screening

1. Screening tests
Dr Mona Aboserea
professor of public health

2. SCREENING tests :
DEFINITION
Is the application of relatively simple and rapid test to a
large number of apparently healthy people in order to
classify them as likely or unlikely to have the disease.
Screening: Considered a secondary preventive
intervention

3. • Assessing a new diagnostic test is done versus an
accepted reference test known as “gold
standard”. By this gold standard individuals are
labeled as either diseased or free.
• Then, sensitivity, specificity, predictive value of
positives & predictive value of negatives can be
calculated for the new test.
Researches to study the performance of a diagnostic
test
“ screening test’’

5. -Disease/disorder should be
An important public health problem
High prevalence
Serious outcome
-Early Detection in a symptomatic (pre-clinical)
individuals is possible
-Early detection and treatment can affect the
course of disease (improve outcome)
-Effective ttt is available for the detected disease.
- Screening tests are simple, acceptable to population, inexpensive,
noninvasive, not painful, and relatively accurate
Criteria for Use of a Screening Test

6. Screening Test
Concerned with a Functional Definition of
Normality versus Abnormality
Screening Test
Normal
Abnormal

7. Special requirements of Screening Tests
A-Valid: ability of the screening test to do
what is supposed to do. Measured by;
–Sensitivity: ability to detect +ve cases among
truly +ve
–Specificity: ability to detect –ve cases among
truly –ve
– Positive Predictive Value: proportion testing positive
who actually have the disease
– Negative Predictive Value: proportion testing negative
who do not have the disease

8. Special requirements of Screening Tests
b-Reliable: It is the ability of the test to give
same results when repeated under standard
conditions.

9. Group (a(
True Positive
Group (b(
False Positive
Group (c(
False Negative
Group (d(
True Negative
The 2x2 Table describes screening test outcomes:
Disease
present
Disease
absent
Positive result
Negative result
Application of Screening to
Populations
Diagnostic test or gold standardScreening test

10. Group (a(
True Positive
Group (b(
False Positive
Group (c(
False Negative
Group (d(
True Negative
Characteristics of Screening
Tests
1) Sensitivity: proportion of those with disease
who test positive in the screened group
(a)
(a) + (c)
Positive
result
Negative
result
Disease
present
Disease
absent
Total

11. Group (a(
True Positive
Group (b(
False Positive
Group (c(
False Negative
Group (d(
True Negative
Characteristics of Screening
Tests
2) Specificity: proportion of those without disease who
test negative in screened group
(d)
(b) + (d)
Positive
result
Negative
result
Disease
present
Disease
absent
Total

12. 200
True positive
0
False positive
0
False negative
800
True negative
Disease
present
Disease
absent
Positive
result
Negative
result
n = 200 n = 800
The Ideal Situation--100%
Agreement

13. 170
True Positive
30
False Positive
30
False Negative
770
True Negative
Disease
present
Disease
absent
Positive
result
Negative
result
n = 200 n = 800
A More Likely Outcome

14. 3) Positive Predictive Value (PPV): The
likelihood that a positive test result indicates
the presence of the disease
(a)
(a) + (b)
Characteristics of Screening
Tests
Group (a(
True Positive
Group (b(
False Positive
Group (c(
False Negative
Group (d(
True Negative
Positive result
Negative result
Disease
present
Disease
absent
Total

15. 4) Negative Predictive Value (NPV): The
likelihood that a negative test result
indicates the absence of the disease
(d)
(c) + (d)
Characteristics of Screening Tests
Group (a(
True Positive
Group (b(
False Positive
Group (c(
False Negative
Group (d(
True Negative
Positive result
Negative result
Disease
present
Disease
absent
Total

16. Criteria for Evaluating a Screening
Test
•Validity : provide a good indication of who does and does
not have disease
-Sensitivity of the test
-Specificity of the test
•Reliability=reproducibility=precision : gives
consistent results when given to same person under the same
conditions
‘’Agreement within and between examiners’’

17. Validity of Screening Test (Accuracy)
- Sensitivity: Is the test detecting true cases of
disease? (Ideal is 100%: 100% of cases are
detected)
-Specificity: Is the test excluding those without
disease? (Ideal is 100%: 100% of non-cases are
negative)

18. • Advantages
• Disadvantages
Screening test
Gold standard test
test diseased healthy Total
positive a b a + b
negative c d c + d
Total a + c b + d a + b + c + d

19. Advantages:
Magnitude of disease can be precisely
assessed, where pre-symptomatic cases are
not missed.
Early detected cases can be effectively
controlled, with better prognosis, no or
minimal consequences and less burden on
health services.
Screening test

20. Disadvantages:
Not 100% accurate test. Detected cases may
be actually free of the disease (called false
positive), on the other hand the test may
diagnose actually diseased persons (false
negative). The higher the percentages of
false positive and false negatives the worse
is screening test.
Screening test

21. 1-Mass screening:
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 type 2 DM in certain community.
Types of Screening program

22. 2-Selective screening: They are used to
detect specific disease in people who are
known to be at high risk.
e.g. screening for bone thickness among
females at menopause, type 2 DM in obese
persons, chest X-ray for males heavy
smokers with chronic cough.
Types of Screening program cont.

23. 3-Opportunistic 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
Types of Screening program cont.

24. • Exercise .
A medical research team conduct a trial to
find if high plasma level of breast carcinoma
promoting factor (BCPF) could be used to
diagnose breast cancer.
• Out of 1600 patients included in the study,
600 demonstrated by breast biopsy (the gold
standard) to have breast cancer (D+) and
1000 were found to be disease –free(D-)
• Out of the 600 demonstrated to have breast
cancer, 570 were positive by BCPF(T+) and
Out of the 1000 were found to be disease –
free, 850 were negative by BCPF(T-)

25. Feedback of Exercise .8:- It is an example of studying
the performance of a new diagnostic test
PATHOLOGY
STUDIED
TEST
Breast
cancer(D+)
No breast
cancer (D-)
Total
Marker (+)
(T+)
570 (TP) 150 ( FP) 720
Marker (-)
(T-)
30 (FN) 850 (TN) 880
Total 600 1000 1600

26. • Feedback of Exercise .8(cont.):-
• Sensitivity, specificity, predictive value positive &
predictive value negative can be calculated
• Sensitivity= 570/600 = 0.95 = 95%
• Specificity=850/1000 = 0.85 = 85%
• Predictive value positive= 570/720 = 0.79=79.2%
• Predictive value negative= 850/880 = 0.97=96.6%

27. Remember Steps in the
Research Process
1.Define the Problem
2. Review the literature
3-Conduct research on the problem.
4. Define the population
5-clearly define terms & concepts

28. Steps in the Research Process
6. Conduct experiments, collect data.
7-Analyze the data
8. Present the results
9-Dissemination.