1. Criteria of Screening for diseases Prepared By Dr
Criteria of Screening for diseases Prepared By Dr.Haitham Issa Hawler Medical University College of Medicin Department of Community Medicine

2. Contents:
Objective of screening
Criteria of the screening test
Criteria of disease to be screened
Sensitivity test
Specificity test

3. Objectives:
To discuss the criteria of the diseases to be screened
To discuss the criteria of the tests used in screening

4. Screening for Disease
Concept of screening
Definition:
The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other
procedures in apparently healthy individuals.
The original screening programmes were for diseases
such as T.B , syphilis or selected groups such
as antenatal mothers.
Today screening is considered a preventive care
function.

5. Aims and objectives of screening:
1- To sort out from a large group of apparently healthy persons those likely to have the disease or at increased risk of the disease under study.
2- To bring those who are "apparently abnormal" under medical supervision and treatment.
3- Screening is carried out in the hope that earlier diagnosis and subsequent treatment alters the natural history of the disease in those who are identified as "positive".

6. Natural History of Disease
Detectable sub clinical disease
Onset of symptoms
Point of Exposure
Susceptible Host
Subclinical Disease
Clinical Disease
Stage of Recovery, Disability, or Death
Screening
Diagnosis sought

7. Iceberg phenomenon of disease
The pattern of disease in hospitals is quite different from that in a community. The iceberg phenomenon is used to describe disease in the community. The floating tip represents what the physician sees in his practice. The submerged portion of the iceberg represents the hidden mass of disease (subclinical cases, carriers, undiagnosed cases), and its detection and control is a challenge to modern techniques in preventive medicine.

9. Screening:
is testing for infection or disease in populations or in individuals who are not seeking health care; as, serological testing for AIDS virus in blood donors, neonatal screening, premarital screening for syphilis.

10. use of clinical and/or laboratory tests to detect
Case Finding
use of clinical and/or laboratory tests to detect
disease in individuals seeking health care for other
reasons; as, the use of VDRL test to detect
syphilis in pregnant women.

11. Diagnostic tests Use of clinical and/or laboratory procedures to
confirm or refute the existence of disease in
patients with signs and symptoms presumed to
be caused by the disease; for example, VDRL
testing of patients with lesions suggestive of
secondary syphilis; endocervical culture for N.
gonorrhoeae.

12. Screening and Diagnostic Tests
A screening test is only an initial examination. positive test results
are referred to a physician for further diagnostic work-up and
treatment.
Screening and diagnostic tests may be contrasted as in this table:

13. Diagnostic test Screening Done on those with indications or sick.
1- Done on apparently healthy
person.
Applied to single patients
2- Applied to groups
Based on evaluation of a number of symptoms, signs and laboratory findings.
3- Based on one criteria
More accurate
4- Less accurate.
More expensive
5- Less expensive
Used as a basis for treatment
6- Not a basis for treatment
The initiative comes from the patient with a complaint.
7- The initiative comes from the investigator.

14. Screening differs from periodic health examinations in the following aspects:
-(1) capable of wide application.
-(2) relatively inexpensive.
-(3) requires little physician-time.

15. This figure depicts the natural history of many disease processes
This figure depicts the natural history of many disease processes. The disease goes through an asymptomatic, but detectable phase. This is the time during which screening can be applied.

18. CRITERIA FOR SCREENING
Based on two considerations:
- The DISEASE to be screened, and
- The TEST to be applied. .

19. The dis. to be screened should fulfill the following criteria:
1- It should be an important health problem,
severe, relatively common.
2- there should be a recognizable early
asymptomatic stage.
3- the natural history of the disease should be
adequately understood.
4- there is a test that can detect the disease
prior to the onset of signs &symptoms.

20. 5- Facilities should be available for confirmation of the D
6- There is an effective Rx.
7- There should be an agreed-on policy concerning whom to treat as patients.
8- There is good evidence that early detection and treatment reduces morbidity and mortality.
9- The expected benefits of early detection exceed
the risks and costs
10. In general, there should be a high prevalence of pre-clinical (early-stage) disease.

21. Characteristics of screening test
The test should be:
1. Highly sensitive and specific.
2.Applicable and acceptable to a large number of individuals.
3.Simple; accomplished easily and quickly.
4.Harmless to individuals being tested.
5.Inexpensive.

22. Parameters of screening test
Parameters of screening test measure the clinical usefulness of the test.
To determine the parameters fourfold table is used, the results are tabulated according to the persons true disease status which determined by diagnostic test.

23. Calculating Measures ( fourfold table)
True Diagnosis
Test Result
Disease
No Disease
Total
a(TP)
b(FP)
a+b
Positive
Negative
Sensitivity = a/(a+c); the probability of having a positive test if you are positive
Specificity = d/(b+d); the probability of having a negative test if you are negative
Positive Predictive Value = a/(a+b); the probability of having the disease if you test positive
Negative Predictive Value = d/(c+d); the probability of not having the disease if you test negative
Prevalence = (a+c)/(a+b+c+d)
Accuracy (efficiency of test) = (a+d)/(a+b+c+d)
c (fN)
d(TN)
c+d
Total
a+c
b+d
a+b+c+d

24. Parameters of screening test (cont.)
1.Sensitivity
2.Specificity
3.Positive predictive value
4.Negative predictive value

25. 1.Sensitivity Definitions:
It is screening tests ability to identify correctly those individuals who truly have the disease.(TP)
It is the number of individuals with the disease whose screening tests are positive to the total number of individuals with the disease under study .

26. 1.Sensitivity (cont.) Sensitivity usually expressed as a percentage.
It is independent of the disease prevalence in population being tested.

27. Calculation of sensitivity:
Sensitivity (%)= a / (a+c) x 100
Where:
a=number of individuals with the disease and +ve screening test (true +ve).
c= number of individuals with the disease and -ve screening test (false-ve).
a+c= to total number of individuals with the disease.

28. 2.Specificity Definitions:
It is the tests ability to identify correctly those individuals who truly do not have the disease.(TN)
It is the number of the individuals without the disease whose screening test are –ve to total number of individuals without the disease under study.

29. 2.Specificity(cont.) Specificity usually expressed as a percentage.
It is independent of the disease prevalence in population being tested.

30. Calculation of Specificity
Specificity (%) = d / (b+d) x 100
b =number of individuals without disease and +ve screening test (false +ve).
d=number of individuals without disease and -ve screening test (true -ve).
b+d=total number of individuals without the disease.

31. 3.Positive predictive value
Definitions:
It is the tests ability to identify those individuals who truly have the disease (true +ve) among all individuals who have +ve screening test.
It is the number of individuals with the disease whose screening test are +ve to total number of whose screening test are +ve.

32. 3.Positive predictive value(cont.)
Positive predictive value usually expressed as a percentage.
Increases with increasing prevalence.

33. Calculation of Positive predictive value (PPV)
PPV (%) = a / (a+b) x 100
A+b = Total number of individuals with a +ve screening test.

34. 4.Negative predictive value (NPV)
Definitions:
It is the tests ability to identify those individuals who truly do not have the disease (true -ve) among all individuals who have -ve screening test.
It is the number of individuals without the disease whose screening test are -ve to total number of whose screening test are -ve.

35. Negative predictive value(cont.)
Negative predictive value usually expressed as a percentage.
NPV decreased with increasing prevalence.
Calculation:
NPV (%) = d / ( c + d )
C+d = the total number of individuals with a –ve screening test.

36. Effects of Prevalence Sensitivity=95% Specificity=95%
Population’s
Prevalence
0.1%
1.0%
2.0%
5.0%
50%
Predictive Value of a Positive Test
1.9%
16.1%
27.9%
50%
95%

37. Total
Diagnosis
Screening test result
Not diseased
Diseased
a+b
b (false +ve)
a (true +ve)
positive
c+d
d (true -ve)
c (false -ve)
negative
a+b+c+d
b+d
a+c
Evaluation of a screening test is by these measures:
1- Sensitivity = a/ (a + c) X 100
2- Specificity = d/ (b + d) X 100
3- Predictive value of a positive test = a/(a + b) X 100
4-Predictive value of a negative test = d/ (c + d) X 100

38. Which is Preferred: High Sensitivity or High Specificity?
If you have a fatal disease with no treatment (such as for early cases of AIDS), optimize specificity
If you are screening to prevent transmission of a preventable disease (such as screening for HIV in blood donors), optimize sensitivity

39. Uses of screening
1- Case detection:
- It is identification of unrecognized disease, which does not arise from a patient's request.
- e.g. of this type of screening are screening for bacteruria in pregnancy, breast cancer, cervical cancer, diabetes mellitus, pulmonary TB.

40. 2- Control of disease: .
- People are examined for the benefit of others.
- e.g., screening of immigrants from infectious diseases such as tuberculosis and syphilis to protect the home population; and screening for streptococcal infection to prevent rheumatic fever.
- The screening programme may, by early diagnosis permit more effective treatment and reduce the spread of infectious disease and/or mortality from the disease.

41. 3-Research purposes:
- Screening may sometimes be performed for research purposes. For example Screening may aid in obtaining more knowledge about the natural history of many chronic diseases as cancer and hypertension, for example, initial screening provides a prevalence estimate and subsequent screening, an incidence figure.

42. 4Educational opportunities:
Apart from possible benefits to the individual and the acquisition of information of public health relevance, screening programmes (as for example, screening for diabetes) provide opportunities for creating public awareness and for educating health professionals. -

43. IFMSA-Kurdistan, World Diabetes Day - Family Mall -
14th of November 2012

44. IFMSA-Kurdistan, World Diabetes Day - Family Mall -
14th of November 2012

45. IFMSA-Kurdistan, World Diabetes Day - Fakhir Mergasuri high school -

46. Types of screening 1- Mass screening :
- It is screening of a whole population or a subgroup, as for all
adults.
- It is offered to all, irrespective of the particular risk individual may
run of contracting the disease in question.
- Mass screening for disease received strong support in the past. However, when a number of mass screening procedures were subjected to critical review, there appeared to be little justification for their use in many instances

47. 2- High risk or selective screening: Screening will be most productive if applied selectively to high-risk groups, the groups defined on the basis of epidemiological research. For e.g., since cancer cervix tends to occur relatively less often in the upper social groups, screening for cancer cervix in the lower social groups could increase the yield of new cases. More recently, epidemiologists have extended the concept of screening for disease to screening for "risk factors", as elevated serum cholesterol is associated with a high risk of developing coronary heart disease.

48. Application of two or more screening tests in
3- Multiphasic screening:
Application of two or more screening tests in
combination to a large number of people at one
time (e.g., chemical and haematological tests on
blood and urine specimens, lung function
assessment, and measurement of visual acuity),
all of which can be performed together.

49. Screening Guidelines for the Early Detection of Breast Cancer, American Cancer Society
Yearly mammograms are recommended starting at age 40.
A clinical breast exam should be part of a periodic health exam, about every three years for women in their 20s and 30s, and every yeara for women 40 and older.
Women should know how their breasts normally feel and report any breast changes promptly to their health care providers. Breast self-exam is an option for women starting in their 20s..
The American Cancer Society states that women aged 40 and older should have an annual mammogram and clinical breast exam (CBE) as part of a periodic health exam. Women should know how their breasts normally feel and report any changes to their health care provider. A breast self-examination (BSE) is an option for women starting in their 20s.

55. Screening Guidelines for the Early Detection of Cervical Cancer, American Cancer Society
Screening should begin after a women begins having vaginal intercourse .
Screening should be done every year with regular Pap tests or every two years .
The American Cancer Society cervical cancer screening guidelines state that women should begin screening approximately three years after she begins having vaginal intercourse, but no later than 21 years of age. Screening should be done every year with regular Pap tests or every two years using liquid-based tests. At or after age 30, women who have had three normal tests in a row may get screened every 2-3 years. Women 70 and older who have had three or more consecutive normal Pap tests in the last 10 years may choose to stop cervical cancer screening.

58. A fecal occult blood test (FOBT) every year
Screening Guidelines for the Early Detection of Colorectal Cancer, American Cancer Society
Beginning at age 50, men and women should follow one of the following examination schedules:
A fecal occult blood test (FOBT) every year
A flexible sigmoidoscopy (FSIG) every five years
Annual fecal occult blood test and flexible sigmoidoscopy every five years
A double-contrast barium enema every five years
A colonoscopy every ten years
The American Cancer Society recommends that beginning at age 50, men and women should receive a fecal occult blood test (FOBT) every year, or a flexible sigmoidoscopy (FSIG) every five years, or an annual FOBT and FSIG every five years (preferred to either method alone), or a double-contrast barium enema every five years, or a colonoscopy every ten years.

59. References:
1. Park K. Textbook of Preventive & Social Medicine. 16th edition. Banarsidas Bhanot India 2008.
2. Webb P and Bain Ch. Essentials of epidemiology. 2nd edition, Cambridge University Press, Cambridge UK 2011.
3. Fletcher R W. Clinical Epidemiology The essentials. Fourth edition. Lippincott Williams & Wilkins. Philadelphia, USA 2005.