Measuring health and Disease/
The measure of health and disease is fundamental to the practice of
epidemiology.
A variety of measures are used to characterize the overall health
of populations.
Population health status is not fully measured in many parts of the
world,
and this lack of information poses a major challenge for
epidemiologists.
Defining health and disease
The most ambitious definition of health is that proposed by WHO in
1948: “health
is a state of complete physical, mental, and social well-being and
not merely the
absence of disease or infirmity.”1 This definition – criticized
because of the difficulty
in defining and measuring well-being – remains an ideal. The World
Health Assembly
resolved in 1977 that all people should attain a level of health
permitting
them to lead socially and economically productive lives by the year
2000. This commitment
to the “health-for-all” strategy was renewed in 1998 and again in
2003.2
Practical definitions of health and disease are needed in
epidemiology, which
concentrates on aspects of health that are easily measurable and
amenable to
improvement.
Definitions of health states used by epidemiologists tend to be
simple, for example,
“disease present” or “disease absent” The development of
criteria to establish the presence of a disease requires a definition
of “normality” and
“abnormality.” However, it may be difficult to define what is
normal, and there is
often no clear distinction between normal and abnormal, especially
with regard to
normally distributed continuous variables that may be associated with
several diseases
For example, guidelines about cut-off points for treating high blood
pressure are
arbitrary, as there is a continuous increase in risk of
cardiovascular disease at every
level A specific cut-off point for an abnormal value is based on an
operational definition and not on any absolute threshold. Similar
considerations apply
to criteria for exposure to health hazards: for example, the
guideline for a safe blood
lead level would be based on judgment of the available evidence,
which is likely to
Diagnostic criteria
Diagnostic criteria are usually based on symptoms, signs, history and
test results. For
example, hepatitis can be identified by the presence of antibodies in
the blood;
asbestosis can be identified by symptoms and signs of specific
changes in lung function,
radiographic demonstration of fibrosis of the lung tissue or pleural
thickening,
and a history of exposure to asbestos fibres. Table 2.1 shows that
the diagnosis of
rheumatic fever diagnosis can be made based on several manifestations
of the disease,with some signs being more important than others.
In some situations very simple criteria are justified.
For example, the reduction of mortality due to bacterial
pneumonia in children in developing countries depends
on rapid detection and treatment. WHO’s casemanagement
guidelines recommend that pneumonia case
detection be based on clinical signs alone, without auscultation,
chest radiographs or laboratory tests. The only
equipment required is a watch for timing respiratory rate.
The use of antibiotics for suspected pneumonia in children–
based only on a physical examination – is recommended
in settings where there is a high rate of bacterial
pneumonia, and where a lack of resources makes it impossible
to diagnose other causes.5
Likewise, a clinical case definition for AIDS in adults
was developed in 1985, for use in settings with limited diagnostic
resources.6 The
WHO case definition for AIDS surveillance required only two major
signs (weight
loss ≥ 10% of body weight, chronic diarrhoea, or prolonged fever) and
one minor
sign (persistent cough, herpes zoster, generalized lymphadenopathy,
etc). In 1993,
the Centers for Disease Control defined AIDS to include all HIVinfected individuals
with a CD4+ T-lymphocyte count of less than 200 per microlitre.
Measuring disease frequency
Several measures of disease frequency are based on the concepts of
prevalence and
incidence. Unfortunately, epidemiologists have not yet reached
complete agreement
on the definitions of terms used in this field. In this text we
generally use the terms
as defined in Last’s Dictionary of Epidemiology .11
Population at risk
An important factor in calculating measures of disease frequency is
the correct estimate
of the numbers of people under study. Ideally these numbers should
only include
people who are potentially susceptible to the diseases being studied.
For instance,
men should not be included when calculating the frequency of cervical
cancer