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Erica J Wallis Clinical
Pharmacology and Therapeutics, Royal Hallamshire Hospital, Sheffield
S10 2JF
Correspondence
to: L E Ramsay d.colley{at}sheffield.ac.uk
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Abstract |
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Objective:
To examine the accuracy of a new version of the Sheffield table designed to aid decisions on lipids screening and
detect thresholds for risk of coronary heart disease needed to
implement current guidelines for primary prevention of cardiovascular disease.
When hydroxymethyl glutaryl coenzyme A (HMG Co-A) reductase
inhibitors (statins), antihypertensive drugs, and aspirin are used for
primary prevention of coronary heart disease or cardiovascular disease,
the absolute risk determines benefit to the individual, cost
effectiveness, proportion of the population treated, and the total cost
of treatment.1-5 Joint guidelines by four British societies6 and British Hypertension Society
guidelines7 recommend aspirin and treatment of mild
hypertension when a risk of coronary heart disease is 15% over 10 years. For hypertension treatment this risk is considered equivalent to
a risk of cardiovascular disease of 20% over 10 years.7
Statins are also justified when coronary risk is 15% over 10 years,
but because of resource implications the guidelines recommend treatment
when coronary risk is Several risk assessment methods based on the Framingham risk
function,
3 6 10-12
including the Sheffield
table,
13 14
are widely used. We modified the Sheffield
table to identify coronary risk thresholds specified in the new
guidelines
Note: This table was amended on 29 March to correct five
values of the total:HDL cholesterol ratio that were wrong in the
originally published
table. The revised values are those for men aged 40, 38, 36, 34, and 32
years in the yellow column (15%) furthest to the left. A written
correction will appear in a subsequent issue of the BMJ.
Sheffield table
Population data
Risk assessment with table
Statistical analysis
Population
Accuracy for coronary and cardiovascular risk thresholds
Table 1.
Design:
Comparison of decisions made on the basis of
the table with absolute risk of coronary heart disease or
cardiovascular disease calculated by the Framingham risk function. The
decisions related to statin treatment when coronary risk is
30%
over 10 years; aspirin treatment when the risk is
15% over 10 years; and the treatment of mild hypertension when the cardiovascular risk is
20% over 10 years.
Setting:
The table is designed for use in general practice.
Subjects:
Random sample of 1000 people aged 35-64 years from the 1995 Scottish health survey.
Main outcome measures:
Sensitivity, specificity, and
positive and negative predictive values of the table.
Results:
13% of people had a coronary risk of
15%, and 2.2% a risk of
30%, over 10 years. 22% had mild
hypertension (systolic blood pressure 140-159 mm Hg). The table
indicated lipids screening for everyone with a coronary risk of
15%
over 10 years, for 95% of people with a ratio of total cholesterol to
high density lipoprotein cholesterol of
8.0, but for <50% with a
coronary risk of <5% over 10 years. Sensitivity and specificity were
97% and 95% respectively for a coronary risk of
15% over 10 years; 82% and 99% for a coronary risk of
30% over 10 years; and
88% and 90% for a cardiovascular risk of
20% over 10 years in
mild hypertension.
Conclusion:
The table identifies all high risk people for lipids screening, reduces screening of low risk people by more than
half, and ensures that treatments are prescribed appropriately to those
at high risk, while avoiding inappropriate treatment of people at low risk.
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Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
30% over 10 years as a priority, with
treatment when coronary risk is 15% to be given when and where
resources permit.
6 7
Absolute coronary risk relates only
weakly to single risk factors such as blood pressure or lipid
concentrations, and it is estimated best by counting and weighting
major coronary risk factors using risk functions derived from
epidemiological studies.
8 9
namely, 15% and 30% over 10 years
and to improve
accuracy we based it on the ratio of total cholesterol to high density
lipoprotein cholesterol (TC:HDL ratio) rather than on cholesterol
concentration alone.15 We report the accuracy of this
table for identifying risk of coronary heart disease of 15% and 30%
over 10 years in a general population; examine whether coronary risk of
15% over 10 years is an acceptable surrogate for cardiovascular risk
of 20% over 10 years in mild hypertension; and evaluate the table as a
tool for selective lipids screening.
Definitions of heart disease

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Fig 1.
New Sheffield table
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Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
The Sheffield table was constructed by using the Framingham
function8 to compute TC:HDL ratios conferring coronary
risks of 15% and 30% over 10 years from age, sex, smoking, diabetes,
and systolic blood pressure. The upper limit for the TC:HDL ratio was
set at three standard deviations above the population mean. As
before, systolic blood pressure was dichotomised to 160 mm Hg for
those with "hypertension" and 139 mm Hg for "no
hypertension." The table and instructions (fig 1) are designed as a
one page guide to screening, assessment of coronary risk, treatment
with aspirin and statins, and treatment for mild hypertension according to current guidelines.
6 7
The 1995 Scottish health survey is a cross sectional survey of a
stratified random sample of the Scottish population aged 35-64 years.16 From 4910 people screened we excluded those with
no lipids measurement (946); requiring secondary prevention (339); with
incomplete data (549); and taking lipid lowering drugs (19). From the
3057 people with complete data we studied a random sample of 1000 people representative of those aged 35-64 years in the Scottish
population who might require primary prevention. Using age, sex, blood
pressure, smoking habit, diabetes status, and TC:HDL ratio and assuming
absence of left ventricular hypertrophy, we calculated coronary and
cardiovascular risks for each individual using the Framingham function.
Seven doctors who were blind to calculated risk estimates used the
new table to carry out risk assessments. Each of the 1000 people had
their coronary risk assessed by two different doctors; thus each doctor
assessed two sevenths of the population sample. Each doctor was given
the person's age, sex, blood pressure, smoking habit, diabetes status,
and TC:HDL ratio and recorded three decisions: (a) was
measurement of the TC:HDL ratio indicated? (b) was
coronary risk
15% over 10 years? and (c) was
coronary risk
30% over 10 years? There were seven errors in 6000 decisions (0.1%); error rates for all seven assessors were between 0%
and 0.7%. These errors were reconciled for final decisions by the table.
Using Framingham estimates of coronary heart disease as the gold
standard, we calculated the sensitivity, specificity, and predictive
values with 95% confidence intervals for the table for coronary risks
of 15% and 30% over 10 years. In the people with mild hypertension
(systolic blood pressure 140-159 mm Hg) we examined the accuracy of
coronary risk of 15% over 10 years for predicting cardiovascular risk
of 20% over 10 years.
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Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Of the 1000 people studied 56.2% (562) were women; 29.9% (299)
smoked; and 1.6%16 were diabetic. The mean age was 49 years; mean blood pressure was 132/75 mm Hg; mean cholesterol concentration was 6.0 mmol/l; mean high density lipoprotein cholesterol was 1.45 mmol/l; and the mean TC:HDL ratio was 4.5. Altogether, 21.7%
(217) of people had mild hypertension, and 7.0% (70) had systolic
blood pressure of
160 mm Hg. Mean 10 year coronary and cardiovascular risks according to the Framingham risk function were
7.2% and 10.4% respectively, and the 10 year coronary risk was
15% in 13.3% (133) of people and
30% in 2.2% (22).
The Sheffield table had 97% sensitivity and 95% specificity for
coronary risk of
15% over 10 years. The predictive value of a
negative test was 99.5% and of a positive test 73%, with all those
with false positive results having a coronary risk of 10.0-15.0% over
10 years (fig 2). For coronary risk of
30% over 10 years the
sensitivity was 82% and the specificity 99% (table 1). False negative
results were all only marginally above the 30% threshold, and those
with false positive results all had coronary risk of
20% over 10 years (fig 2). In those with systolic blood pressure of 140-159 mm Hg,
coronary risk of
15% over 10 years according to the table had 88%
sensitivity and 90% specificity for predicting cardiovascular risk
20% over 10 years (table 1). Those classified incorrectly all lay
close to the 20%
threshold.

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Fig 2.
Accuracy of new Sheffield table for predicting
risk of coronary heart disease of 15% over 10 years and 30% over 10 years in 1000 people assessed for primary prevention. For the 15%
threshold, sensitivity was 97% and specificity 95%; for the 30%
threshold, sensitivity was 82% and specificity 99%
Screening on basis of Sheffield table
According to this table, lipids would have been measured in
70% of this population (in 100% with coronary risk of
15%, in
97% with coronary risk of 5.0-14.9%, and in 46% with coronary risk
<5% over 10 years). The proportion of people who would have been
screened was higher in men than in women and increased with age (from
61% of men and 11% of women aged 35-44; to 100% of men and women
aged 55-64 years) (fig 3). The proportion of people screened increased
as the TC:HDL ratio increased (table 2). This reflects clustering of
hyperlipidaemia with other risk factors and is not a specific function
of the table. The screening rate in people with a TC:HDL ratio of
8.0 was high (94%), so that only two people above this level would
not have been screened, unless a family history of hyperlipidaemia was
suspected (see notes in figure
1).
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Discussion |
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Accuracy of table
The table identified correctly 97% of those with a risk of
coronary heart disease of
15% over 10 years; these people might
require treatment with aspirin and (where resources permit) statins for
primary prevention.6 High risk people not identified were
only marginally above the 15% threshold, and decisions that coronary
risk was below 15% over 10 years were 99.5% correct. The table
incorrectly identified for treatment 5% of people with coronary risk
below 15% over 10 years, but all had coronary risks of 10-15%, which
is a risk level at which statin treatment is safe.17 No
one with very low risk was identified for treatment.
30% over 10 years.
6 7 18
The table identified correctly 82% of those at such a risk, with those not identified for treatment only marginally above the threshold. One per
cent of the population were identified incorrectly as having coronary
risk
30% over 10 years, but all of these had a risk of 20-30%.
Coronary risk increases with age, and the table can be used to look
forward in time. Analyses of sensitivity and specificity ignore this
and underestimate the information provided by the table.
Dichotomising blood pressure
Most Framingham based risk methods offer a wide range of blood
pressures3-12 and seem more accurate than this table, but
our results indicate that little accuracy is sacrificed by
dichotomising blood pressure, even when uncontrolled hypertension is
ignored. The table is designed for use only after the control of
moderate to severe hypertension, with assessment for aspirin and
statins postponed until this is achieved. False negatives would not
have occurred had it been used in this way. The apparent accuracy for
blood pressure offered by other methods is misleading. In people whose
hypertension has been treated, pretreatment blood pressure
overestimates long term risk,19 whereas blood pressure taken while a person is taking treatment underestimates the risk because the risk remains higher than is predicted during
treatment.
19 20
The Sheffield table assumes that coronary
risk assessment is done after hypertension has been controlled, and it
approximates the persistently increased coronary risk in people
receiving treatment by using systolic blood pressure 160 mm Hg for risk calculation.
20% over
10 years in people with mild hypertension, with 88% sensitivity and
90% specificity.
Use of table as screening tool
In the United Kingdom selective lipids measurement in those at
high risk has been preferred to population screening, but this may need
reappraisal, given new evidence for the statins. The Sheffield table
identified for screening everyone with a coronary risk of
15% over
10 years without the need for general screening. Everyone aged
55
years, and almost everyone aged 45-54, needed screening. Savings from
selective screening will be attained only in younger people. At age
35-44 years, 65% of people (39% of men, 89% of women) need not be
screened, and few people aged under 35 would be screened. Selective
screening may miss some people with extremely high lipid concentrations
resulting from familial hyperlipidaemia; the Sheffield table, however,
detected most people with severe hyperlipidaemia because screening
aimed at those with high coronary risk coincidentally also reaches
those with high lipid concentrations. Among 1000 people, only two with
a ratio of total cholesterol to high density lipoprotein cholesterol of
8.0 were missed; they had ratios of 12.4 and 12.6 and would
generally be treated with a statin if detected. Unless diagnosed
through their family history, detection would require additional
routine screening of 297 people not otherwise screened, including 65% of people aged 35-44 years. The value of detecting these relatively uncommon individuals needs to be weighed against the additional cost,
resources, and harm from "labelling" (when "well" people become
"patients") as a result of general screening.
Targeting treatment at absolute risk
Compared with decisions based on blood pressure or lipids
thresholds alone, methods that entail simple counting of risk
factors21-23 improve the accuracy of risk assessment
significantly9 yet still identify for treatment some
people at very low risk15 who may be harmed by treatment
with, for example, aspirin, while failing to treat some with
exceptionally high risk. Framingham based methods are a step towards
ensuring that those at high risk get treatment and those at low risk
are not endangered. The Framingham estimates of coronary risk seem
acceptably accurate for the British population,24 but
additional risk factors, such as left ventricular hypertrophy, family
history, familial hyperlipidaemia, and ethnic status, influence
coronary risk (see notes in figure 1). Framingham based methods should
therefore guide but not dictate treatment decisions. The Sheffield
table identifies those who definitely should be offered treatments, but
it should not be used to deny treatment to people close to treatment
thresholds.
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What is already known on this topic
New guidelines for prescribing of statins, aspirin, and treatment of mild hypertension for primary prevention recommend targeting treatment according to absolute risk of coronary heart disease Doctors need simple but accurate methods for estimating such risk What this study addsA new Sheffield table has been developed to identify the coronary risk thresholds in current guidelines In a random sample of the population aged 35-64 years without atherosclerotic disease, estimates of coronary risk by this table were accurate when compared with coronary risk calculated using the Framingham risk function The sensitivity and specificity values were high for coronary risk of
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Acknowledgments |
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We thank Phil Sanmuganathan and Rod Williamson, who performed risk assessments, and the original depositors and data archive for access to data from the Scottish health survey 1995. Those who conducted the survey and the original analysis of the data bear no responsibility for their further analysis or interpretation. We acknowledge Crown copyright material (data collected in the Scottish health survey) reproduced by permission of the controller of HMSO.
Contributors: All authors contributed to the design of the study, development of the table, risk assessments, writing of the manuscript, and approval of the final version. IUH generated the Sheffield table from the Framingham equation. EJW coordinated development of the format of the table. IUH and EJW prepared the population data. KRY prepared study materials and coordinated the risk assessments. PG and PRJ performed the statistical analysis. LER designed the study, drafted the manuscript, and will act as guarantor for the study.
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Footnotes |
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Funding: None.
Competing interests: None declared.
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References |
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(Accepted 5 November 1999)
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