Screening of a disease

1. SCREENING OF A DISEASE
Dr Jazeela Mohamed Siddique
Senior Resident
Department of Community Medicine

2. Definition
• The search for unrecognized disease or defect
• By means of rapidly applied tests, examinations or
other procedures
• In apparently healthy individuals
INTRODUCTION

3. PROBLEM STATEMENT
Iceberg phenomenon of disease

4. PREVENTION AND
NATURAL HISTORY OF
DISEASE
Biological
onset of
disease
Symptoms
appear
Pre-clinical phase
Diagnosis Therapy
begins
Clinical phase
outcome
Promoting
health and
primary
prevention
Tertiary
prevention
Rehabilitation,
support
Potential improvement by
screening
Secondary
prevention
Screening and
early detection

5. Flow chart for Screening
Apparently Healthy
Screening
Probably have disease Probably do not have
disease
Diagnostic Test
Diseased Not Diseased Periodic Screening 5

6. SCREENING TEST DIAGNOSTIC TEST
Apparently healthy Indications or sick
Applied to groups Applied to single patients all diseases
are considered
Test results are arbitrary and final Diagnosis not final but modified; sum
of all evidence.
Based on one cut-off point Based on evaluation of number of
symptoms, signs and lab findings
Less accurate More accurate
Less expensive More expensive
Not a basis for treatment Used as a basis for treatment
The initiative comes from the
investigator or agency providing care
The initiative comes from a patient
with a complaint

7. Concept of lead time
Disease onset
detection
First
possible
point
Final critical
diagnosis OUTCOME
Usual time of
diagnosis
Screening time
Lead time
A
B
7
A - usual outcome of the disease
B - advantage gained by early detection of the disease
B-A - benefit by the screening programme.

8. USES OF SCREENING
CASE DETECTION Prescriptive screening
People are screened for their own
benefit.(cancer, DM, HTN)
CONTROL OF DISEASE Prospective screening
People are screened for the benefit of
others.(HIV,STD)
RESEARCH to know the natural history
of disease
EDUCATION public awareness

9. TYPES OF SCREENING
1. Mass screening
2. High risk or selective screening
3. Multiphasic screening

10. Mass screening
• Screening of whole population
• Irrespective of risk of
contracting the disease
• Eg: visual defects in school
children
• Indiscriminate mass screening
not useful unless backed up by
a suitable treatment
High risk screening
• screening of selected high-risk
groups in the population
• Eg: screening for HIV in risk
groups
• Effective and economical use
of resources
TYPES OF SCREENING

11. •Two or more screening tests in combination to a large no of
people at one time
• Eg: Chemical and hematological tests on blood and urine
specimens, LFT, audiometry, and measurement of visual acuity
Multiphasic screening

12. CRITERIA FOR SCREENING
• Disease to be screened
• Screening test
– Acceptability
– Repeatability
– Validity
– Yield
– simplicity, safety, rapidity, ease of administration
and cost

13. • Important health problem high prevalence
• Recognizable latent or early asymptomatic stage
• Natural history of the condition should be known
• Test that can detect the disease prior to onset of signs &
symptoms
• Facilities to confirm diagnosis
DISEASE TO BE SCREENED - CRITERIA

14. • Effective treatment should be there
• There should be an agreed-on policy concerning whom to treat
as patients
• Evidence that early detection & treatment reduces morbidity
& mortality
• Expected benefits greater than risk and cost of screening
DISEASE TO BE SCREENED - CRITERIA

15. 1.ACCEPTABILITY
Cooperation and acceptable to population
SCREENING TEST - CRITERIA

16. 2. REPEATABILITY/ RELIABILITY/ PRECISION/
REPRODUCIBILITY
• The test must give consistent results when repeated more
than once on the same individual or material, under the same
conditions.
• Depends on 3 major factors:
➢ Observer variation.
➢ Biological variation
➢ Errors relating to technical methods.

17. 3.VALIDITY (ACCURACY)
• The term validity refers to what
extent the test accurately
measures which it purports to
measure.
• Ability of the test to distinguish
those who have the disease
from those who do not.
• Two components - Specificity
and sensitivity

18. 4. YIELD
It is the amount of previously unrecognized disease that is
diagnosed as a result of screening effort.
Yield depends up on :
▪ sensitivity
▪ specificity
▪ prevalence of the disease
▪ community participation
▪ availability of medical care

19. EVALUATION OF A SCREENING TEST
Screening test
results
Cancer Diagnosis(Sx biopsy) Total
Diseased Not diseased
Positive(FNAC) a (14)
( true positive)
b(8)
(false positive)
a + b (22)
Negative(FNAC) c (1)
(False negative )
d (91)
( true negative)
c + d (92)
Total a + c(15) b + d (99) a+b+c+d (114)

20. 1. Sensitivity
• It is defined as the proportion of the diseased people who
were correctly identified as positive by the test.
TP/ TP+FN = a/ a+c x100 = 14/ 14+1 x100 = 93%
• For the FNAC test,93% of all the patients with breast cancer
had positive test results
EVALUATION OF A SCREENING TEST

21. 2. Specificity
• It is defined as the proportion of non diseased people who are
correctly identified as negative by the test.
TN/TN+FP = d / d+b x100 = 91/91+8 x 100 =92%
• If the test is highly specific ,a positive test result should
strongly suggest the presence of the disease of interest.

22. 3. Positive Predictive value
• The "predictive value of a positive test" indicates the
probability that a patient with a positive test result has, in
fact, the disease in question.
PPV = TP/TP+FP X 100 =a / a+b x 100 =
14/ 14+8 x100= 64%

23. 4. Negative Predictive value
• The "predictive value of a negative test" indicates the
probability that a patient with a negative test result doesn't
have disease in question.
• NPV= TN /TN+FN x100 = d / d+c x100 =91/91+1x100=99%

24. 5. Percentage of false positives
• Means that patients who do not have the disease are told
that they have the disease.
• FP/ FP + TN *100
= b/ b+ d *100
= 8 / 8+91x100 =8%

25. 6. Percentage of false negatives
• Means that patients who actually have the disease are told
that they do not have the disease.
• FN/ TP + FN *100
= c/ a+ c *100
= 1/1+14 x100 = 6.6%

26. CHANGING CUT-OFF POINTS:
Where to draw cut off points

32. • High disease prevalence  set low cut off point
• Disease is lethal  set low cut off point
Where to draw cut off points

33. THANK YOU
https://www.slideshare.net/JazeelaMohamedSiddiq