The Effect of Numeracy Level on Completeness of Home Blood Pressure Monitoring

Abstract

Home blood pressure monitoring (HBPM) readings predict the increased risks of cardiovascular events and end‐organ damage independent of office blood pressure (BP). Numeracy (the ability to handle numbers) may limit the feasibility of patients' performing HBPM. The authors analyzed data from 409 adults recruited from 12 North Carolina primary care clinics who completed a three‐item numeracy assessment, the Rapid Estimate of Adult Literacy in Medicine–Short Form health literacy assessment, and HBPM over 2 weeks. Among the 409 participants, 73% were college graduates and 69% had adequate numeracy. Completion of HBPM was greater among those with adequate numeracy (96.2% vs 93.7%; P=.009) and did not correlate with health literacy scores. More participants with adequate numeracy reported completion of ≥85% of readings than those with low numeracy (95% vs 88%; P=.018). Adequate numeracy, but not high literacy, is associated with more complete HBPM reporting. Whether higher numeracy is associated with more accurate self‐reported readings is an area of future research.

Hypertension increases the risk of cardiovascular disease (CVD) and stroke‐related morbidity and mortality.1, 2, 3 It is highly prevalent in the United States and accounts for $46.4 billion annually in direct and indirect health‐related costs.4 Between 2003 and 2010, nearly 70 million Americans had hypertension, and, among those, about one half had it under control.5 Appropriate identification and treatment of high blood pressure (BP) is an important public health issue as it may significantly reduce cardiovascular comorbidity and mortality.

Traditionally, clinicians diagnose and manage hypertension using BP measurements performed in the office setting. Self‐measured BP at home, or home BP monitoring (HBPM), is a useful strategy for providing clinicians with out‐of‐office measurements of BP. Furthermore, HBPM readings predict risk of cardiovascular events and end‐organ damage independent of office BP.6, 7, 8 With HBPM, the patient uses a portable device in the home setting to measure BP. The recommended timing and frequency of measurements may vary, but one systematic approach using HBPM for acquiring a patient's “usual” BP calls for two sets of multiple BP measurements during a day for several consecutive days.9 HBPM can be useful not only for diagnosing hypertension but also for tracking BP over time. However, its use is limited by patient effort and understanding.

Health literacy is the ability by which an individual can attain, process, and understand health information to make educated health decisions.10 In particular, numeracy, one component of overall health literacy, may affect the way patients process numerical information or the patient's ability to successfully complete health‐related tasks in and out of the medical setting.11, 12, 13, 14 Low numeracy skills have been shown to interfere with patients' self‐efficacy and health‐related skills.15 Numeracy level has been shown to vary even in highly educated and literate populations,11, 16 and, because some home BP monitors require patients to measure and record BP values with time, numeracy level may be more relevant to the successful completion of HBPM and may be more relevant than print literacy alone (Table 1). To our knowledge, the association of numeracy with quality and completeness of home BP monitoring has not been examined.

Table 1.

Topics Previously Known on Numeracy and Health‐Related Outcomes

Topics Known on Numeracy and Outcomes	Main Points of This Research
A person's ability to interpret health information, a person's self‐efficacy, and reported quality‐of‐life decrease with lower numeracy, although the strength of evidence is low15	Assess the relationship between numeracy and completion of home blood pressure monitoring
There is insufficient evidence in the effect of numeracy on accuracy of risk perception, ability to take medications, disease prevalence, health knowledge, and use of healthcare services15	Identify factors that may mediate the role between numeracy and health‐related skills
Numeracy level has been shown to vary even in highly educated and literate populations11, 16	Identify potential barriers to home blood pressure monitoring

The purpose of this study is to investigate the relationship between numeracy level and completeness of home BP reporting and identify factors that mediate this possible relationship with a specific focus on demographic and socioeconomic characteristics.

Methods

Overall Design and Study Participants

This cross‐sectional study was part of a larger study in which we recruited 420 participants from 12 primary care clinics in central North Carolina between October 2010 and June 2013. We also posted flyers in a clinical research center affiliated with the University of North Carolina at Chapel Hill. Participants had to be at least 30 years of age with most recent clinic systolic BP between 120 mm Hg and 149 mm Hg and diastolic BP between 80 mm Hg to 95 mm Hg, be able to read and speak English, and be able to attend study visits. We enrolled participants 30 years and older since they would potentially have elevated BPs that may lead to meaningful clinical outcomes or would have absolute cardiovascular risk high enough to justify risk‐reducing therapies. We excluded patients who were pregnant, had persistent atrial fibrillation or other arrhythmias, had known heart disease including coronary artery disease, had a history of dementia or cognitive disorders, had a diagnosis of diabetes, or took antihypertensive medications. We also excluded potential participants if their first research visit systolic BP was ≥160 mm Hg or ≤110 mm Hg or diastolic BP was ≥100 mm Hg or ≤70 mm Hg. This study was approved by the University of North Carolina institutional review board, and informed consent was obtained from each participant. The study complied with all aspects of the Health Insurance Portability and Accountability Act (HIPAA).

Health Literacy and Numeracy Assessment

We assessed participant health literacy using the Rapid Estimate of Adult Literacy in Medicine‐Short Form (REALM‐SF)17 and numeracy using a commonly cited three‐item numeracy measure.18 Each of these measurement tools has been previously validated and widely reported.17, 18 Participant literacy scores were determined by the pronunciation of and time to read medical words (eg, menopause, antibiotics, exercise, jaundice, rectal, anemia, and behavior). A score of 0 corresponded with a third‐grade reading level or below, while a score of 7 corresponded with at least a high school education and an ability to read most patient education materials.17 Participant numeracy scores were calculated by answering three questions that assessed basic familiarity with probability, ability to convert a percentage into a proportion, and ability to convert a proportion back into a percentage.18 Higher numeracy scores correspond with greater accuracy in interpreting numerical information and applying risk reduction by this measure.18 These measurements were conducted following initial research office BP measurements in order to avoid the possibility of influencing BP. They were also conducted on separate visits to minimize questionnaire burden. Using the three‐item numeracy scale, we defined “adequate numeracy” as a score of 2 or 3 and “low numeracy” as a score of 0 or 1.

Office BP Measurements

We obtained three research office BP measurements from the nondominant arm with the participant seated with their feet on the floor. We used an automatic oscillometric monitor to record measurements at 1‐minute intervals using an appropriate cuff size after an initial 5 minutes of rest to minimize variability in measurements.19

Home BP Monitoring

We asked participants to perform out‐of‐office BP measurements in between office visits during two consecutive weeks. We used the Omron 705 CP, an independently validated automatic monitor, for all home BP measurements.20, 21 This device uses automatic inflation and deflation to measure and report systolic and diastolic blood pressures and pulse pressures. It has the ability to store up to 28 recordings and print out readings in numerical or graphical form. Participants were instructed to apply the BP cuff to the non‐dominant arm with the proper cuff size determined by upper arm circumference. If the participant's arm circumference was too large and could not be accommodated by the available BP cuffs, he or she was provided with a Braun Vital Scans Plus wrist BP monitor to obtain home measurements.22 This wrist BP monitor measures and displays only systolic and diastolic blood pressures with each use. It has the ability to store up to 90 readings and can average the most recent three readings. An in‐office test measurement was performed to demonstrate adequate fit and comfort. All subjects were given standardized oral and written instructions on how to the use the BP device. They were instructed to rest in the seated position for 5 minutes, and then make three measurements at 1‐minute intervals, and to record values in the mornings and evenings for five consecutive days.9, 23 For every measurement, participants were asked to record in writing the date, time, and systolic and diastolic BP readings onto a pre‐printed form. “Completeness” of home BP measurements was calculated as the percentage of total number of recorded measurements that were supposed to be taken by the participant during the 2 week period that participants performed HBPM. Thus, a participant who reported 60 systolic and 60 diastolic BPs during 2 weeks would have 100% completeness

$$\left(\frac{\begin{array}{c}\hfill 2measures(1\text{SBP}+1\text{DBP})\times 3\text{iterations}\times \\ \hfill \text{twice a day}\times 5\text{days}\times 2\text{measurement periods}\end{array}}{120\text{total possible measurements}}\right).$$

Other Measures

We collected information on race, ethnicity, marital status, education, health status, employment status, and household income.

Statistical Analysis

For primary analyses, participants were grouped according to their numeracy score as adequate numeracy or low numeracy (defined above). Participant characteristics were calculated by numeracy level. We then used analysis of covariance to assess group differences in completeness of HBPM among individuals with “low” and “adequate” numeracy levels. In an initial model we adjusted for several potential confounders with numeracy level as the exposure and mean percentage of home BP recordings complete as the outcome. There was a strong correlation between education levels and participant numeracy scores (Table S1; Goodman and Kruskal's γ=0.63; Pearson's χ²=92.6; P<.001). Given this correlation, we did not include education in our model as it may serve as an explanatory factor between numeracy and HBPM. In our final adjusted model, we omitted covariates that had relatively equal distribution among those with adequate and low numeracy and covariates that did not have a meaningful effect on adjusted home BP reporting percentages (ie, statistically not significant or clinical difference <5%) by numeracy level. Covariates in the final model included sex, race, marital status, health status, income level, and literacy level.

To further evaluate the relationship of numeracy level with HBPM completeness, individuals were stratified with the outcome of ≥85% vs <85% of completeness of home BP reporting. Although studies indicate variable numbers of minimum HBPM measurements needed to make a clinical decision,9 we selected this threshold to provide an average estimate of home BPs while minimizing reporting bias by participants. The odds ratio for having completeness of BP ≥85% was then calculated using a logistic regression model.

As a secondary analysis, we also assessed the correlation between health literacy and numeracy scores, and analyzed the relationship between health literacy scores and completion of HBPM reporting using simple linear regression. The differences in completed HBPM reporting by low health literacy (score of 5 or less) and high health literacy (score 6 or 7) were compared post hoc using Student t test. We also assessed the difference in reported numerical home BP values between numeracy groups. We compared the outcomes mentioned above between participants with low numeracy level with those with adequate numeracy level using the two‐sample Student t test for continuous variables or chi‐square for categorical variables. All statistical analyses were performed using Stata 13 (StataCorp LP, College Station, TX).

Power Estimation

This study sample of 420 participants was available as part of a larger BP measurement study. This relatively large sample gave us an 80% power to detect a 5% difference in the effect of numeracy groups on completeness of BP measurement, assuming normal distribution of numeracy scores across the sampled population.

Results

Characteristics of Sample

A total of 409 participants performed HBPM and completed the numeracy level assessment (99.5%). The other 11 participants did not complete the numeracy assessment and thus were not included in the analysis. Fourteen participants (3%) required a wrist BP monitor for home BP measurements. The mean age of all participants was 47.9 years (Table 2). In this study population, 31% had low numeracy. More participants in the low numeracy group than the adequate numeracy group were female (79% vs 45%) and black (44% vs 17%), and the distribution in education levels was lower in the low numeracy group with fewer college graduates than in the adequate numeracy group (51% vs 84%). The mean research office BP was 125/78 mm Hg (±34/32 mm Hg) among those with low numeracy level and 129/81 mm Hg (±25/23 mm Hg) among those with adequate numeracy level. Reported morning home BP averages were 128/80 mm Hg (±10/7 mm Hg) among those with adequate numeracy vs 129/80 mm Hg (±11/11 mm Hg) among those with low numeracy (∆BP of −1/0 mm Hg; P=.5/.6; Table 3). Home BP averages reported in the evening were 130/80 mm Hg (±10/8 mm Hg) in the adequate numeracy group vs 131/81 mm Hg (±11/8 mm Hg) in the comparison group (∆BP of −1/−1 mm Hg; P=.7/.1).

Table 2.

Study Participant Characteristics by Numeracy Level

Demographics	Total (N=409)	Numeracy Level		P Valuea
		Low (0–1) (n=126)	Adequate (2–3) (n=283)
Age, mean (SD), y	47.9 (12.0)	47.9 (11.7)	47.9 (12.1)	.99
Female, No. (%)	228 (56)	100 (79)	128 (45)	<.001
Race, No. (%)				<.001
White	306 (75)	71 (56)	235 (83)	<.001
Black	87 (21)	51 (40)	36 (13)	<.001
Asian	11 (3)	2 (2)	9 (3)	.36
Other	5 (1)	2 (2)	3 (1)	.65
Ethnicity, No. (%)
Hispanic	16 (4)	8 (6)	8 (3)	.09
Non‐Hispanic	393 (96)	118 (94)	275 (97)
Marital status, No. (%)				.002
Married	237 (58)	57 (45)	180 (64)	.001
Widowed	9 (2)	6 (5)	3 (1)	.018
Living with partner	30 (7)	15 (12)	15 (5)	.018
Separated/divorced	73 (18)	27 (21)	46 (16)	.2
Never married	60 (15)	21 (17)	39 (14)	.4
Education, No. (%)				<.001
Some high school	5 (1)	4 (3)	1 (0)	.017
High school grad	24 (6)	17 (13)	7 (2)	<.001
Some college	79 (19)	41 (33)	38 (13)	<.001
College grad	301 (74)	64 (51)	237 (84)	<.001
Health, No. (%)				<.001
Excellent	79 (19)	19 (15)	60 (21)	.15
Very good	198 (48)	49 (39)	149 (53)	.010
Good	109 (27)	44 (35)	65 (23)	.012
Fair	22 (5)	13 (10)	9 (3)	.003
Poor	1 (0)	1 (0)	0 (0)	.13
Employed, No. (%)	321 (78)	92 (73)	229 (81)	.073
Household income, No. (%), $				<.001
<15,000	25 (6)	14 (11)	11 (4)	.005
15,000–19,999	9 (2)	6 (5)	3 (1)	.018
20,000–24,999	15 (4)	7 (6)	8 (3)	.17
25,000–29,999	17 (4)	7 (6)	10 (4)	.34
30,000–34,999	18 (4)	3 (2)	15 (5)	.19
35,000–39,999	18 (4)	12 (10)	6 (2)	.001
40,000–49,999	29 (7)	12 (10)	17 (6)	.19
50,000–79,999	94 (23)	38 (30)	56 (20)	.019
80,000–99,999	51 (13)	13 (10)	38 (13)	.39
≥100,000	132 (32)	13 (10)	119 (42)	<.001
Literacy score, mean (SD)	6.87 (0.46)	6.76 (0.68)	6.92 (0.32)	.001
Clinic blood pressure, mean (SD), mm Hg	128/80 (28.5/25.9)	125/78 (34.1/31.7)	129/81 (25.5/22.8)	.2 .3

^a P value calculated using chi‐square test for categorical variables and t test for continuous variables.

Table 3.

Reported Home Blood Pressures by Numeracy Level

Demographics	Total (N=409)	Numeracy Level		P Valuea (SBP/DBP)
		Low (0–1) (n=126)	Adequate (2–3) (n=283)
Overall
Morning BP, mean (SD), mm Hg	128/80 (11/9)	129/80 (11/11)	128/80 (10/7)	.5/.6
Evening BP, mean (SD), mm Hg	130/80 (11/8)	131/81 (11/8)	130/80 (10/8)	.7/.1
Week 1
Morning BP, mean (SD), mm Hg	129/80 (11/8)	129/81 (12/8)	128/80 (11/7)	.5/.2
Evening BP, mean (SD), mm Hg	130/80 (11/8)	131/81 (12/8)	130/79 (11/8)	.4/.1
Week 2
Morning BP, mean (SD), mm Hg	128/80 (11/12)	129/80 (12/18)	128/80 (11/8)	.6/.9
Evening BP, mean (SD), mm Hg	130/80 (11/8)	130/81 (11/8)	130/80 (11/8)	.9/.1

Abbreviations: BP, blood pressure; DBP, diastolic blood pressure; SBP, systolic blood pressure; SD, standard deviation.

^a P value calculated using t test.

Home BP Reporting

The home BP reporting averages are shown in Table 4. The unadjusted mean completeness of HBPM reporting among those with low numeracy level was 93.7% vs 96.2% among the adequate numeracy group. After adjusting for sex, race, marital status, health status, income level, and literacy level, the difference in mean completeness of HBPM reporting between both groups was small at 2.6% (93.6% vs 96.2%; P=.02). There was no relationship between completeness of HBPM reporting and health literacy scores (r=0.0002, P=.8), and when stratified by low vs high literacy scores, the difference in completion rates between groups was not statistically significant (99% vs 95%, P=.09).

Table 4.

Home BP Completeness by Numeracy Level

	Numeracy Level		P Value
	Low (0–1) (n=126)	Adequate (2–3) (n=283)
HBPM, mean (SE), %a	93.7 (0.008)	96.2 (0.005)	.009
HBPM, mean (SE),%b	93.6 (0.009)	96.2 (0.005)	.020

Abbreviations: BP, blood pressure; HBPM, home blood pressure monitoring, SE, standard error.

^aUnadjusted using t test.

^bAdjusted for sex, race, marital status, health status, income level, and literacy level using linear regression.

Participants completing a minimum of 85% of reported HBPM are shown in Table 5. Of those with low numeracy level, 11.9% did not complete at least 85% of HBPM reporting compared with 5.9% in the adequate numeracy level group (P=.018), with an odds ratio of 2.4 (95% confidence interval, 1.1–5.1).

Table 5.

Eighty‐Five Percent or Greater Home Blood Pressure Reporting by Numeracy Level

	Numeracy Level		P Value
	Low (0–1) (n=126)	Adequate (2–3) (n=283)
≥85% reporting, %a	88.1	94.7	.018

^a P value calculated using chi‐square test.

Discussion

In this cross‐sectional study, we examined how adults who were not taking BP medications performed on a numeracy assessment and home BP reporting. Participants with lower numeracy scores completed the requested home BP reporting less often than those with higher numeracy scores, which held true after adjusting for sex, race, marital status, health status, income level, and literacy level. One third of our participants exhibited low numeracy despite having overall high literacy scores and education levels, which is consistent with other studies characterizing numeracy deficits in educated populations.11, 16 Although the absolute difference in completed home BP reporting by numeracy level was 3%, there was no difference in BP in the low and high numeracy groups. Further, within our study population, we found no clear relationship between health literacy scores and completion of home BP reporting.

This finding must be interpreted in the context of our sample in which college graduates comprised 51% and 84% of participants in the low and adequate numeracy groups, respectively. These levels of educational attainment among the numeracy groups are higher compared with the general US population, in which 34% of those aged 25 to 29 years have a college degree.24 The difference in home BP completion rates by numeracy level may be substantially higher in cohorts that are more representative of the US population and among those not recruited to participate in a BP measurement study.

Numeracy has more recently been studied alongside literacy as a set of essential skills that may play a role in the patient's understanding of health risks, knowledge, and medical decision‐making.11, 12, 13 Although most studies have assessed numeracy in the context of oral and written communication, data examining the relationship between numeracy and the patient's ability to perform health‐related tasks are insufficient.15 Our study findings suggest that low numeracy may be an additional barrier to successful HBPM completion. We hypothesize that the association between low numeracy level and less HBPM completion could, in part, be attributed to a fear of working with numbers. Existing studies report that other barriers to successful HBPM completion include failure of recognized benefits, lack of knowledge of cuff use, time required for monitoring, forgetfulness, lack of personal assistance, and misunderstanding of how to report.25, 26 Future studies are needed to further elucidate these potential explanations. We did not collect data to distinguish among them.

Low health literacy has been associated with poorer ability to take medications appropriately, difficulty in interpreting health messages, reduced use of some preventive health services, increased emergency department visits and hospitalizations, and increased mortality among older populations.15, 27 Health literacy has also been shown to play a role in increased prevalence of hypertension and reduced hypertension‐related knowledge.28, 29, 30 Within our study population, we found no clear relationship between health literacy scores and completion of home BP reporting. Although differences in health literacy scores were statistically significant between the numeracy groups, both groups averaged at or above a high school reading level, minimizing the likelihood that a clinical difference in participant ability to read the health education materials explains the difference in home BP reporting.

Incomplete reporting of HBPM potentially limits the accurate identification of uncontrolled or masked hypertension in patients who would benefit from antihypertensive therapy,6, 31 limits monitoring of the effectiveness of therapy in treated uncontrolled patients to prevent secondary complications of chronic hypertension,6, 7, 8 and may lead to misclassification and potential overtreatment among patients with acceptable out‐of‐office BPs. Addressing barriers such as numeracy level may improve HBPM adherence and appropriate classification of patients with borderline high BPs.

Previous studies have determined that approximately 43% of primary care patients with hypertension perform HBPM, and, of these, the primary reasons for self‐measuring BP were out of curiosity to know their BPs (55.2%) or under advisement or oversight by a physician (29.6%–35.2%).32, 33 Those who were more likely to perform HBPM were older, had a history of transient ischemic attack or stroke, or had higher hypertension knowledge.33 In one study of primary care patients with hypertension, approximately one third reported using their monitor at least a few times per month, <30% reported using it every day, and some measured their BP only when they had certain symtoms.33 Less than one third indicated that they reported the BP measurements to their physician.32 The inconsistent use of HBPM may diminish effective management of hypertension among patients who are not yet at their target BP compared with its demonstrated effective use in other studies.34 Part of the problem may lie in patient understanding and manipulation of numerical BP values in the management of hypertension. It is worth noting that studies demonstrating the utility of home BP monitoring have utilized specific measurement protocols, and to assume that less regimented home monitoring contributes to better management may not be justified.

Study Limitations

In interpreting results, readers should be aware that our study has several limitations. We included only untreated patients with borderline elevated BPs, and the results may not be generalizable to populations with a more uniform distribution of clinic BPs or to patients with treated hypertension. HBPM was also performed over two separate weeks, and participants could have gained knowledge and skills when performing the health‐related task during the first week that may have influenced completion rates during the second week (Table S2). In addition, by participating in this research study about BP, participants may have been more motivated to complete home BP measurements. With regards to our numeracy assessment, we administered a three‐item widely used measure to categorize our study population into low and adequate numeracy groups. The outcome of home BP measurement completion may differ by use with other assessment tools and numeracy level stratifications. Regarding the quality of home BP measurements, we were unable to assess accuracy of reported home BPs because of the limited number of measurements that could be stored in the home BP monitor device memory and ability to be certain that readings remaining in memory always belonged to the participant. However, studies assessing the reliability of self‐reported BP have found a significantly higher proportion of erroneous reporting among those with elevated BP and heart rates that possibly could change management,35 which warrants further investigation into numeracy's role on reliability of reported home BPs.

Study Strengths

To our knowledge, this is the first study to report that higher numeracy level is associated with more complete reporting of home BPs. In this relatively well‐educated cohort, home monitoring was successfully accomplished even among those with low numeracy. Given the differences in demographic characteristics between our numeracy groups, numeracy level may also play a role in disparities among race, marital status, health status, and household income level. Numeracy may also serve as a predictor of poorer completion of other health‐related skills involving numerical information, and addressing low numeracy may help overcome similar barriers that also affect completion of HBPM. Furthermore, patients with low numeracy may benefit from strategies for reducing reliance on numeracy, such as using home BP monitors with device memory that automatically records BPs for the physician to review and telemetry.

Conclusions

Adequate numeracy, but not high literacy, is associated with more complete reporting of HBPM, although the difference is small. Further studies are required to examine the relationship between numeracy level and the quality of HBPM in order to assess whether numeracy level is associated with accuracy of reporting and other health outcomes.

Disclosures

No authors have any conflicting relationships to declare.

Supporting information

Table S1. Numeracy Level and Education Level

Table S2. Completeness of Home Blood Pressure Monitoring for Weeks 1 and 2 by Numeracy Level

J Clin Hypertens (Greenwich). 2015;17:39–45. DOI: 10.1111/jch.12443. © 2014 Wiley Periodicals, Inc.