1. Dr. Tauseef Ismail Assistant Professor Dept of C Med. KGMC
Screening
Dr. Tauseef Ismail Assistant Professor
Dept of C Med.
KGMC

2. Introduction
Screening is the identification of unrecognized disease by application of rapid tests to separate well persons who probably have the disease from those who probably do not have the disease.
A screening test is not intended to be diagnostic.
Persons with positive results should be referred for diagnosis and treatment.

3. Characteristics of a Good Screening test
Inexpensive
Easy to administer
Minimal discomfort
Reliable (consistent)
Valid (distinguishes diseased & non-diseased)

4. Diagnostic test & Screening test
A diagnostic test is used to determine the presence or absence of a disease when a subject shows signs or symptoms of the disease.
„ A screening test identifies asymptomatic individuals who may have the disease.
„ The diagnostic test is performed after a positive screening test to establish a definitive diagnosis.

5. Types of screening Mass screening Targeted screening
 involves screening a large population (e.g., chest x-rays for TB)
Targeted screening
Selected groups who are anticipated to have an increased prevalence of the condition for which screening has been instituted (e.g. Measuring the blood cholesterol in relatives of people with familial hyperlipidemia )
OR groups with specific exposures is often used in environmental and occupational health (e.g., battery workers).

6. Multiple or multiphasic screening involves the use of several screening tests on the same occasion (e.g., an annual health check-up).
Case-finding (or opportunistic screening) is a form of screening restricted to patients who consult a health practitioner for some other purpose (the GP may take your blood pressure when you come for your 'flu shot)

7. Some common Screening tests
Pap smear for cervical dysplasia or cervical cancer
Fasting blood cholesterol for heart disease
Fasting blood sugar for diabetes
Blood pressure for hypertension
Mammography for breast cancer
PSA test (prostate specific antigen) for prostate cancer
Fecal occult blood for colon cancer
Ocular pressure for Glaucoma
PKU test for Phenylketonuria in newborns
TSH for hypo & hyperthyroid
PSA: Prostate-specific antigen

8. Quality of Screening Tests
Depends on:
Validity – Ability of the test to distinguish between who has a disease and who does not
A perfect test would be perfectly valid
Reliability – Repeatability of a test
A perfectly reproducible method of disease ascertainment would produce the same results every time it was used in the same individual.

9. Validity of Screening Test
It has Two components;
Sensitivity – the ability of the test to identify correctly those who HAVE the disease; the search for diseased persons
Specificity – the ability of the test to identify correctly those who DO NOT HAVE the disease; the search for well persons
Sensitivity and specificity quantify a test’s accuracy in the presence of known disease status.

10. - + a + b - c d True Disease Status Screening Test Results of
a = true positive
b = false positive
c = false negative
d = true negative

11. - + a + b - c d True Disease Status Screening Test
Results of
Screening Test + - + a b - c d
Sensitivity: The probability of testing positive if the disease is truly present
Sensitivity = a / (a + c)

12. - + a + b - c d True Disease Status Screening Test
Results of
Screening Test + - + a b - c d
Specificity: The probability of screening negative if the disease is truly absent
Specificity = d / (b + d)

13. + - 132 + 983 - 45 63650 Sensitivity: a / (a + c) Sensitivity =
Breast Cancer
Physical Exam
and Mammo-
graphy + - +
132
983 - 45
63650
Sensitivity: a / (a + c)
Sensitivity =
Specificity: d / (b + d)
Specificity =

14. - + 132 983 + 45 63650 - Breast Cancer Physical Exam and Mammo- graphy
Sensitivity: a / (a + c)
Sensitivity = 132 / ( ) = 74.6%
Specificity: d / (b + d)
Specificity = / ( ) = 98.5%

15. Sensitivity: Screening by physical exam and
mammography will identify 75% of all true breast
cancer cases.
Specificity: Screening by physical exam and
mammography will correctly classify 98.5% of all non-breast cancer patients as being disease free.

16. Repeatability
When there is no satisfactory standard against which to assess the validity of a measurement technique, then examining its repeatability is often helpful.

17. Reliability (Repeatability) of Tests
Can the results of a test be replicated?
If a test is valid but NOT reliable, results are meaningless
Factors that contribute to variation between test results:
Intra-observer variation – variation within individual subjects by the same observer
e.g. blood pressure variation in an individual
Inter-observer variation – variation between those reading test results
Intra-Subject variation- blood pressure, blood sugar, serum cholesterol

18. Inter-observer Variation
Two observers may not always arrive at the same results
The extent or magnitude of disagreement is important…if disagreement is large the results are less meaningful or less reliable
Variation between observers can be quantified by calculating a percent agreement

19. Overall percent agreement
Analyzing repeatability
Overall percent agreement
Reading No 1
Reading No 2
Abnormal
Suspect
Doubtful
Normal A B C D
Abnormal +
Suspect E F G H + I J K L
Doubtful +
Normal M N O P
A + F + K + P
× 100
Percent agreement =
Total Readings

20. Inter-Observer Agreement in Grading Severity of Cataract
Reliability (reproducibility)
Agreement within and between examiners
________________________________________________
Inter-Observer Agreement in Grading Severity of Cataract
Examiner 1: Grade
Examiner 2 <1 1-< < <4 4
<
1-<
2-<
3-<
% Agreement = 83.3%

21. Predictive Value of a test
Reflects diagnostic power of a test
The probability that a patient with a positive test has in fact the disease in question
Positive PV
The probability that a patient with a negative test has in fact not got the disease in question
Negative PV

22. Comparing the definitions
Sensitivity: The probability of testing positive if the disease is truly present
Specificity: The probability of screening negative if the disease is truly absent
Positive PV: The probability that a patient with a positive test has in fact the disease in question
Negative PV: The probability that a patient with a negative test has in fact not got the disease in question

23. a + b c + d Disease Status P A + Screening − test a b c d
Positive predictive value = probability of the person having the disease when the test is positive
= a/a+b
Negative predictive value= probability of the person not having the disease when the test is negative
= d/c+d

24. a b c d
sensitivity
specificity
PPV
NPV

25. Criteria for Screening Programs
There are WHO guidelines for deciding when screening is appropriate ,drawn up by Wilson and Jungner in 1968;
Condition should be an important health problem
The natural history should be well understood.
There should be a detectable early stage.
There should be a suitable test for the early stage.
The test should be acceptable to the population to be screened.
The cost should be balanced against benefits.

26. A school nurse routinely tests all new children ages 6-14 for strabismus (having crossed eyes). The prevalence of this condition at this age group is approximately 5 %. The nurse’s examination has a sensitivity and specificity of 90%. She refers those who test positive to the ophthalmologist; whose examination also has sensitivity and specificity of 90%.
Q-1. If this nurse examines 2000 children, she is going to refer to the ophthalmologist:
Q-2. The predictive value of a positive test by the nurse is:

27. For Further Reading