Application of relatively simple & rapid test to a large number of apparently healthy people in order to classify them as likely or unlikely to have the disease.

1. Dr. Dalia El-Shafei
Assistant Professor, Community Medicine Department,
General overview

2. Application of relatively simple & rapid test to a
large number of apparently healthy people in order
to classify them as likely or unlikely to have the
disease.
Screening

4. Asymptomatic Symptomatic

6. Help to use measures that can prevent or delay the
development of disease or at least improve the prognosis.
An initial examination to identify disease in the pre-
clinical phase, it is usually not diagnostic & requires
appropriate specific investigations, follow up and treatment.
Can be a step after sampling or we can screen all individuals
at the same place as in survey studies or a cluster sample.

7. Types of screening program
•Large numbers of people are screened for the presence of a disease without
specific reference to their individual risk of having or developing the
condition.
•As screening for hypertension in certain city.
Mass screening
•They are used to detect specific disease in people who are known to be at
high risk.
•For example screening for bone thickness among females at menopause,
chest X-ray for males heavy smokers with chronic cough.
Selective screening
•Applied when the opportunity arises
• As for example in pre-placement examination, examination of students at
school entry and soldiers who join the army by general clinical examination,
urine and stool analysis, blood picture, dental examination and visual acuity
Opportunistic screening

8. Advantages
Early detection of
disease among
subclinical cases
for better
prognosis.
Identify individuals
who may be at risk
of some health
hazards.
Identify individuals
who are carriers of
disease & thus
protect the
population by
isolating these
carriers.
Magnitude of the
disease can be
precisely assessed,
where pre-
symptomatic cases
are not missed.

9. Disadvantage
Not 100%
Accurate test
Detected cases may
be actually free of
disease
False +ve
Miss diagnosis of
truly diseased persons
False -ve

10. Requirements for screening test
General
Specific
Valid Reliable

11. General requirements
• There must be available treatment for detected cases, it is
unethical to diagnose disease early among people and you have
nothing to do for them.
• Simple
• Not invasive, not painful and
• Accepted by people.
• Performed in the least possible time
• Not necessarily by medical, but by trained personnel.
• Inexpensive for mass application

12. Special requirements
• Valid: It is the ability of the screening test to do what
it is supposed to do, and it is measured by its
sensitivity & specificity.
• Reliable: It is the ability of the test to give same
results when repeated under standard conditions.

13. The groups screened are divided into 4 sections
according to their results in comparison to more
specific accurate test “Gold standard test”.
4Outcomes
Diseased &+ve by
screening
Diseased &-ve by
screening
Non-diseased & +ve
by screening
Non-diseased & -ve
by screening

14. Gold standard test
Total-ve+ve
PVP=a/a+b
x100
a +bb
(false
+ve)
a
(true +ve)
+ve
PVN=d/c+d
x100
c+dd
(true -ve)
c
(false -ve)
–ve
a+b+c+db+da+cTotal
Accuracy=
a+d/a+b+c+d
Specificity =
d/b+d x 100
Sensitivity =
a/a+c x 100
Screeningtest
The higher the sensitivity & specificity, the more valid
is the screening test

15. • % of truly -ve cases
detected from all
-ve individuals
diagnosed by
screening test
• % of truly +ve cases
detected from all
+ve individuals
diagnosed by
screening test
• Ability of the
screening test to
detect -ve cases
among truly -ve
• Ability of the
screening test to
detect +ve cases
among truly +ve
Sensitivity Specificity
Predictive
value
negatives
“PVN”
Predictive
value
positives
“PVP”

16. Choosing a screening test is not easy, it need trial of
many tests for diagnosis and that which gives the
highest sensitivity & specificity will be the best.

17. Example:
Total-ve+ve
45
55
30(false+ve)
45(true –ve)
15(true+ve)
10(false -ve)
+ve
–ve
10075 (free)25 (dis.)Total
 Sensitivity = 15/25 x 100 = 60%
 Specificity = 45/75 x 100 = 60%
 Predictive value +ve = 15/45x 100= 33.3%
 Predictive value –ve =45/55x 100= 81.8%
 Accuracy= 15+45/100 x 100=60%
Mammography (gold standard)
Selfexam
(screening)
So, breast self-examination screening test has low validity
because of low sensitivity & specificity