remains a

major problem. Interventions should aim, at least, to

improve social support. With few associated factors found

and yet low adherence, an individualized approach seems

indicated.

Keywords Adherence � Dialysis � Electronic

measurement � Medication � Phosphate binders �Phosphatemia

Introduction

Patients with end-stage renal disease (ESRD) have a reduced

phosphate excretion, resulting in hyperphosphatemia, which

is associated with vascular calcification. Vascular calcifica-

tion is a major contributor to cardiovascular disease, the

Electronic supplementary material The online version of thisarticle (doi:10.1007/s40620-014-0062-3) contains supplementarymaterial, which is available to authorized users.

Y. P. M. Van Camp (&) � B. Van Rompaey � M. M. Elseviers

Faculty of Medicine and Health Sciences, Centre for Research

and Innovation in Care (CRIC), Universiteit Antwerpen, CDE

R334, Universiteitsplein 1, 2610 Wilrijk, Belgium

e-mail: yoleen@gmail.com; yoleen.vancamp@uantwerpen.be

B. Vrijens

Department of Biostatistics and Medical Informatics,

Universite de Liege, Liege, Belgium

B. Vrijens

MWV Healthcare, Vise, Belgium

I. Abraham

Center for Health Outcomes and Pharmacoeconomic Research

(HOPE), University of Arizona, Tucson, AZ, USA

B. Van Rompaey

Department of Healthcare, Artesis University College of

Antwerp, Antwerp, Belgium

123

J Nephrol

DOI 10.1007/s40620-014-0062-3

leading cause of death in ESRD, making phosphate control a

crucial treatment goal [1–3].

The majority of dialysis patients require oral phosphate

binders [4]. Despite the development of numerous oral

phosphate binders, phosphatemia control has not improved

significantly over the past decade. Poor medication

adherence—the process by which patients take their med-

ications as prescribed [5]—has been identified as an

important contributing factor [6–8] and is challenging due

to the intake pattern (during meals and snacks), high pill

burden, side-effects and lack of noticeable effect.

Addressing nonadherence requires insight into both its

prevalence and associated factors, as a basis for the

development of interventions to improve adherence [6].

Many studies have measured adherence through patient

self-report, pill count or prescription refill data which have

proven to be not the most reliable methods [9]. Electronic

monitoring is one, if not the main, option in that it gener-

ates objective data [9]. A variety of (non)adherence

determinants (demographic, clinical and psychosocial)

have been studied but the associations are often inconsis-

tent. Further, many predictors likely to be important

determinants have not been fully explored (such as medi-

cation regimen, patients’ beliefs and social support) [6].

The purpose of this study was therefore (1) to estimate

the extent of phosphate binder nonadherence using elec-

tronic monitoring, and (2) to identify potential determi-

nants of adherence.

Subjects and methods

Study design

This article reports the results of an observational analysis

based on the baseline period of a multi-center randomized

clinical trial testing a nurse-led intervention to enhance

adherence to phosphate binder therapy. The first 2 months

of the 1-year trial included a blind, observational, baseline

period (without any randomization or intervention)—on

which we report here. Approval was obtained from the

Ethics Committee of the Antwerp University Hospital,

Belgium (B300201111744).

Study sample

The total sample was powered for the intervention study

and 135 dialysis patients were included. Considering

adherence as a random event with 0.5 probability, the

above sample size resulted in a 9 % precision of the point

estimate of the prevalence of adherence (95 % CI 41–59).

Patients were recruited at three Belgian hemodialysis

centers in November 2011. Patients were eligible if they

were: (1) adults (aged C18 years), (2) receiving chronic

hemodialysis C1 month, (3) treated with phosphate binders,

and (4) Dutch-speaking. When prescribed a combination of

phosphate binders, the types of phosphate binder therapy

monitored were the new generation (sevelamer or lanthanum

carbonate). Exclusion criteria comprised: (1) professional

medication care, (2) cognitive impairment, or (3) nursing

home residents. Participants signed an informed consent.

Data collection

At inclusion, medical and pharmacological data were col-

lected from patients’ medical charts and patients filled out

questionnaires surveying: (1) demographics, (2) attitudes,

(3) knowledge, (4) social support, and (5) health and

quality of life [using the validated short form (SF)-36

health survey] [10]. Patient attitudes investigated included

problems experienced with intake (e.g. inconvenience due

to phosphate binders’ taste, shape or size), perceived side-

effects, belief in effectiveness and self-reported adherence

(always, mostly, mostly not). Knowledge about the therapy

was measured by ten multiple-choice questions about the

phosphate cycle, phosphate binder pharmacodynamics, and

dietary recommendations. Social support was scored by 11

questions assessing perceived support in general and sup-

port with taking phosphate binders specifically. Both the

knowledge and social support tests were developed by the

authors in collaboration with nephrologists and dialysis

nurses and are added as ESM appendices. Phosphatemia

and calcemia were gathered from patients’ charts. We

collected all measures taken as part of the routine care,

which was weekly in one center (averaged per month) and

monthly in the other two centers.

The primary outcome variable was phosphate binder

adherence, measured electronically with the medication

event monitoring system (MEMS�). Drug containers were

filled with patients’ phosphate binders and then capped

with MEMS. Patients were to take their phosphate binders

only and directly from the container at the time of intake

and to open the container only for an intake. The microchip

in the MEMS cap registered date and time of each opening

(i.e. the presumed intake). The dosing history data were

downloaded from the MEMS and centralized through a

secured server. The MWV Healthcare adherence platform�

stored the adherence data, and released them only at the

end of the baseline study.

Data analysis

Data were analyzed using SPSS Statistics 20�. The

adherence summary data were adjusted for regimen chan-

ges and hospitalization periods were excluded. When

patients dropped out (e.g. because they died), their

J Nephrol

123

adherence data were analyzed up until the week of drop-out

and thereafter kept as missing values. Mean adherence was

calculated as the ratio of taken/prescribed doses, averaged

per week [e.g. patients with a ter in die regimen (21 doses/

week) and taking all 21 would have a mean adherence of

21/21 = 100 % in that week]. As additional phosphate

binders are taken during snacks rich in phosphate, mean

adherence might exceed 100 %. ‘Being adherent’ was

defined as having missed weekly\1 total daily dose, which

equals an intake[6/7 (quaque die regimen), [12/14 (bis in

die) or[18/21 (ter in die), or [85.7 % (all regimens). Over

the 2-month period, we defined adherent patients as those

being adherent in each of the 8 weeks monitored (i.e.

8 weeks with a mean adherence [85.7 %).

Differences between adherent and nonadherent

patients were identified using independent sample t tests

(continuous variables) and v2 tests (categorical).

Fig. 1 Flow chart of study, showing details of patient recruitment.

MEMS medication event monitoring system

Table 1 Characteristics of study subjects (n = 135)

Socio-demographic characteristics

Age [mean years (range)] 67 (25–90)

Gender

Male 64 %

Living

With partner 50 %

Alone 30 %

With partner and children 13 %

With parent(s) or other 5 %

Profession

Retired 69 %

None 24 %

Working 7 %

Social supporta [mean score (SD)] 76 % (22)

With dialysis treatment 87 %

With phosphate binders intake 42 %

Health characteristics

Renal diagnosis

Renal vascular disease 31 %

Diabetic nephropathy 20 %

Polycystic kidney disease 14 %

Etiology unknown 8 %

Glomerulonephritis 6 %

Otherb 21 %

Dialysis months [mean (range)] 36 (1–286)

Dialysis high care 59 %

Dialysis Mon–Wed–Fric 64 %

On kidney transplant list 13 %

Body mass index [mean (range)] 26 (16–44)

Alcohol-use 31 %

Smoking 13 %

Physical activity never 74 %

Comorbidities [mean (range)] 2 (0–5)

Hypertension 75 %

Vascular disease 41 %

Heart failure 37 %

Diabetes 35 %

Hospitalization(s) in year \inclusion 66 %

Number [mean (range)] 1 (0–5)

Total days [mean (range)] 17 (1–180)

SF-36 (health and quality of life scale)

Mental score [mean % (range)] 51 (27–70)

Physical score [mean % (range)] 38 (12–63)

Phosphatemia [mean mg/dl (range)] 4.9 (2.4–8.7)

Calcemia [mean mg/dl (range)] 9.1 (6.8 – 10.7)

Pharmacological characteristics

Phosphate binder therapy monitored

Types

Calcium acetate 46 %

Sevelamer 20 %

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123

Pearson’s correlations were applied to calculate corre-

lations. Differences and correlations were judged sta-

tistically significant when the p value B0.05. Odds

ratios of influencing factors of adherence were calcu-

lated by logistic regression, building a multivariate

model to identify the factors explaining the variance in

adherence, with the significance of the odds ratios

judged by the 95 % CI.

Results

Study sample

Figure 1 shows a flow chart of the study and patient

recruitment process. In total, 161 subjects were eligible and

the non-response rate was 16 %, bringing the total number

of enrolled patients to 135. Patients’ characteristics are

described in Table 1. The phosphate binders most fre-

quently monitored were calcium acetate and sevelamer.

One in five patients experienced side-effects (all gastro-

intestinal), and also one in five reported intake inconve-

niences mainly related to the tablet size (60 %) and taste

(30 %).

Adherence to phosphate binders

An overall mean (standard deviation, SD) of 78 % (29) of

the prescribed doses was taken over the 2-month period.

The average adherence per week ranged from 76 to 82 %.

About half of the patients were adherent (missing \1 total

daily dose per week) (Fig. 2). When looking at the entire

8-week period, one-fifth of patients (22 %) were consis-

tently adherent (on all 8 weeks).

Factors associated with adherence to phosphate binders

Table 2 lists the determinants of adherence. Being adherent

was associated with living with a partner and with having a

higher social support, both of these being interrelated

(patients living with a partner had an 8 % higher social

support score than those living alone or with others (80 vs.

72 %; p = 0.055). A higher SF-36 physical score was

associated with being adherent, and experiencing intake

inconveniences (e.g. unpleasant taste) negatively affected

adherence (p = 0.056); 26 % of the variance in being

adherent was determined by the social support and SF-36

physical scores (p \ 0.001).

Association between self-reported and electronically

measured adherence

Figure 3 shows adherence according to MEMS versus self-

reported adherence. Patients self-reporting to always

adhere to their treatment, to be mostly adherent, or to be

mostly nonadherent had a respective mean MEMS adher-

ence of 77, 69 and 32 %.

Lanthanum carbonate 13 %

Sevelamer carbonate 11 %

Calcium carbonate (CaCO3) 10 %

Regimen 3x/day 73 %

Perceived side-effect(s) 21 %

Intake is inconvenientd 19 %

Belief in effectiveness 79 %

Self-reported total adherencee 58 %

Second phosphate binder 32 %

Knowledgef [mean (SD) out of 10] [5.6 (2.7)/10]

Oral daily pill burden [mean (range)] 12 (2–27)

Chronic analgesic use 36 %

Chronic sleeping pill use 32 %

Chronic antidepressant use 19 %

a Based on 11 questions assessing perceived support with the he-

modialysis treatment in general (transport to dialysis centre, someone

to talk to, etc.) and support with taking phosphate binders medications

specifically (laying out medication, reminding to take pills, etc.)b Other causes (all \5 %) included mainly analgesic nephropathy,

immunoglobulin (Ig)A nephropathy, glomerulosclerosis, Wegener’s

granulomatosis, lupus, trauma and tuberculosisc Patients were either dialysed on Monday, Wednesday and Friday or

on Tuesday, Thursday and Saturdayd Because of the phosphate binder’s taste, shape or sizee Self-reported adherence categories included: always (58 %), mostly

(37 %) and mostly not (5 %)f Assessed by ten multiple choice questions about phosphate cycle,

phosphate binder pharmacodynamics and dietary recommendations to

control serum phosphate

Fig. 2 Proportion of adherent patients per week (missing \1 total

daily dose per week)

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123

Association between phosphatemia and adherence

The correlation between mean adherence to treatment

and phosphatemia was inversely proportional (y =

-0.008x ? 5.5, r = -0.191, p = 0.026). As displayed in

Fig. 4, adherent patients had significantly lower mean

serum phosphate levels than nonadherers in 608 observa-

tions (all the available phosphatemia values as collected

from patients’ charts) (p \ 0.001). Over the entire period,

mean phosphatemia was 0.55 mg/dl lower in adherent than

nonadherent patients (4.76 vs. 5.31 mg/dl).

Discussion

Main findings

Every week, only half of the patients were adherent. For all

8 weeks taken together, roughly one in five patients was

adherent. Factors associated with being adherent were

higher physical quality of life, higher social support score

and living with a partner (the latter two interrelated). Intake

‘inconveniences’ negatively affected adherence. The social

support and SF-36 physical scores explained 26 % of the

Table 2 Determinants of

adherence

a Being adherent is defined as

missing weekly \1 total daily

dose (i.e. 8 weeks with a mean

adherence [85.7 %)b Based on eleven questions

assessing perceived support

with the hemodialysis treatment

in general (e.g. transport to

dialysis centre, someone to talk

to, …) and support with taking

phosphate binders medications

specifically (e.g. laying out

medication, reminding to take

pills,…)c Assessed by ten multiple

choice questions about

phosphate cycle, phosphate

binder pharmacodynamics and

dietary recommendations to

control serum phosphated Nagelkerke r2 = 0.256

Adherenta p Univariate Multivariate

Yes

(30)

No

(105)

Odd’s ratio

(95 % CI)

Odd’s ratio

(95 % CI)

Socio-demographics

Age: mean years 70 67 0.273 1.014 (0.985–1.043)

Gender: female 33 % 36 % 0.844 0.905 (0.384–2.137)

Living: with partner 70 % 44 % 0.013 2.942 (1.231–7.033)

Profession: retired 77 % 67 % 0.327 1.596 (0.623–4.086)

Social supportb: mean score (%) 87 74 \0.001 1.043 (1.014–1.073) 1.053

(1.020–1.087)d

Health characteristics

Renal diagnosis: vascular disease 21 % 34 % 0.173 0.506 (0.188–1.364)

Dialysis: mean months 33 37 0.547 0.996 (0.982–1.009)

Dialysis: high care 60 % 59 % 0.925 1.040 (0.455–2.380)

On kidney transplant list 17 % 13 % 0.555 1.400 (0.456–4.301)

Body mass index: mean 27.3 25.9 0.726 1.052 (0.975–1.136)

Smoking 7 % 15 % 0.217 0.393 (0.085–1.815)

Physical activity: never 67 % 76 % 0.307 0.633 (0.262–1.529)

Comorbidities: mean number 2.6 2.3 0.230 1.139 (0.838–1.549)

Hospitalization(s) in \year 60 % 68 % 0.398 0.679 (0.301–1.613)

SF-36 physical: mean score 43 37 0.004 1.060 (1.015–1.107) 1.078

(1.027–1.132)d

SF-36 mental: mean score 53 51 0.409 1.016 (0.979–1.055)

Pharmacological characteristics

Phosphate binder

Type

Calcium acetate 43 % 47 % 0.836 0.858 (0.379–1.946)

Regimen: 3x/day 87 % 72 % 0.085 2.635 (0.847–8.198)

Number of tablets per day 3.5 3.3 0.710 1.047 (0.823–1.332)

Second phosphate binder 43 % 29 % 0.134 1.886 (0.816–4.358)

Perceived side-effects 17 % 22 % 0.518 0.704 (0.243–2.045)

Intake is inconvenient (e.g.

taste)

7 % 22 % 0.056 0.252 (0.056–1.136)

Belief in effectiveness 87 % 77 % 0.248 1.950 (0.619–6.142)

Knowledge scorec: mean 5.8/

10

5.5/

10

0.536 1.049 (0.902–1.220)

Oral daily pill burden: mean 4.6 4.7 0.587 1.024 (0.940–1.116)

Antidepressant use 17 % 20 % 0.667 0.790 (0.270–2.311)

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variance in being adherent. Adherent patients had a sig-

nificantly lower phosphatemia than nonadherers.

Adherence to phosphate binders

In our study, only about 50 % patients were adherent to

their treatment. Two reviews, published in 2008 and 2009,

found comparable mean proportions of 51 and 67 % [6, 8].

The vast majority of the included studies measured

adherence through self-report and/or serum phosphate and

only one study used MEMS. One in six patients preferred

not to participate in our study. It is likely that non-

responding patients might be nonadherent. The actual ‘‘real

life’’ proportion of nonadherent patients might thus amount

to 66 %, presuming all non-responders were nonadherent.

Since the appearance of those reviews [6, 8] little

research has been conducted on phosphate binder

(non)adherence. A 2010 study using self-report identified

21 % of patients as nonadherent [11]. A 2009 case series

study (n = 7), also using MEMS, found a mean adherence

of 77 % (calcium acetate) and 80 % (sevelamer)—com-

parable to the 78 % found in our study [12]. More studies

using objective (e.g. electronic) adherence measurement

are needed to map the real extent of phosphate binder

nonadherence.

Adherence measurement

We only measured intake adherence, and not other com-

ponents such as initiation (taking the first prescribed dose).

Neither did we calculate the number of doses taken on

time, as phosphate binders are taken during meals and

snacks. The measurement was restricted to 2 months and

we applied traditional analysis techniques, not fully taking

into account the longitudinal character of the adherence

data.

Comparing our results with the existing literature is

difficult because adherence is defined with great variation

[13]. The cut-off value most often used is 80 %, but we feel

this value has limited clinical relevance—meaning as much

missing a weekly dose of ‘‘1.4’’ (quaque die prescription).

We therefore tried to use an, in our opinion, more con-

ceivable/meaningful approach, defining adherence as hav-

ing missed \1 total daily dose per week.

Factors associated with adherence to phosphate binders

Most of the questionnaires used were developed by the

researchers and have not been validated. Few factors were

associated with adherence which is in line with previous

research [6, 11]. Although often found to significantly

influence adherence [6, 14, 15], the phosphate binder reg-

imen was not retained in our model, possibly because most

patients had a ter in die regimen. Neither did we find the

type of phosphate binder or the number of tablets per day to

be adherence-influencing factors.

As many psychosocial factors are likely to influence

adherence [6, 16, 17], we tested beliefs, social support,

therapeutic knowledge and antidepressant use but only

social support and the SF-36 physical score resulted to be

adherence determinants. A recently conducted study also

found the SF-36 physical score to be higher in adherent

patients [18]. As social support was the only ‘‘modifiable’’

adherence determinant, adherence enhancing interventions

should aim to improve this aspect, e.g. by involving an

informal caregiver or by enhancing contact between fellow

patients.

Fig. 3 Mean adherence to phosphate binders according to MEMS vs.

self-reported adherence. MEMS medication event monitoring system

Fig. 4 Mean phosphatemia (error bars ±1 standard error) in adher-

ent vs. nonadherent patients

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123

Future research

The results reported in this article are part of a multi-center

randomized clinical trial. In the next phase of the study

patients will be randomized. The intervention group will be

given a first and one-time intervention consisting of edu-

cation, social support and skills (e.g. cue-dose) training,

and thereafter a monthly follow-up intervention (individ-

ualized counselling). The results of the study will be

available in winter 2013.

Conclusion

Phosphate binder nonadherence continues to be a major

problem and it is a crucial aspect that needs to be addressed

in order to improve phosphatemia control in hemodialysis

patients. Interventions to improve adherence need to be

developed and should aim to enhance—at least—social

support. Finding few factors associated with nonadherence

and yet a high prevalence of nonadherence, we believe an

individualized approach is indicated. When reporting

adherence, we advocate a meaningful approach—stepping

away from the widely used 80 % cut-off towards a more

conceivable definition, such as the number of weekly or

monthly doses missed.

Acknowledgments Sincere thanks to Vocatio vzw for acknowl-

edging and supporting the curriculum of Yoleen Van Camp. We

greatly appreciate the professional and technical support by MWV

Healthcare� and the cooperation of all the participating patients.

Many thanks for the efforts of the local study nurses, assisting master

students and nephrologists Paul Arnouts, Koen Bouman, Ronald

Daelemans, Katja De Grande, Elfie Deprez, Wendy Engelen, Heidi

Hoeben, Ann Scheipers, Leen Torremans, Daniel Van Caesbroeck,

Ludo Van Doorslaer, Sarina Van Loock, Jeannine Van Loon, Bob

Van Santbergen and Sandra Vervynckt.

Conflict of interest None.

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