Journal of Human Hypertension (2006) 20, 593–598& 2006 Nature Publishing Group All rights reserved 0950-9240/06 $30.00
www.nature.com/jhh

ORIGINAL ARTICLE

Self-monitoring of home blood pressure
with estimation of daily salt intake
using a new electrical device
K Yamasue1, O Tochikubo1, E Kono2 and H Maeda3
1

Department of Preventive Medicine, Yokohama City University Graduate School of Medicine, Yokohama,Japan; 2Kono ME Research Institution, Kawasaki, Japan and 3HEC Science Clinic, Yokohama, Japan

We investigated a simple device to monitor daily salt
intake at home and examined the relationship between
salt excretion and morning blood pressure in orderto enable patients to better manage daily salt intake
and hypertension. The correlation between 24-h urinary
salt excretion and measured value with salt monitor
from overnight urine was significant (n ¼ 224, r ¼ 0.72,
Po0.001). A total of 46 volunteers participated for morethan 3 weeks by measuring daily salt intake and morning
blood pressure. The relationship between predicteddaily salt excretion and blood pressure was examinedwith use of 3-day moving average. Mean salt excretion and systolic blood pressure (SBP) significantly
decreased by the end of the trial (i.e., salt excretion
decreased from 158731 to 149730 mmol/day and SBP
from 137717 to 133716 mm Hg). Of 46 participants, 18(39%) had a significant correlation between predicteddaily salt excretion and blood pressure (r40.4, Po0.05,n421), indicating sodium sensitivity. An additional 17%
had a positive correlation that did not reach statisticalsignificance (0.2orp0.4), and the remaining 44% had
no correlation (rp0.2). Mean decrease in blood pressure
per decrease in salt (g) (17 mmol) intake in the 18
participants with a significant correlation was 3.3 mm Hg(SBP) and 1.5 mm Hg (diastolic blood pressure), which
was higher than that reported for other studies.
Hypertensive patients not using medication showed
the largest decrease. We conclude that daily monitoringof salt intake and morning blood pressure will be usefulfor management of hypertension.
Journal of Human Hypertension (2006) 20, 593–598.
doi:10.1038/sj.jhh.1002049; published online 18 May 2006

Keywords: salt restriction; sodium dietary; urinary salt excretion; overnight urine; blood pressure monitoring

Introduction
High-salt (NaCl) intake is a well-known risk factor
for hypertension, cardiovascular disease1–8 andstomach cancer.9,10 The Japanese Ministry of Healthand Welfare recommends a salt intake of less than
10 g/day, whereas the World Health Organization’s
recommendations restrict salt intake to 5 g/day orless and US,11 UK and Japanese Society of Hypertension recommendations are 6 g (100 mmol)/day orless. Nevertheless, many people are still consuming
more salt than recommended. Most people are notaware how much salt they consume.
Monitoring nutritional salt intake through sodium
excretion has been popular, because the main routefor sodium (Na) excretion is through the urine.12–14
To collect a 24-h urine and measure the content is
an accurate but difficult method for most patients,
especially working people. In a previous study,
Correspondence: K Yamasue, Department of Preventive Medicine,
Yokohama City University Graduate School of Medicine, 3-9
Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan.
E-mail: t046055c@yokohama-cu.ac.jp
Received 10 February 2006; revised 4 April 2006; accepted 12
April 2006; published online 18 May 2006

creatinine and sodium content were measured ineither a spot urine or overnight urine sample and24-h sodium excretion (Na24) was estimated fromthe ratio of 24-h predicted creatinine based on bodyweight, body height and age15,16 or on lean bodymass.17 However, measuring creatinine content athome has a high cost. A 24-h sodium excretion(Na24) value can also be estimated from overnighturinary sodium excretion (UNan).18–20 We consider
it ideal to measure salt intake at home in order tocontrol average daily salt intake. Salt titrator sticks18
and tapes21 that detect chlorine ion are easy to use,
but they are not accurate depending on the amountof urine applied to the tape and error in visual
perception of colour change. To calculate overnighturinary sodium excretion, it is necessary to checkurine volume and integrate it into sodium concentration.
Numerous studies have shown that some people
are not sensitive to salt. Sensitivity and resistance tothe effects of salt have been evaluated by the degreeof blood pressure change between high-salt intake
and low-salt diet.22–24
Recent reports suggest that morning surge isclosely related to incidence of cardiovascular andHome monitoring of BP and daily salt intake
K Yamasue et al594

cerebrovascular diseases.25,26 Blood pressure andsalt intake differ in individuals and on different
days. Thus, it would be useful to measure morning
blood pressure and salt intake with the new saltmonitor and to examine the relationship between
blood pressure and salt intake for a relatively
long period. The aim of this study was to makeparticipants accustomed to measuring morning
blood pressure and salt intake to control hypertension in a way that is suitable for each individual.

abVolume sensor(resistance chip)
Urine cup

Salt monitor
Display

Materials and methods
Study subjects

Two hundred and fifty healthy adults (53.9716
years; male 62, female 188) participated to measure
24 h urinary salt excretion and overnight urinary salt
excretion using ion electrode method.
Two hundred and twenty-four healthy adults(52.7715.9 years; male 70, female 154) participated
to measure 24-h urinary salt excretion by ion
electrode method and overnight urine by new saltmonitor.
A total of 46 adults (60.1711.5 years; male 33)
including 17 people who were taking antihypertensive medication participated to measure daily salt
intake by new salt monitor using overnight urine
and measure daily blood pressure. The main subgroup of people with systolic blood pressure (SBP)
not less than 130 mm Hg, were recruited. The study
protocol was approved by the ethics committee of
Yokohama City University School of Medicine andall subjects gave informed consent to participate
before enrolment.

15mm

50mm

Conductivity sensor

Estimation of salt intake with the new saltmonitoring system

The salt monitor we developed consists of a 1 l urine
cup (10-cm diameter; 15-cm height) and an electrical device with volume and conductivity sensors(Figure 1). The volume sensor, which consists of
50 small resistant chips, measures overnight urine
volume in the cup. The conductivity sensor consistsof two gold-plated nickel metal plates. A temperature compensating circuit is used because conductivity changes with temperature. The NaCl
concentration was adjusted by a correlation formula
to be between the value obtained with the ion
electrode method and the value for the conductivity
method because measurement of concentrationby conductivity is affected by other electrolytes
such as potassium. Vn (overnight urine volume) was
measured by resistance sensor and NaCln (overnighturinary sodium chloride concentration) was measured by conductivity sensor; then, values were
integrated. Formula (1) was derived by the regression analysis between NaCl24 (24-h salt excretion)
and NaCln  Vn by the ion electrode. The value
for NaCl24 was estimated by Formula (1) using
Journal of Human Hypertension

Figure 1 Salt monitor for estimation of daily salt intake. (a)Conductivity sensor and volume sensor. (b) Salt monitor andurine cup (1-l volume). Daily salt excretion (g/day) is shown on
the display.

a computer calculator and displayed on themonitor panel:
Y ¼1:95X þ 4:5g=dayðY : NaCl24 ; X : NaCln Vn Þ½1:95X þ 77:6mmol=dayÞ

ð1Þ

Before going to bed, participants voided completely
and discarded the urine. Overnight urine wascollected in the 1-l urine cup. After awakening,participants voided and placed the urine in theurine cup, adding any urine they had voided
overnight. Participants were asked to collect theurine for approximately 8 h. Then they set the saltmonitor and recorded the display value. The
relationship between NaCl24 from 24-h urine determined by ion electrode method and NaCl24
estimated from overnight urine by the new saltmonitor and ion electrode method was examined in
224 subjects.

Home monitoring of BP and daily salt intake
K Yamasue et al595

Daily monitoring of salt intake and morning
blood pressure

Statistical analysis

Values are expressed as mean7s.d. or mean7s.e.m.
Statistical comparisons between mean values at
the first 7 days of the study and values for thelast 7 days were performed with paired t-test.
Simple linear regression analysis was used toestimate the relationship between blood pressure
and salt excretion for each participant. Analyses
were performed using Microsoft Excel Statistics
2004 for Windows (SSRI Co. Ltd, Japan, Tokyo)
and SPSS version 11 (SPSS Inc., Chicago, IL, USA).
A P-value less than 0.05 was considered statisticallysignificant.

Results
The correlation between 24-h urinary salt excretion
with ion electrode method and measured value withnew salt monitor using overnight urine was significant (r ¼ 072, Po0.001) (Figure 2). The durationof participation of 46 participants varied from 21
to 66 days (mean, 28 days). Mean salt excretionand SBP for all participants decreased significantly
from 158731 mmol (average for first 7 days) to
149730 mmol (average for last 7 days) and from137717 to 133716 mm Hg, respectively. Study
participants (31/46, 67%) significantly decreased
salt excretion.
A significant correlation was found for 18 of the
46 subjects over the duration of the study; an
example is shown in Figure 3. Unmodified data

NaCl24 from 24hr urine byion electrode method (g)Forty-six participants measured daily salt intake
by the new salt monitor using overnight urine, andmeasured blood pressure before breakfast and taking
morning medicine using an arm cuff equipped forautomatic inflation, after they had been sitting for 1–
2 min. Participants were asked to monitor salt intake
and blood pressure more than 3 weeks. During thefirst 2 weeks, they ate as usual. Subjects who could
not decrease salt excretion to less than 137 mmol
(8 g)/day were asked to reduce salt intake fromthe beginning of the third week. The relationshipbetween blood pressure and salt excretion was
examined using the 3-day moving average, whichsmoothes a time series by averaging each successive
group of data points from the beginning to the endof the study.
Hypertension was defined as an average SBP in
the first 3 days of 140 mm Hg or greater, diastolicblood pressure (DBP) 90 mm Hg or greater or useof antihypertensive medication. Participants who
showed a significant correlation in either measurement without modification or 3-day moving average
were identified as salt sensitive (group A), andsubjects without a significant correlation but r40.2
were placed into group B. Other subjects wereassumed to be salt resistant.

25
20
15
10
5
0

0

5

10

15

20

25

PrNaCl24 by salt monitor (g)Figure 2 Relationship between values of 24-h urinary sodium
chloride excretion (PrNaCl24) predicted from overnight urine
by new salt monitor and 24-h urinary sodium chloride
excretion (NaCl24) by ion electrode method (n ¼ 224, r ¼ 0.72,
Po0.001).

(left) and 3-day moving averages (right) showed asignificant correlation, especially the 3-day moving
average. Participants were placed into group A, B
or C based on correlation level (r, correlation
coefficient between morning blood pressure andestimated 24-h salt excretion):
A: r40.4 (Po0.05); B: 0.2or%0.4, 0.05%Po0.3; C:
r%0.2 (P^0.3):
As shown in Table 1, 39% of subjects belonged togroup A, with 17% in group B and 44% in group C.
People who were hypertensive without medication were largely in group A. In group A, seven of
18 subjects showed significant correlation only with
3-day moving averages. The regression coefficientsand the slope (mm Hg/salt (g, 17 mmol)) of regression line for group A are shown in Table 2. For
these subjects, mean decrease in blood pressure was
3.3 mm Hg SBP and 1.5 mm Hg DBP. For hypertensive subjects without medication, mean decrease inblood pressure was 4.3 mm Hg SBP and 2.5 mm Hg
DBP. Among subjects who were taking antihypertensive medication, before and during the study,
mean decrease in blood pressure was 3.1 mm Hg
SBP and 1.3 mm Hg DBP. In normotensive individuals, mean decrease was 2.2 mm Hg SBP and
0.4 mm Hg DBP.

Discussion
This study evaluated a simple method for estimationof 24-h urinary salt excretion to monitor daily
salt intake at home; additionally, we examined therelationship between salt excretion and morning
blood pressure in each individual more than 3
weeks. Based on our formula (1) to estimate daily
salt intake from overnight urine (see Materials
and methods), minimum salt excretion was 4.5 g(77.6 mmol)/day. Some people are known to excrete
Journal of Human Hypertension

Home monitoring of BP and daily salt intake
K Yamasue et al

160

160

150

150

140

140
SBP (mm Hg)

SBP (mm Hg)

596

130
120
110

130
120
110

100

100

90

90

80
100

140

180

220

260

80
100

140

180

220

260

PrNaCl24 (mmol)

PrNaCl24 (mmol)

Figure 3 Example of relationship between SBP and 24-h urinary salt excretion (PrNaCl24) predicted by salt monitor (64-year-old man;
34 days of data). r ¼ 0.42 (Po0.05) (left panel) and r ¼ 0.5 (Po0.001) for 3-day moving average (right panel).

Table 1 Three correlation levels between(PrNaCl24) and morning blood pressure

saltexcretion

Group

A

B

C

Hypertensive subjects
No medication (untreated)
On antihypertensive medication
Normotensive subjects
All subjects
Ratio (%)

13
7
6
5
18
39

4
2
2
4
8
17

15
6
9
5
20
44

A: r40.4 (Po0.05); B: 0.4Xr40.2 (0.05pPo0.3); C: rp0.2 (PX0.3).
r, regression coefficients of the correlation between salt excretion andmorning blood pressure; PrNaCl24, 24-h urinary salt excretionpredicted by salt monitor.

below 3 g (50 mmol)/day and one report suggeststhat salt intake should be reduced to 3 g (50 mmol)/day.27 In our study design phase, we studied powerfunction (2) instead of linear function (1).
Y ¼ 5:76ðXÞ0:53 ðg=dayÞ;Y ¼ 21:92ðXÞ0:53 ðmmol=dayÞð2Þ

However, there was no difference in mean squareerror of the estimate between formulas. In people
with lower salt excretion, formula (2) resulted invalues nearer 24-h excretion, but in people withhigher salt excretion, formula (1) came nearer tothe 24-h value. Because few of our subjects had abaseline salt intake less than 85 mmol (5 g), we
adopted formula (1) at the first stage to attain an
intake less than 100 mmol (6 g). The correlation
between 24-h urinary salt excretion and measured
value with our salt monitor was significant, but
not sufficient. The difference between our new saltmonitor and ion electrode method was small andregression coefficients were 0.72 and 0.75, respectively. This was largely owing to a difference in the
proportion of overnight urine and 24-h urine
volumes among individuals, a difference that isdue to individual circadian rhythms. Additional
Journal of Human Hypertension

contributors would be inter-individual differencesin renal function, interval between dinnertime andbedtime, antidiurenic hormone (ADH) function andin sweating. Some studies have demonstrated goodcorrelations between overnight urinary salt and 24-h
urinary salt,18–20 whereas others found an absenceof correlation28,29 and some suggest seven or 14
measurements are needed to replace 24-h urine
collection.19,30 Urinary salt excretion is affected not
only by the previous day’s salt intake, but also
by the salt intake of the previous 3 or 4 days.14 We
examined the relation of 24-h urinary salt excretion and overnight urinary salt excretion for eightsubjects over 14 days. The correlation coefficientof the 3-day moving average was high (mean0.8570.08; range, 0.76–0.98). Thus, we expected
to be able to manage daily relative salt intake,although there was a possibility that the predictedsalt value would differ from the real value, if a
relatively long period was required for representative results and we used the moving 3-day moving
average.
Daily blood pressure changes not only because ofsalt intake but also because of temperature change,31
day of the week32 and change of sympathetic
nervous system activity. As morning blood pressurein particular fluctuates, it takes a long interval
to examine accurately the relationship between
salt excretion and morning blood pressure. We
decided on 3 weeks as the minimum duration ofparticipation in the trial. As a result, the relationship between salt intake and blood pressure
seemed to be clear for each individual. The target
for salt excretion differed for each individual
based on correlations between salt excretion on thepanel and morning blood pressure. One advantageof the current study was that it allowed saltsensitive individuals to see their own salt–bloodpressure pattern, as well as to see the gains madein blood pressure with reduction in salt intake.
Self-monitoring of morning blood pressure andHome monitoring of BP and daily salt intake
K Yamasue et al
597

Table 2 The regression coefficients and the slope of regression line between morning blood pressure and daily salt excretion in group A
SBP

Hypertensive subjects
No medication (untreated)
Antihypertensive medication
Normotensive subjects
All subjects

DBPra (mm Hg/g)ra (mm Hg/g)

0.5570.04
0.670.04
0.470.03
0.5970.08
0.5670.04

3.770.7
4.371.1
3.170.8
2.270.8
3.370.5

0.5270.05
0.5970.08
0.4570.04
0.4470.13
0.5470.04

2.070.6
2.571.0
1.370.3
0.470.2
1.570.4

Values are mean7s.e.m. r, regression coefficients; a, slope of regression line (mm Hg/salt excretion (g, 17 mmol)).
Abbreviations: DBP, diastolic blood pressure; SBP, systolic blood pressure.

daily salt intake for more than 3 weeks, as was
performed in the current study, will be useful forthe physicians and patients to identify who is salt
sensitive and who is salt resistant and to decide
on target goals, including goals for patients whoalready take antihypertensive medication. The
mean decrease in SBP with reduction of salt
excretion in our group A was higher than thatreported for other studies.23,27,33 It is thought thatsome of these studies included both salt-sensitiveand salt-resistant subjects and our data included
the 3-day moving average, and the blood pressure
data were morning blood pressure, which would
explain in part why the mean decrease in bloodpressure of our data in group A was higher thanthat of the other data. In our study, about 44%of the subjects seemed to be salt resistant. Epidemiological studies have suggested that salt intake
is related to risk for gastric cancer,9,10 an importantmalignancy in Japan. Therefore, regardless ofsalt sensitivity in terms of blood pressure, it isimportant to reduce salt intake. The new saltmonitor will be useful for all patients who need toregulate salt intake.
In this study, mean salt excretion decreased
significantly and 67% of subjects reduced salt
intake, but the reduction level was not sufficient.
We relied on each individual to change dietary andother life-style factors.
We are now mounting a communication modulein the salt monitor. Patients will be able to send dataon daily salt intake, as well as other data such as
blood pressure, electrocardiography tracings andweight to the medical centre. By using this method
and providing more guidance to patients, greater salt
reduction is expected, along with possible gains inother dietary factors such as daily calories andcalories from fats.
In conclusion, the new salt monitor using overnight urine is a simple device for effectively
monitoring salt intake over long periods of time.
Self-monitoring of home blood pressure and daily
salt intake for more than 3 weeks could help
physicians and patients manage risk factors for
hypertension in a way that is suitable for eachindividual.

What is known on this topic
K A 24-h sodium excretion value can be estimated fromovernight urinary sodium excretion.18–20
K Salt titrator sticks and tapes are known as simple method todetect the concentration of salt in overnight urine.18,21
K Sensitivity and resistance to the effects of sodium were
evaluated by the degree of BP change between high-salt dietand low-salt diet.22–24
What this study adds
K A new simple method to monitor daily salt intake wasdeveloped.
K Self-monitoring of morning blood pressure and daily salt
intake by this device for more than 3 weeks and examining
the relationship with use of 3-day moving average will be
useful to identify who is salt sensitive.
K Measuring daily morning blood pressure and overnight urineby this device at home will be useful for the management ofhypertension suitable for each individual.

Acknowledgements
We express our gratitude to Dr Kouichi Hirao of HEC
Science Clinic for obtaining data used in our study.
This study was supported in part by the Japan
Health and Research Institute.

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