2. DEFINITION of Screening Test
The search for unrecognized
disease or defect by means of
rapidly applied tests, examinations
or other procedures in apparently
healthy individuals.
3. Historical Background
The benefits of screening for disease prevention were first
demonstrated in the 1940s by the use of mass miniature
radiography (MMR) for the identification of individuals
with tuberculosis (TB). After the end of the Second World
War, when effective treatment for TB was introduced, the
use of MMR became widespread in many western countries.
Gradually the concept of screening
began to be considered equally
applicable to the prevention of other
diseases.
4. INTRODUCTION
The concept of screening in health care – that is,
actively seeking to identify a disease or pre-disease
condition in individuals who are presumed and
presume themselves to be healthy – grew rapidly
during the twentieth century and is now widely
accepted in most of the developed world. Used
wisely, it can be a powerful tool in the prevention
of disease. But it is essential to observe the long-
established principles and criteria and resist the
introduction of screening practices that do not
meet these requirements.
5. The criteria for screening are based on two
considerations
The DISEASE to be screened and
The TEST to be applied
6. The Disease to be Screened
The condition sought should be an important health
problem.
The condition should have a recognizable latent or
early symptomatic stage.
The natural history, including development from latent
to declared disease, is adequately understood.
There should be a suitable diagnostic test that is
available, safe and acceptable to the population
concerned.
7. There should be an agreed policy, based on respectable
test findings and national standards, as to whom to
regard as patients, and the whole process should be a
continuing one.
There should be an accepted and established treatment
or intervention for individuals identified as having the
disease or pre-disease condition and facilities for
treatment should be available.
The cost of case-finding (including diagnosis and
treatment) should be economically balanced in relation
to possible expenditure on medical care as a whole.
8. The Screening Test
The test must satisfy the criteria of :
Simplicity
Acceptability
Repeatability
Validity and
Cost
9. Simplicity
The test should be simple to perform, easy to
interpret and, where possible, capable of use by
paramedics and other personnel.
e.g.. Blood and urine tests and ECG for early
detection of hypertension.
10. Acceptability
Since participation in screening is voluntary, the
test must be acceptable to those undergoing it.
In general tests that are painful, discomforting
or embarrassing are not likely to be acceptable.
Eg. Screening for prostrate cancer might not be
acceptable to a large proportion of the
community.
11. Repeatability
The test should give consistent results in
repeated trials.
It is influenced by three factors:
Observer variation
Biological/ Subject variation
Errors relating to technical methods
12. Observer Variation
Intra-observer variation-variations by a single observer
on the same subject at same time. These can be
minimised by taking average of several readings.
Inter-observer variation-variations between different
observers on the same subject.
eg. Readings of BP or X rays.
These can be minimised
a) standardization of procedures
b) intensive training of observers
c) making use of two or more observers
13. Biological variation
There is biological variability associated with many
physiological variables. eg. BP, blood sugar
These may be due to
a) Changes in the parameters observed
b) Variations in the way patients perceive their
symptoms and answer.
c) Regression to the mean i.e. tendency for values at
extremes to regress to the mean or average on
repeat measurement.
14. Errors relating to technical methods.
These variations are inherent in the method.
These may be due to
a) defective instruments
b) erroneous calibration
c) faulty reagents etc.
In such circumstances results of a single test
may be unreliable.
15. Validity(Accuracy)
The test must give a true measurement of the
condition or symptom under investigation.
Eg. Though glycosuria is a useful screening test,
Glucose Tolerance Test is a more accurate test
Validity has two components
a) Sensitivity
b) Specificity
16. Sensitivity:
Ability of a test to identify correctly all those who have
the disease i.e. true positives.
Specificity:
Ability of a test to identify correctly all those who do
not have the disease i.e. true negatives
17. Details of GTT for detection of Diabetes
Screening test
results
Diagnosis
Diseased Not Diseased
TOTAL
Positive 40 20
(a) (b)
60
(a+b)
Negative 100 9840
(c) (d)
9940
(c+d)
140 9860
(a+c) (b+d)
10000
(a+b+c+d)
True Positives=40
Sensitivity=a / a+c 100=28.57%
True negatives=9840
Specificity=d / b+d 100=99.79%
18. Screening tests with high sensitivity give fewer false
negatives and thus prevent false reassurance to patients
actually having the disease.
Screening tests with high specificity give fewer false
positives and hence prevent unnecessary anxiety and
expense for those not having the disease.
Ideally a screening test must be 100%sensitive and
100%specific. But there is no such test.
If the disease is a serious one and if early detection is
useful for the patient then a more sensitive test must be
chosen at the cost of specificity.
19. Other important terms in Validity of a screening
test are:
a) Predictive value of a positive test-indicates
probability that a patient with a positive test result
has, in fact the disease in question.
b) Predictive value of a negative test- indicates
probability that a patient with a negative test result
doesn’t have the disease in question.
The Predictive value of a positive test increases if the
prevalence of the disease is more and decreases if its
less.
20. c) Yield- amount of previously unrecognized disease that
is diagnosed as a result of screening effort.
It depends on many factors : sensitivity, specificity,
prevalence of disease and participation of individuals.
21. Cost
And finally the expense of the test must be
considered in relation to the benefits of early
detection of the disease.
It should be economically balanced in relation to
possible expenditure on medical care as a whole.
23. Antenatal and neonatal screening
Two issues should be emphasized in the context of
antenatal screening.
The first is that care must be taken not to medicalize
this usually normal stage of life where most pregnancies
have a successful outcome.
The second is that there must be full, balanced and
understandable information available for pregnant
women and properly trained health professionals with
time to provide and/or explain it. This is important for
all pregnant women but particularly for those who
experience difficulty and defect.
24. Some of the routine screening tests in
antenatal period
Anaemia
Blood group and RhD status Blood test Early in pregnancy with
Hepatitis B effective follow-up for any
HIV abnormalities identified
Risk factors for pre-eclampsia
Rubella immunity
Syphilis
Asymptomatic bacteriuria Urine test As above
Fetal anomalies Ultrasound and Between 18 and 20 weeks
Anencephaly blood test, if with effective follow-up
Spina bifida indicated
Chromosome abnormalities Quadruple serum Second trimester with
Down syndrome tests, ultrasound effective follow-up
25. Screening procedures in the neonatal period can be
divided into those that are part of routine
screening for all newborn babies either by
clinical examination or biochemical tests and
those procedures for conditions such as hearing
loss that will require separate testing.
26. Common screening tests in neonatal period
Bloodspot
Phenylketonuria
Congenital hypothyroidism Must be properly evaluated
Cystic fibrosis In process of introduction for all neonates
Sickle cell disease
Physical examination
Congenital heart disease Adequate training programmes in
Congenital cataract physical examination must be developed
Cryptorchism
Congenital dislocation of the hip/ Use of ultrasound as primary screening
developmental dysplasia of the hip test to be evaluated
Other congenital malformations
Other tests
Hearing impairment Implementation ongoing
27. Childhood and adolescence
Screening and surveillance (or observation) in childhood are
important in following up difficulties already identified and in
diagnosing disorders for which effective treatment is available.
It should be a seamless extension of antenatal and neonatal care
and provides the opportunity for establishing a basis for good
health in later life with appropriate advice on healthy eating,
home and road safety, and immunization.
All reasonable steps should be taken at this early stage to
promote good health and prevent illness.
The more deprived and disadvantaged children and those who
have recently arrived from abroad as refugees or asylum seekers
may have missed out on earlier medical and dental checks and
strenuous efforts should be made to identify them to make sure
that any omissions or inequities are minimized.
28. Screening in adolescents and young adults is another crucial
area that needs to be approached with sensitivity.
This is a period of life when formal contact with the health
service is infrequent for most individuals.
But it is also a time when individuals are coping with
profound physical and emotional changes and seeking their
independence, but often lack the experience and judgment
to use it wisely.
The focus of screening and surveillance at this sensitive
stage should take account of what adolescents and young
adults themselves feel they require and how it can be most
effectively provided. Among their main needs are:
• accessible confidential health services;
• greater involvement in planning services;
• health education that reflects their experiences,
especially about drugs and alcohol;
• specialized advice centers for those with drug
problems.
29. Screening in adults
Media interest in health is insatiable, and anyone
who reads the newspapers, watches television or
listens to radio can hardly fail to be aware of the
various diseases that may be lying in wait for
them. Of course, it is of benefit if potential
health problems can be identified early and
treated, or at least alleviated.
eg. screening for breast cancer, cervical cancer,
colorectal cancer, coronary heart diseases etc.
30. Screening in Elderly
A system of regular surveillance and case-finding in
primary care would seem to be the most appropriate
form of screening, particularly in those aged 75 and
over, but the resource implications of this must be
confronted.
Several simple tests, such as
identifying difficulties with
sight or hearing or problems with feet,
can make a huge difference to the comfort and quality
of life.
31. The problem of Borderline
Blue- normal
Pink- diseased
Purple- borderline
Bimodal Distribution-Some diseases such as
phenylketonuria, are bimodal. In such cases the
shaded area consists of a mixture of persons
with and without the disease.
In such cases the point of intersection is taken as
cut off between normal and diseased.
32. Yellow-Borderline
Red-Diseased
Unimodal Distribution-many physiological variables
such as BP, blood sugar show this distribution. In such
cases there is no sharp cut off between normal and
diseased.
Placing cut off at a low level increases sensitivity and
decrease specificity while placing at a high level
increases specificity and decrease sensitivity.
33. Evaluation of Screening Programmes
In any screening programme, as with any other service
programme, adequate steps must be taken to ensure that the
original objectives are being met and that the methodology
meets appropriate standards.
The ideal method for evaluating a screening programme is the
randomized controlled trial in which individuals in a population
are allocated, at random, either to a group that is screened or to a
group that receives only its normal medical care. Randomized
controlled trials are expensive and difficult to manage and may
also be ethically questionable in situations where the control
group is denied treatment for the condition in question.
Other methods of evaluation are through Uncontrolled trials,
Case control studies etc.
34. CONCLUSION
Finally if the criteria of screening tests are followed
appropriately, screening could prove to be a
powerful tool for controlling morbidity and
mortality from a wide range of diseases.
Newer fields such as genetic screening are on the
rise which would help the cause.
