Number 59 n October 5, 2012

Blood Pressure Randomized Methodology Study
 

Comparing Automatic Oscillometric and Mercury
 

Sphygmomanometer Devices: National Health and
 

Nutrition Examination Survey, 2009–2010
 by Yechiam Ostchega, Ph.D., R.N., Division of Health and Nutrition Examination Surveys;
 

Guangyu Zhang, Ph.D., Office of Research and Methodology; Paul Sorlie, Ph.D., National Institutes of Health,
 

National Heart, Lung, and Blood Institute; and Jeffery P. Hughes, M.P.H.; Debra S. Reed-Gillette, M.S.;
 

Tatiana Nwankwo, M.S.; and Sarah Yoon, Ph.D., R.N., Division of Health and Nutrition Examination Surveys
 

Abstract

Objectives—The mercury sphygmomanometer has been the gold standard used for

obtaining blood pressure (BP) for the National Health and Nutrition Examination

Survey (NHANES) from 1960 to the present. However, due to environmental concerns

and an increased use of automated oscillometric BP devices, NHANES has been

exploring an alternative to using the standard mercury sphygmomanometer (mercury)

to measure BP.

Methods—The accuracy of Omron HEM–907XL BP readings was compared with

that of mercury BP device readings for gender, age group, race and ethnicity, and body

mass index categories and cuff-size subgroups. Each person had three BP

measurements per device recorded sequentially. The order of the devices and readers

were randomly assigned. A total of 6,460 participants had three valid systolic readings,

and 6,338 had three valid diastolic readings.

Results—Omron and mercury measurements were correlated (r = 0.92, systolic

BP; r = 0.79, diastolic BP). Overall, the mean between-device differences (Omron and

mercury) were –1.6 mm Hg for systolic and –0.6 mm Hg for diastolic (p < 0.05 for

both). The mean between-device differences were less than or about 2 mm Hg for each

subgroup: gender, age group, race and ethnicity, and body mass index categories, and

cuff-size subgroups. The exceptions were mean systolic between-device differences for

those using the extra-large BP cuff (–3.1 mm Hg) and obese individuals (–2.6 mm Hg),

and the mean diastolic between-device differences for the underweight group (–3.5 mm

Hg). Assuming mercury to be the gold standard, between-device agreements for the

frequency of high BP (140/90 mm Hg or more) and stage II high BP (160/100 mm Hg

or more) were above chance (κ = 0.72 for both). Omron underestimated the high BP

frequency by 2.28% and stage II high BP frequency by 0.77%.

Conclusions—Lower estimates of high BP by the Omron device may require

adjusting future national prevalence estimates accordingly to account for betweendevice differences.

Keywords: National Health and Nutrition Examination Survey • mercury

oscillometric devices

Introduction

For many years, the standard

instrument for measuring blood pressure

(BP) in the National Health and

Nutrition Examination Survey

(NHANES) has been the mercury

sphygmomanometer. In recent years,

because of increased environmental

concerns about the disposal of mercurycontaminated medical waste and the risk

of spills from mercury

sphygmomanometers, clinical settings

(e.g., doctor’s offices, clinics, and

hospitals) have begun phasing out the

mercury devices. In addition, in a

number of cross-sectional state surveys

(1–4), an automatic oscillometric device

has replaced the mercury device in

assessing BP. In 1999, the

Environmental Protection Agency and

the American Hospital Association

published a report in which both

agencies agreed to eliminate mercurycontaining waste from the health care

industry by 2005 (5). In that report, both

agencies’ recommended best practices

were discussed and, based on those

recommendations, federal hospitals

eliminated mercury-containing

instruments, including

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Center for Health Statistics 
Page 2 National Health Statistics Reports n Number 59 n October 5, 2012

sphygmomanometers. Similarly, the

National Institutes of Health’s ‘‘Mad as

a Hatter’’ Mercury-Free Campaign goal

is to eliminate all unnecessary uses of

mercury in NIH facilities (6). Along

these lines, NHANES has been

considering alternatives to the mercury

sphygmomanometer for future survey

cycles.

Among the automatic oscillometric

devices, the Omron BP monitor is

designed and utilized as a clinical BP

monitor and was used by the National

Heart, Lung, and Blood Institute study

known as Coronary Artery Risk

Development in Young Adults or

CARDIA (7). Omron was previously

validated in multiple studies (8,9); all

past validation studies were performed

using small sample sizes and in a study

setting. Therefore, in 2006–2007,

NHANES conducted a study using a

convenience sample of 509 persons in

the mobile examination center (MEC)

that compared the mercury

sphygmomanometer and the Omron

HEM–907XL BP monitor, following the

Association for the Advancement of

Medical Instrumentation’s (AAMI) 2002

criteria as a guideline (10,11). With the

exception of diastolic BP in youths aged

13–19 years [mean difference –1.77 mm

Hg; standard deviation (SD), 8.65], the

Omron device met the AAMI validation

criteria (11).

In contrast with the first study,

which used a convenience sample, the

motivation for the second study was to

determine whether the results of the first

study could be replicated in the usual

NHANES MEC standardized data

collection environment with regular

survey participants. Therefore, in early

2009, a BP randomized methodology

study was conducted during a 2-year

NHANES data collection cycle (2009–

2010) to compare the Omron and

mercury devices with the ultimate goal

of assessing whether the mercury

sphygmomanometer can be replaced

with the Omron device,

The overall objective of this study

was to compare mercury

sphygmomanometer (mercury) readings

to Omron HEM–907XL (Omron)

readings using a randomized study

design in a standard NHANES

environment. The specific objectives of

the analysis were twofold. The first

objective was to compare the measured

values of BP obtained by Omron with

those obtained by mercury overall and

by gender, age, race and ethnicity, BP

cuff size, body mass index (BMI), and

irregular heart rate. The second

objective was to compare high BP

classification agreement between the

devices.

Methods

Study population

The 2009–2010 BP randomization

study participants were from the

2009–2010 NHANES. NHANES is a

cross-sectional national health survey of

the civilian, noninstitutionalized U.S.

population, conducted by the Centers for

Disease Control and Prevention’s

National Center for Health Statistics

(NCHS). Descriptions of the sample

design and data collection methods for

NHANES are available on the survey

website (12). Survey participants were

interviewed in their homes and then

examined in the NHANES MEC. The

BP randomization study was part of the

NHANES MEC data collection cycle for

2009–2010. The NCHS Ethics Review

Board approved the BP randomization

study protocol.

Sample selection

All of the participants in the BP

randomization study were regular

NHANES MEC participants. The BP

randomization study started

approximately 21⁄2 months after the

beginning of the 2009 NHANES data

collection cycle, consequently missing

three data collection sites. The late start

date was related to logistic and training

issues. A total of 7,410 participants aged

8 years and over were eligible to

participate in the BP randomization

study. Of these, 343 were excluded due

to having all BP values missing, and 7

were excluded because an observer

made too few measurements. For

analysis of systolic BP, an additional

722 individuals were excluded due to

having fewer than three systolic

readings, leaving 3,231 males and 3,229

females. For analysis of diastolic BP, an

additional 600 were excluded due to

having fewer than three diastolic

readings, leaving 3,166 males and 3,172

females (Table 1).

Following AAMI standards, the

term ‘‘observer’’ is used throughout this

report to denote a health technician or

physician trained to accurately obtain

BP values using the mercury device

(10,11). A total of 6,460 persons aged 8

years and over were eligible to

participate in the BP study. Participants

had three valid mercury and Omron

systolic readings (3,231 males and 3,229

females) and 6,338 persons had three

valid mercury and Omron diastolic

readings (3,166 males and 3,172

females). For more details about the

sample selection, see Table 1.

The overall mean age of the study

participants was 40 years (age range

8–80 years and over). No statistically

significant difference was observed

between participants and eligible

nonparticipants across age (40.2

compared with 41.6 years, p = 0.05),

BMI (27.3 compared with 27.7, p =

0.05), and midarm circumference (31.6

compared with 31.5 cm, p ≥ 0.05).

When compared with males, a greater

percentage of females did not participate

in the study (males 9.52% compared

with females 11.45%, p < 0.01). A

greater percentage of non-Hispanic

black persons did not participate in the

study compared with non-Hispanic

white persons, Hispanic persons, and

other racial and ethnic categories

(non-Hispanic black, 12.87%; nonHispanic white, 9.92%; Hispanic,

9.54%; and other, 11.52%; p < 0.01).

Study design

Study participants were randomly

assigned to have their BP taken either

by a MEC physician or by a health

technician. Next, the order of which

device was to be used initially was

further assigned (i.e., each person

randomly received an Omron reading or

mercury reading first) (Figure 1). When

using the Omron, the ‘‘hide’’ mode

feature was utilized. Once measurements
Study randomization 

First-level 

randomization 

Second-level 

Enter study randomization

Participant 

Physician 

1. Omron,

 Mercury 

2. Mercury,

Omron 

1. Omron,

 Mercury 

2. Mercury,

Omron 

Health 

technician 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology study. 

National Health Statistics Reports n Number 59 n October 5, 2012 Page 3

Figure 1. Randomized sampling methodology for blood pressure study comparing Omron

and mercury devices: National Health and Nutrition Examination Survey, 2009–2010

were captured, the three readings were

recorded by a staff member, who did not

know the readings from the other

instrument. This specific design

approach was taken for three reasons:

first, to minimize observer bias; second,

to minimize the order effect (i.e., first

reading being always higher) (13); and

third, to minimize the possibility that

Omron readings may affect the mercury

readings. Therefore, BP values were

keyed off the Omron device by persons

who were effectively blind to the

mercury results.

The data confirmed that the

randomization schema worked well,

with 49% of the readings being done by

a health technician and 51% being done

by a physician. Additionally, 50% of

participants had their BP measured

initially with the Omron followed by the

mercury, and the other 50% had the

mercury followed by the Omron. The

slightly greater percentage of physicians

than technicians taking a BP reading

was related to the fact that the

physicians started taking study BP

measurements approximately 1 month

earlier than the technicians. The

physicians were trained and started the

randomized study 21⁄2 months after the

start of the 2-year cycle, whereas the

technicians started collecting data 11⁄2

months later. Therefore, the first 11⁄2

months of the study had only one

randomization schema, namely device

order. The random order of

measurements (mercury first or Omron

first) resulted in equal means of age,

BMI, and arm circumference in the two

groups (p > 0.05 for all). Additionally,

the distribution of gender, BMI, race

and ethnicity, and cuff-size groups also

showed equal distribution between the

randomized orders (p > 0.05 for all) (14).

Equipment

The Omron HEM–907XL is a

digital upper-arm electronic blood

pressure monitor that is designed to be

used in clinical settings (15). According

to the manufacturer (15), the HEM–

907XL is an updated version of the

HEM–907, specifically because it has

been upgraded to allow inflation of a

larger-size cuff (15). The algorithm

range was expanded to accept the

measurement for a larger arm

circumference while still maintaining its

validated margin of error (± 0.3%). The

Omron HEM–907XL automatic

measurements are based on smart

‘‘inflate’’ technology (IntelliSense),

where inflation is driven by a pumping

system, and deflation is driven by an

electromagnetic control valve that

allows rapid air release. The

measurement scale for this oscillometric

device ranged from zero to 280 mm Hg.

A special function ‘‘hide’’ mode

conceals blood pressure values from a

person’s view, which may reduce

participant anxiety (15).

The medical wall-mounted mercury

sphygmomanometer (Baumanometer)

was used as the standard comparison

device to the Omron HEM–907XL. The

study employed only one brand of cuffs:

the standard mercury Baumanometer

cuffs [small adult (17–22 cm), adult

(22–32 cm), large adult (32–42 cm), and

extra-large adult (42–50 cm)] were used

to perform all blood pressure

measurements with both the mercury

sphygmomanometer and the Omron

HEM–907XL.

The calibration technique

performed to check the HEM–907XL

involved connecting the mercury

sphygmomanometer and the Omron

device via a T-tube connector and

setting the Omron HEM–907XL MODE

selector to ‘‘CHECK.’’ Explicitly, the

pressure values displayed on the monitor

were compared with the ones on the

mercury manometer at different BP

points (300, 250, 200, 150, 100, 60, and

0 mm Hg) using the four Baumanometer

cuff sizes wrapped around a cylinder.

According to the manufacturer’s

guidelines, comparative readings needed

to be within ± 3 mm Hg to meet the

calibration procedure criteria (15).

Measurement of BP

Performing the BP readings

required participants to be seated in a

chair with back support, with both feet

resting comfortably on the floor, and

both forearms supported on a level

surface at heart level. The appropriate

BP cuff size (small adult, adult, large

adult, extra-large adult) was selected

according to the midarm circumference

of the participant (16). After a 5-minute

rest, the study participants had their BP

measured. Three systolic and diastolic

measurements were obtained by the first

assigned device, and the next three

systolic and diastolic measurements

were obtained by the second device. For

mercury measurements, a fourth reading

was taken if one or two of the other
Page 4 National Health Statistics Reports n Number 59 n October 5, 2012

readings were not obtained. Both the

mercury individual determination

(systolic/diastolic) and Omron individual

determination (systolic/diastolic) were

repeated at 30-second intervals. The

transition from one device BP

determination to the other also was

obtained within 30 seconds.

Fourteen observers were involved in

the BP randomization study (10 health

technicians and 4 physicians). Fifty-one

percent of all BP readings were done by

the physicians; the majority of those BP

readings (90%) were done by two of the

four physicians. The mean number of

readings per observer was 462, median

293, minimum 35, and maximum 1,836.

Three criteria were used to assess the

observer effect on the BP readings of

the devices: a) individual observer’s

mean difference of the between-device

readings, b) end-digit preference, and c)

BP cuff selection. All criteria assessing

observer effect were based on our

previous work assessing BP quality

assurance in NHANES (17). The 14 BP

observers’ between-device readings did

not substantially deviate from each

other; 75% of between-observer

differences were within 3 mm Hg for

systolic, and 70% were within 3 mm Hg

for diastolic (differences are in absolute

values). The between-device values by

individual observer for systolic BP

reading ranged from a mean of –3.4 mm

Hg to 2.7 mm Hg, and for diastolic BP

reading, the values ranged from –3.6

mm Hg to 3 mm Hg.

For the four mercury systolic BP

determinations, all of the end-digits

were at or about 20% preferences. As

for mercury diastolic BP values, the

fourth determination had the largest

end-digit preference for zero (27%). The

fourth reading was always taken if one

or two of the other readings were not

obtained. Therefore, this reading was the

most difficult to obtain, which could

account for the greater than 20%

end-digit preference for zero. After

rounding the Omron readings to even

end-digit values, the end-digit

preferences of the three Omron systolic

and diastolic readings were analyzed.

All of the end-digit preferences were in

the 19%–20% range.

The observers’ cuff selections were

compared with midarm circumference

values obtained during the body

measurements exam in the MEC. A total

of 541 persons, or 8%, had discrepant

cuff size. Discrepant cuff size is defined

as an observer selecting a BP cuff that

is different from a cuff size selected

with the body measure component of

midarm circumference values. The

discrepancy resulted in more participants

with a larger cuff than needed rather

than a smaller cuff. Specifically, among

adult cuff selections, 6.13% were

overcuffed and given a large adult cuff

size, while only 0.48% were

undercuffed and given a small adult cuff

size. Among large adult cuff selections,

2.81% were overcuffed and given an

extra-large adult cuff size, and 1.81%

were undercuffed and given an adult

cuff size. The above notwithstanding,

our findings show that 92.56% of the

small adult/child BP cuffs, 93.39% of

the adult BP cuffs, 95.30% of the large

adult BP cuffs, and 94.53% of the

extra-large BP cuffs were correctly

selected. Only three individuals required

an infant cuff.

Other measurements

Height in centimeters and weight in

kilograms were measured in the MEC

following a standard protocol. BMI was

calculated as weight in kilograms over

height in meters squared (kg/m2

).

Midarm circumference was determined

by having the participant stand erect

with his or her feet together, his or her

right arm flexed at a 90° angle at the

elbow, and his or her palm facing up.

On the right scapula, the observer

located and marked, with a horizontal

line, the uppermost edge of the posterior

border of the acromion process. The

observer held the zero end of the

measuring tape at this mark and

extended the tape down the posterior

surface of the arm to the tip of the

olecranon process. The observer made a

horizontal mark at the midpoint at the

posterior aspect of the arm and

measured the arm circumference.

Statistical analyses

All statistical analyses were

performed using the software products

SAS 9.2 for Windows (Cary, N.C.) and

STATA 11.1 (College Station, Texas).

Assessing device

differences

The statistics used to assess device

differences were based on an AAMI

recommendation for noninvasive

sphygmomanometers—clinical

validation of automated measurement

devices—and previous validation studies

(11,12). For each person, the average of

the three measurements was calculated

from the same device, which leads to

four readings per subject denoted as

systolic mercury, systolic Omron,

diastolic mercury, and diastolic Omron.

The between-device differences for

systolic BP and diastolic BP were

assessed separately. All of the

differences were calculated as Omron–

mercury. The correlation of BP readings

between the two devices was calculated

using the Pearson correlation coefficient.

A graphic display of the differences of

between-device readings (Omron –

mercury) compared with the

corresponding averages [(Omron +

mercury/2)], a Bland and Altman graph,

was created to assess the relationship

between the two devices (18). The

Bland and Altman scatter-plot graphic

displays were overlaid with a local

regression line (LOESS) to further

assess the relationship between the two

devices. More specifically, LOESS was

used as a smoothing algorithm to allow

greater flexibility to fit a regression line

in the face of possible outliers in the

data (19).

In addition, absolute between-device

differences were calculated and

displayed graphically, following both the

AAMI and the international European

protocol for device comparison (11,20).

For more details about differences

between devices, we also compared BP

values across devices by selected

percentiles (1, 10, 25, 50, 75, 90, 95,

and 99%).

The mean difference (Omron minus

mercury) of the test device (Omron) and
National Health Statistics Reports n Number 59 n October 5, 2012 Page 5

the comparison device (mercury) were

calculated within strata:

1. By gender.

2. By four age groups (8–18, 19–39,

40–59, and 60 and over).

3. For three race-and-ethnicity selfreported categories (Hispanic,

non-Hispanic white, and nonHispanic black).

4. For four cuff sizes (child/small adult,

adult, large adult, extra-large adult).

5. For four BMI categories in persons

aged 18 and over [underweight (less

than 18.5 kg/m2

), normal weight

(less than 25.0 kg/m2

), overweight

(25.0 to 29.9 kg/m2), and obese (30

kg/m2 or more)].

6. By presence of an irregular heart

rate.

Paired T tests were used, separately,

to test if the mean between-device

differences were equal to zero for

systolic and diastolic across the

categories listed above. The α-level for

a significant test was considered to be

p < 0.05.

Comparing high BP

classification agreement

As in our previous work, we used

agreement statistics to assess betweendevice agreement for high BP

classification (12). Survey participants

aged 18 and over who had three valid

mercury and Omron systolic readings

and three valid mercury and Omron

diastolic readings were included in the

analysis. Classifications of high BP (i.e.,

BP greater than or equal to 140/90 mm

Hg) and stage II high BP (i.e., BP

greater than or equal to 160/100 mm

Hg) were compared based on mercury

and Omron readings (21). Note that high

BP was defined by BP values only,

regardless of whether the study

participant took antihypertensive

medication. The agreement parameters

estimated were sensitivity, specificity,

and kappa agreement. Mercury was

considered the gold standard.

Results

Comparing measured values

of BP (Omron – mercury)

Figure 2 is a bar graph that shows

the percent distribution of the absolute

differences between the two device

measurements within 0–2, 3–5, 6–10,

11–15, and 16+ mm Hg categories.

Absolute agreement within 5 mm Hg is

considered the acceptable threshold for

between-device agreement (11). In our

study, the absolute between-device

differences were within 5 mm Hg for

62.43% of the systolic BP readings and

56.71% for diastolic BP readings. The

absolute agreement within 2 mm Hg

was 31.24% for systolic readings and

27.72% for diastolic readings.

Figures 3 and 4 show scatter plots

of the mercury and Omron BP readings,

averaged over the three measurements,

for systolic and diastolic separately. The

respective correlations were statistically

significant (r = 0.92 systolic, r = 0.79

diastolic; p < 0.001 for both). The

scatter plots were overlaid by a unity

and a regression line. In general, the

regression line was below the unity line

for systolic BP, suggesting that Omron

underestimated the mean of systolic BP

compared with mercury; for diastolic Percent

0 

4 

8 

12 

16 

20 

24 

28 

32 

0–2 mm Hg 3–5 mm Hg 6–10 mm Hg 11–15 mm Hg ≥ 16 mm Hg

Diastolic BP 

Absolute difference 

Systolic BP 

31.24 31.19 

26.33 

8.13 

3.11 

27.94 28.76 

27.07 

11.06 

5.16 

NOTE: BP is blood pressure in mm Hg. 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology 

study. 

Figure 2. Frequency distribution of absolute between-device differences (Omron minus

mercury) for systolic and diastolic blood pressure values

BP, the regression line was above the

unity line for the lower BP values and

was below the unity line for higher BP

values, suggesting that Omron

overestimated the mean of BP at low

diastolic mercury values and

underestimated the mean of BP at high

mercury diastolic values. Table 2

compares selected percentile values by

device. For systolic BP, mercury always

read higher than Omron. In contrast,

Omron read higher than mercury for

lower diastolic BP (1%–10%), and read

lower than mercury for higher diastolic

BP (25% or more).

Figures 5 and 6 show the

relationship of the between-device

differences with the BP levels. The

differences of the two device readings

were plotted against the corresponding

averages of the two device readings for

systolic and diastolic measurements

separately (Bland and Altman graphs).

Both figures show some extreme values

beyond two SDs, but no discernible

linear relationship could be ascertained.

The Spearman rank correlation between

the absolute difference and the means

was 0.2 for systolic and –0.15 for

diastolic. For systolic BP (Figure 5), the

LOESS slightly deviates from the zero

reference line at higher systolic values,

indicating larger between-device
differences at higher BP values. In

contrast with systolic BP, Figure 6

shows a curvilinear relationship for

diastolic BP. Lower diastolic values

were associated with larger betweendevice differences, but the differences

decreased at higher diastolic values.

The means of the between-device

differences are presented in Tables 3 and

4. Overall, the mean between-device

differences (Omron minus mercury) for

systolic was –1.6 mm Hg (SD = 6.8)

and for diastolic was –0.6 mm Hg (SD

= 7.8). The mean between-device

differences by gender, age group, race

and ethnicity, cuff size, BMI, and

irregular heart rate were less than or

about 2 mm Hg, except those for

extra-large BP cuff (–3.1 mm Hg) and

obese individuals (–2.6 mm Hg) for the

systolic readings, and underweight

individuals for the diastolic readings

(–3.5). For the systolic readings, apart

from persons classified as ‘‘other’’ and

underweight, the mean between-device

differences were statistically significant

for all other subclassifications. As for

diastolic readings, apart from females,

persons aged 8–18 and 60 and over,

non-Hispanic blacks, obese persons, and

persons classified as having irregular

heart rate, the mean between-device

difference was statistically significant

for all other subclassifications.

Comparing high BP

classification agreement

by device

Table 5 shows the between-device

agreement for the classification of

overall high BP (140/90 mm Hg or

more) and stage II high BP (160/100

mm Hg or more), which was defined by

BP taken in the MEC for persons aged

18 and over. The kappa agreements

were 0.72 for both. For overall high BP,

Omron correctly identified 70.28% of

hypertensive individuals and 97.38% of

normotensive individuals. For stage II

high BP, Omron correctly identified

65.24% of stage II hypertensive

individuals and 99.44% of stage II

normotensive individuals. For both high

BP categories, consistently more persons

were correctly identified as nonhigh BP

(higher specificity) than correctly

identified as high BP (relatively lower

sensitivity). The gold standard for the

sensitivity and specificity analyses was

mercury BP determinations. The

percentages of high BP and stage II high

BP based on the Omron measurements

were lower than those based on the

mercury measurements (12.87%

compared with 15.15% for overall high

BP; 2.98% compared with 3.75% for

stage II high BP).

Discussion

The Omron and mercury device

measurements were correlated (r = 0.92

for systolic BP and r = 0.79 for diastolic

BP). Previous studies comparing the

Omron device with mercury consistently

showed lower readings of Omron for

both systolic and diastolic BP (7–10). In

our previous validation study comparing

Omron with mercury, the results showed

that the overall difference between the

two devices was –1.62 mm Hg (SD =

6.14) for systolic and –1.64 mm Hg (SD

= 6.63) for diastolic (10). The results of

the current study show an overall

similar underestimation using the Omron

device for both systolic BP (–1.6 mm

Hg, SD = 6.8) and diastolic BP (–0.6

mm Hg, SD = 7.8). Previous studies

have also suggested that increased

systolic values are associated with

increased discrepancy between the

Omron and mercury devices (7–10). The

results from our earlier study suggested

that both systolic and diastolic values

were affected by increased average

mercury BP values (12). The current

Regression Unity

Mean of 3 Omron systolic BP values

Mean of 3 mercury systolic BP values

70

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

80 90 100 110 120 130 140 150 160 170 180 190 200 210

NOTE: BP is blood pressure in mm Hg.

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology

study.

Figure 3. Scatter plot of Omron and mercury systolic blood pressure values overlaid by

regression and unity lines

Page 6 National Health Statistics Reports n Number 59 n October 5, 2012
National Health Statistics Reports n Number 59 n October 5, 2012 Page 7

Regression Unity 

Mean of 3 Omron diastolic BP values 

Mean of 3 mercury diastolic BP values 

0 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

110 

120 

130 

10 20 30 40 50 60 70 80 90 100 110 120 

NOTE: BP is blood pressure in mm Hg. 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology 

study. 

Figure 4. Scatter plot of Omron and mercury diastolic blood pressure values overlaid by

regression and unity lines

study further supported these findings

for systolic BP. However, for diastolic

BP, the current study suggests that the

relationship is not linear, but curvilinear,

with a greater between-device difference

at lower diastolic values (below 60 mm

Hg) and little difference thereafter.

Although there was a beyondchance agreement between the

two device readings for high BP

classification, the agreement was more

likely to be for the absence of high BP

(specificity) rather than the presence of

high BP (sensitivity). Our kappa statistic

for overall high BP classification in the

current study was 0.72, a somewhat

higher kappa than the kappa obtained in

the previous study, which was 0.68. One

reason for the low sensitivity and high

specificity of Omron is that Omron

reads lower than mercury in general. As

a result, for persons who had BP at the

borderline of high (140/90 mm Hg),

Omron would read their blood pressure

below 140/90 mm Hg and these subjects

would be identified as normotensive,

thus reducing the sensitivity. Similarly,

for individuals who were measured as

normotensive by mercury, it is more

likely that they would be measured as

normotensive by Omron, which leads to

high specificity. To address this issue,

we can adjust the Omron readings

accordingly. Methods for adjustment

include parametric methods, such as

linear regression, demining regression,

or a nonparametric method such as

smoothing splines.

A total of 541 persons (8%) had a

discrepancy in cuff-size selection when

compared with midarm circumference

values obtained during the body

measurements exam in the MEC.

Comparing the respective results after

removing the individuals with discrepant

cuff size suggested that removing these

persons resulted in negligible changes

(some slight changes after the decimal

point) to no changes in values among

respective BP determinations. Moreover,

the prevalence of high systolic or high

diastolic BP is nearly identical with and

without exclusions. It also shows that at

the high levels of BP, the mean values

of between-device differences are nearly

identical with and without exclusions.

As noted by Jones and others (19),

oscillometric BP devices have problems

obtaining BP readings in persons with

arrhythmias. We attempted to have a

proxy measure for atrial fibrillation by

having the observers palpate the radial

pulse—this crude measure suggested a 2

mm Hg mean between-device difference

for systolic and less than 1 mm Hg

mean difference for diastolic, although

these results need to be interpreted

cautiously. NHANES does not have

complete information for atrial

fibrillation. Available data in NHANES

are one check box (irregular heartbeat)

for the physician to check during the

examination and lists of anti-arrhythmic

medications in the household

questionnaire. Based on NHANES

2007–2008 data only, 171 (1.8%)

persons aged 8 years and over had an

irregular pulse, and 35 persons (0.34%)

in 2007–2008 were taking antiarrhythmic medication.

Both the strengths and the

limitations of the study can be related to

its setting. The study was carried out

during 2 survey years of NHANES,

echoing the recommendations of Jones

and colleagues (22) who advised that

automatic BP devices should be assessed

for accuracy under conditions of routine

and frequent use. Unlike traditional

validation studies, the BP observers

were not continuously monitored for

observer agreement within 4 or 5 mm

Hg, nor did they repeat any measures

with a difference greater than 4 or 5

mm Hg.

The study employed only standard

mercury Baumanometer cuffs to perform
Page 8	 National Health Statistics Reports n Number 59 n October 5, 2012

Mean of 3 mercury and 3 Omron systolic BP values 

-60 

-48 

-52 

-56 

-44 

-40 

-36 

-32 

-20 

-24 

-28 

-12 

-16 

4 

8 

0 

4 

8 

12 

16 

20 

24 

28 

32 

36 

44 

48 

40 

56 

52 

60 

64 

68 

72 

Systolic difference between devices in mm Hg 

LOESS, smooth = 0.5 

70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 

Midpoint 

NOTES: BP is blood pressure in mm Hg. LOESS is local regression line. 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology 

study. 

Figure 5. Scatter plot of between-device differences and mean of mercury and Omron

systolic blood pressure values overlaid by local regression line

all BP measurements by both devices;

our first comparison study followed the

same method. The rationale for not

changing the cuff was to control for the

physiological reaction, which could be

expressed if the cuff was changed in

midmeasurement. Moreover, as

previously described, the cuff sizes were

selected according to predetermined arm

circumference values. Lastly, the Omron

device was calibrated against the

mercury manometer using the four

Bauman cuffs. In all of the 190

calibrations, the comparative readings

were within ± 3 mm Hg of the mercury

manometer readings adhering to the

manufacturer’s guidelines, which are

more stringent that the AAMI guideline

for device comparison (11).

Conclusions

Aside from observer effect on the

BP readings, other factors may

contribute to our findings, such as the

fact that the physical basis for BP

measurement using oscillometric devices

differs from that of standard mercury

measurements, and, therefore, systolic

and diastolic pressure may not actually

represent the same physiologic entity

when measured using these two devices.

The latter notwithstanding, with some

exception the difference between

devices was less than or about 2 mm

Hg. The exceptions were systolic

between-device differences for those

using the extra-large BP cuff (–3.1 mm

Hg) and for obese persons (–2.6 mm

Hg). Another exception was the mean

diastolic between-device difference for

underweight individuals (–3.5 mm Hg).

Between-device agreement for the

frequency of high BP (140/90 mm Hg

or more) and stage II high BP (160/100

mm Hg or more) was above chance (κ 

= 0.72 for both). Omron underestimated

the high BP frequency by 2.28% and

stage II high BP frequency by 0.77%.

The Omron device was used under

controlled circumstances, and results

may not apply to use in other settings

such as the physician office, clinic, or

home. However, this study provided a

great amount of experience using the

Omron device in the usual NHANES

environment. BP readings obtained with

the Omron device provide lower

estimates of high BP; therefore, future

national prevalence estimates may need

to be adjusted accordingly to take into

account the between-device differences.

References

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National Heart, Lung, and Blood
National Health Statistics Reports n Number 59 n October 5, 2012 Page 9

64 

68 

72 

-60 

-48 

-52 

-56 

-44 

-40 

-36 

-32 

-20 

-24 

-28 

-12 

-16 

4 

8 

0 

4 

8 

12 

16 

20 

24 

28 

32 

36 

44 

48 

40 

56 

52 

60 

Diastolic difference between devices in mm Hg 

0 10 20 30 40 50 60 70 80 90 100 110 120 

Midpoint LOESS, smooth = 0.5

Mean of 3 mercury and 3 Omron diastolic BP values 

NOTES: BP is blood pressure in mm Hg. LOESS is local regression line. 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology 

study. 

Figure 6. Scatter plot of between-device differences and mean of mercury and Omron

diastolic blood pressure values overlaid by local regression line

8.

Institute. CARDIA: Coronary Artery

Risk Development in Young Adults.

Available from: http://www.

cardia.dopm.uab.edu/.

White WB, Anwar YA. Evaluation of

the overall efficacy of the Omron

office digital blood pressure HEM–

10. El Assaad MA, Topouchian JA,

Darné BM, Asmar RG. Validation of

the Omron HEM–907 device for

blood pressure measurement. Blood

Press Monit 7(4):237–41. 2002.

11. American National Standard,

ANSI/AAMI SP10:2002. Manual,

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907 monitor in adults. Blood Press

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Ombani S, Riva I, Giglio A, Caldara

G, Groppelli A, Parati G. Validation

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monitors in elderly individuals

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protocol of the European Society of

electronic, or automated

sphygmomanometers. Arlington, VA:

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Medical Instrumentation (AAMI).

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12. Ostchega Y, Nwankwo T, Sorlie PD,

Wolz M, Zipf G. Assessing the

validity of the Omron HEM–907XL

oscillometric blood pressure

Hypertension. Blood Press Monit

12(4):233–42. 2007.

measurement device in a national

survey environment. J Clin

Hypertens (Greenwich) 12(1):22–8.

2010.

13. Grossardt BR, Graves JW, Gullerud

RE, Bailey KR, Feldstein J. The

occurrence of the alerting response is

independent of the method of blood

pressure measurement in

hypertensive patients. Blood Press

Monit 11(6):321–7. 2006.

14. National Center for Health Statistics.

National Health and Nutrition

Examination Survey analytic

guidelines. 2004. Available from:

http://www.cdc.gov/nchs/nhanes/

nhanes2003-2004/analytical_

guidelines.htm.

15. Omron Healthcare, Inc. Omron

instruction manual: Model HEM–

907XL. 2009. Available from:

http://www.omronhealthcare.com/

wp-content/uploads/hem907xl_im.pdf.

16. National Center for Health Statistics.

National Health and Nutrition

Examination Survey (NHANES)

questionnaire and exam protocol.

http://www.cdc.gov/nchs/nhanes/

nhanes_questionnaires.htm.

17. Ostchega Y, Prineas RJ, Paulose-Ram

R, Grim CM, Willard C, Collins C.

National Health and Nutrition

Examination Survey 1999–2000:

Effect of observer training and

protocol standardization on reducing

blood pressure measurement error. J

Clin Epidemiol 56(8):768–74. 2003.

18. Bland JM, Altman DG. Measuring

agreement in method comparison

studies. StatMethods Med Res

8(2):135–60. 1999.

19. Cohen RA. An introduction to PROC

LOESS for local regression. Paper

273. Cary, NC: SAS Institute, Inc.

1999. Available from: http://

www.ats.ucla.edu/stat/sas/library/

loesssugi.pdf.

20. O’Brien E, Atkins N, Stergiou G,

Karpettas N, Parati G, Asmar R, et

al. European Society of Hypertension

international protocol revision 2010

for the validation of blood pressure

measuring devices in adults. Blood

Press Monit 15(1):23–38. 2010.

21. Pickering TG, Hall JE, Appel LJ,

Falkner BE, Graves J, Hill MN, et al.

Recommendations for blood pressure

measurement in humans and
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experimental animals. Part 1: Blood

pressure measurement in humans: A

statement for professionals from the

Subcommittee of Professional and

Public Education of the American

Heart Association Council on High

Blood Pressure Research.

Hypertension 45(1):142–61. 2005.

22. Jones DW, Appel LJ, Sheps SG,

Roccella EJ, Lenfant C. Measuring

blood pressure accurately: New and

persistent challenges. JAMA

289(8):1027–30. 2003.
National Health Statistics Reports n Number 59 n October 5, 2012 Page 11

Table 1. Sample size for blood pressure methodology study: National Health and Nutrition Examination Survey, 2009–2010

Number of Number of

participants participants

Condition included excluded

Eligible to participate in blood pressure study1 ...................... 7,410 . . .

Missing all blood pressure measurements (unit nonresponse) ............ . . . 343

Measurements obtained by excluded observer2 ..................... . . . 7

At least one valid blood pressure reading for both systolic and diastolic3 ..... 6,660 . . .

At least two valid blood pressure readings for both systolic and diastolic3 ..... 6,542 . . .

At least three valid blood pressure readings for both systolic and diastolic3 .... 6,336 . . .

Had three valid systolic mercury and Omron readings ................. 6,460 . . .

Had three valid diastolic mercury and Omron readings................. 6,338 . . .

. . . Category not applicable.
 1

Persons aged 8 years and over.
 2

Observers were excluded if too few observations were made.
 3

With both mercury and Omron devices.
 

NOTE: Mercury is the mercury sphygmomanometer device; Omron is the Omron HEM–907XL device.
 

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology study.
 
Page 12 National Health Statistics Reports n Number 59 n October 5, 2012

Table 2. Blood pressure values across selected percentiles, by device

Mean and percentile Mercury Omron n (Om – Me)

Systolic
 

Mean of three blood pressure readings .......................... 117.3 115.7 –1.6 
 

1% ................................................ 86.7 86.3 –0.4 
 

5% ................................................ 94.0 93.0 –1 
 

10% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98.0 96.7 –1.3 
 

25% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.3 103.7 –1.6
 

50% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.0 113.0 –1.7 
 

75% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.7 124.7 –1.3
 

90% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138.7 138.7 –1.3
 

95% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148.0 148.0 –2
 

99% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.0 172.0 –1.3
 

Diastolic
 

Mean of three blood pressure readings .......................... 66.5 66.0 –0.5 
 

1% ................................................ 36.0 43.3 7.3 
 

5% ................................................ 46.0 49.3 3.3 
 

10% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50.0 52.3 2.3 
 

25% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58.7 58.0 –0.7 
 

50% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66.7 65.0 –1.7 
 

75% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.7 73.0 –1.7 
 

90% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.0 80.7 –1.3 
 

95% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87.3 86.0 –1.3 
 

99% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98.0 96.3 –1.7 
 

NOTE: Mercury (Me) is the mercury sphygmomanometer device; Omron (Om) is the Omron HEM–907XL device.

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 20092010 blood pressure methodology study.
National Health Statistics Reports n Number 59 n October 5, 2012 Page 13

Table 3. Means of between-device differences (Omron minus mercury) for systolic blood pressure values, by gender, age group, race

and ethnicity, cuff size, BMI category, and irregular heart rate

Characteristic Sample size

Mercury

(SD)

Omron

(SD)

Mean

difference

(SD)

Overall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6,460 117.3 (17.6) 115.7 (17.4) † –1.6 (6.8)

Gender

Male ...............................................

Female . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3,231

3229

119.4 (17.0)

115.1 (17.9)

118.3 (16.4)

113.0 (17.9)

† –1.1 (6.5) † –2.1 (7.1)

Age group

8–18 years............................................

19–39 years ...........................................

40–59 years ...........................................

60 years and over .......................................

1,554

1,766

1,598

1,542

104.8 (10.1)

113.3 (12.2)

120.4 (16.2)

131.2 (19.4)

103.6 (9.8)

111.5 (12.0)

118.7 (16.1)

129.4 (19.2)

† –1.2 (5.7) † –1.8 (5.9) † –1.7 (6.7) † –1.8 (8.5)

Race and ethnicity

Hispanic .............................................

Non-Hispanic white ......................................

Non-Hispanic black ......................................

Other races ...........................................

1,829

3,002

1,239

390

114.6 (17.2)

117.9 (17.1)

120.6 (18.8)

114.6 (17.0)

113.2 (17.2)

116.0 (16.6)

118.8 (18.7)

114.0 (17.5)

† –1.4 (6.5) † –1.8 (6.8) † –1.8 (7.1)

–0.6 (6.6)

Cuff size

Child or small adult ......................................

Adult ...............................................

Large adult ...........................................

Extra-large adult ........................................

343

2,089

3,132

893

100.8 (11.9)

112.8 (17.4)

120.6 (16.9)

122.1 (16.2)

99.4 (10.1)

111.7 (16.6)

119.1 (17.0)

119.0 (17.0)

† –1.3 (5.8) † –1.2 (6.3) † –1.5 (6.9) † –3.1 (7.6)

BMI category§

Underweight...........................................

Normal weight .........................................

Overweight ...........................................

Obese ..............................................

78

1,415

1,676

1,813

110.6 (21.7)

117.6 (17.6)

121.6 (17.2)

123.4 (16.9)

109.1 (18.4)

116.2 (16.7)

120.4 (17.2)

120.8 (17.5)

–1.5 (7.2) † –1.5 (6.6) † –1.1 (7.0) † –2.6 (7.4)

Heart rate

Irregular heart rate ....................................... 191 126.7 (19.0) 124.6 (18.5) † –2.1 (6.9)

† p < 0.05. § Excludes persons under age 18.

NOTES: Mercury is the mercury sphygmomanometer device; Omron is the Omron HEM–907XL device. BMI is body mass index. SD is standard deviation.

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology study.
Page 14 National Health Statistics Reports n Number 59 n October 5, 2012

Table 4. Means of between-device differences (Omron minus mercury) for diastolic blood pressure values, by gender, age group, race

and ethnicity, cuff size, BMI category, and irregular heart rate

Characteristic

Sample

size

Mercury

(SD)

Omron

(SD)

Mean

difference

(SD)

Overall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6,338 66.5 (12.8) 66.0 (11.3) † –0.6 (7.8)

Gender

Male ...................................

Female . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3,166

3,172

67.8 (13.3)

65.3 (12.0)

66.8 (11.8)

65.0 (10.6)

† –0.9 (7.7)

–0.2 (7.9)

Age group

8–18 years................................

19–39 years ...............................

40–59 years ...............................

60 years and over ...........................

1,477

1,759

1,592

1,510

57.2 (10.8)

68.0 (11.7)

73.6 (10.8)

66.4 (12.2)

57.1 (7.5)

67.2 (10.2)

72.1 (10.5)

66.7 (10.9)

–0.2 (9.7) † –0.8 (7.3) † –1.5 (6.3)

0.3 (7.6)

Race and ethnicity

Hispanic .................................

Non-Hispanic white ..........................

Non-Hispanic black ..........................

Other races ...............................

1,783

2,952

1,219

384

64.8 (12.8)

66.9 (12.2)

68.1 (13.7)

67.0 (13.0)

64.7 (10.9)

65.9 (10.8)

68.1 (12.3)

65.7 (11.6)

–0.1 (8.0) † –1.0 (7.4)

–0.0 (8.2) † –1.3 (8.0)

Cuff size

Child or small adult ..........................

Adult ...................................

Large adult ...............................

Extra-large adult ............................

306

2,047

3,102

880

54.8 (10.6)

63.3 (11.9)

68.6 (12.2)

71.0 (13.1)

53.4 (7.3)

61.8 (9.8)

68.3 (10.6)

71.8 (11.3)

† –1.4 (9.3) † –1.5 (8.0) † –0.3 (7.3)

†0.8 (8.1)

BMI category§

Underweight...............................

Normal weight .............................

Overweight ...............................

Obese ..................................

78

1,403

1,666

1,791

65.8 (11.6)

67.4 (11.2)

69.0 (11.6)

71.0 (12.6)

62.4 (10.8)

65.5 (9.9)

68.5 (10.4)

71.2 (11.0)

† –3.5 (7.2) † –1.9 (7.0) † –0.5 (6.8)

0.2 (7.4)

Heart rate

Irregular heart rate ........................... 187 67.1 (13.2) 67.6 (12.3) 0.5 (7.7)

† p < 0.05. § Excludes persons under age 18.

NOTES: Mercury is the mercury sphygmomanometer device; Omron is the Omron HEM–907XL device. BMI is body mass index.

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology study.
National Health Statistics Reports n Number 59 n October 5, 2012 Page 15

Table 5. Classification of overall high blood pressure and stage II high blood pressure in

persons aged 18 and over, by mercury and Omron devices

Device and

agreement†

Overall high blood

pressure

Stage II high blood

pressure

Mercury

Yes No

Mercury

Yes No

Omron .......... Yes

No

532

225

111

4,129

122

65

27

4,779

Omron ..........

Mercury .........

12.87

15.15

Percent hypertensive

2.98

3.75

Sensitivity ........

Specificity ........

Kappa ..........

70.28

97.38

0.72

Agreement statistics

65.24

99.44

0.72

† Mercury blood pressure readings are gold standard.

NOTES: High blood pressure is systolic blood pressure of 140 mm Hg or higher, or diastolic blood pressure of 90 mm Hg or

higher. Stage II high blood pressure is systolic blood pressure of 160 mm Hg or higher, or diastolic blood pressure of 100 mm Hg

or higher. Mercury is the mercury sphygmomanometer device; Omron is the Omron HEM–907XL device.

SOURCE: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010 blood pressure methodology study.
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National Health Statistics Reports n Number 59 n October 5, 2012

Acknowledgments

This paper is dedicated to the memory of Dr. Lester R. (‘‘Randy’’) Curtin, a friend and a mentor—you are going to be missed. We would like to

express our gratitude to Michele Chiappa, Technical Writer, Harris Corporation, for the review and initial editing of various drafts of this report.

We also would like to thank the National Institutes of Health’s National Heart, Lung, and Blood Institute for its support and help in this project.

Suggested citation

Ostchega Y, Zhang G, Sorlie P, et al. Blood

pressure randomized methodology study

comparing automatic oscillometric and

mercury sphygmomanometer devices:

National Health and Nutrition Examination

Survey, 2009–2010. National health statistics

reports; no 59. Hyattsville, MD: National

Center for Health Statistics. 2012.

Copyright information

All material appearing in this report is in the

public domain and may be reproduced or

copied without permission; citation as to

source, however, is appreciated.

National Center for Health Statistics

Edward J. Sondik, Ph.D., Director
 

Jennifer H. Madans, Ph.D., Associate Director
 

for Science
 

Division of Health and
 

Nutrition Examination Surveys
 

Clifford L. Johnson, M.S.P.H., Director

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DHHS Publication No. (PHS) 2013–1250 • CS234654