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Dr. Dalia El-Shafei
Assistant Professor, Community Medicine Department,
General overview
Application of relatively simple & rapid test to a
large number of apparently healthy people in order
to classify them as ...
Asymptomatic Symptomatic
Help to use measures that can prevent or delay the
development of disease or at least improve the prognosis.
An initial ex...
Types of screening program
•Large numbers of people are screened for the presence of a disease without
specific reference ...
Advantages
Early detection of
disease among
subclinical cases
for better
prognosis.
Identify individuals
who may be at ris...
Disadvantage
Not 100%
Accurate test
Detected cases may
be actually free of
disease
False +ve
Miss diagnosis of
truly disea...
Requirements for screening test
General
Specific
Valid Reliable
General requirements
• There must be available treatment for detected cases, it is
unethical to diagnose disease early amo...
Special requirements
• Valid: It is the ability of the screening test to do what
it is supposed to do, and it is measured ...
The groups screened are divided into 4 sections
according to their results in comparison to more
specific accurate test “G...
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 -...
• % of truly -ve cases
detected from all
-ve individuals
diagnosed by
screening test
• % of truly +ve cases
detected from ...
Choosing a screening test is not easy, it need trial of
many tests for diagnosis and that which gives the
highest sensitiv...
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
 Sensit...
Screening2
Screening2
Screening2
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Screening2

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.

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Screening2

  1. 1. Dr. Dalia El-Shafei Assistant Professor, Community Medicine Department, General overview
  2. 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
  3. 3. Asymptomatic Symptomatic
  4. 4. 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.
  5. 5. 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
  6. 6. 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.
  7. 7. Disadvantage Not 100% Accurate test Detected cases may be actually free of disease False +ve Miss diagnosis of truly diseased persons False -ve
  8. 8. Requirements for screening test General Specific Valid Reliable
  9. 9. 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
  10. 10. 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.
  11. 11. 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
  12. 12. 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
  13. 13. • % 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”
  14. 14. 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.
  15. 15. 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

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