Advance Concept of Screening

1. Advance concepts in screening
Aslam Aman
MPH, 3rd Batch
School of Health and Allied Science (SHAS)
Pokhara University
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2. Contents
 Introduction
 Review of Screening
 Sequential and parallel tests
 Net gain and net loss
 ROC curve in screening and interpretations
 Youden Index
 Cost consideration
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3. SCREENING
 the identification of unrecognized disease or
defect by the application of tests, examinations
or other procedures.
 sort out apparently well persons who probably
have disease from those who probably do not.
 not intended to be diagnostic.
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4. Definition
 The presumptive identification of those who
probably have disease from those who do not
have by means of rapidly applied tests in
apparently healthy individuals.
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6. WHO - Principles of Screening
1. The condition should be an important health problem.
2. There should be a treatment for the condition.
3. Facilities for diagnosis and treatment should be
available.
4. There should be a latent stage of the disease.
5. There should be a test or examination for the
condition.
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7. 6. The test should be acceptable to the population.
7. The natural history of the disease should be adequately
understood.
8. There should be an agreed policy on whom to treat.
9. The total cost of finding a case should be economically
balanced in relation to medical expenditure as a whole.
10. Case-finding should be a continuous process, not just
a "once and for all" project.
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9. Results of screening test
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10. Criteria for successful Screening test
 Reliability: getting the same results, when the test is
repeated in the same target individuals in the same
setting.
 Acceptability : the test should not be painful,
embarrassing, unsafe, socially not accepted.
 Validity : it should measure what it is intended to
measure. Ability of the test to distinguish between who
has the disease and who does not.
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14.  A 90% sensitivity means that 90% of the
diseased people screened by the test will give a
‘true positive’ and the remaining 10% a ‘false
positive’ results.
 A 90% specificity means that 90% of the non
diseased people screened by the test will give a
‘true negative’ and the remaining 10% a ‘false
negative’ results.
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18. Yield
 The amount of previously unrecognized disease
that is diagnosed and brought to treatment as a
result of the screening program.
 The higher the prevalence of disease is in the
population being screened, the higher the yield.
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21. Screening test in series and parallel
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25. Example of a Two-Stage Screening Program:
Test 2 (Glucose Tolerance Test)
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35.  Example of parallel test: screening of pregnant women for
VDRL, HIV, HBV by serological tests
 Example of sequential test:
 DM- FBS, Glucose tolerance test
 Sickle cell anemia- CBC, Hb electrophoresis
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39.  When simultaneous tests are used, there is a net gain in sensitivity
and net loss in specificity.
 In sequential testing, there is a net loss in sensitivity and a net
gain in specificity.
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40. Receiver operating characteristic
(ROC) Curve
 ROC curve is defined as a plot of test sensitivity or True
positive rate (TPR) as the y coordinate versus its 1-
specificity or false positive rate (FPR) as the x
coordinate, is an effective method of evaluating the
performance of diagnostic tests.
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45.  In an ideal situation, a point
determined by both TPR and FPR
yields coordinates (0,1). This ideal
point indicates that the screening
test has a sensitivity of 100% and
specificity of 100%.
 Screening test with 50% sensitivity
and 50% specificity lies on the
diagonal determined by coordinate
(0,0) and coordinate (1,1).
 A point predicted by a screening
test that falls into the area above the
diagonal represents a good
screening classification.
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46. Interpretation of ROC curve
 Total area under ROC curve is a single index for
measuring the performance of a test. The larger
the AUC, the better is overall performance of the
medical test to correctly identify diseased and
non-diseased subjects. Equal AUCs of two tests
represents similar overall performance of tests
but this does not necessarily mean that both the
curves are identical. They may cross each other.
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47.  Figure depicts three different
ROC curves. Considering the
area under the curve, test A is
better than both B and C, and
the curve is closer to the
perfect discrimination. Test B
has good validity and test C
has moderate.
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48. Uses of ROC curve
 ROC curve helps to choose the critical cut-off value which best
discriminate the presence or absence of a disease.
 ROC curve is used to compare two indicators. The curve that
contains a large area below it is a better predictor than one below
with a smaller area.
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49. METHOD TO FIND THE OPTIMUM CUT
– OFF POINT OF A SCREENING TEST
 Optimal threshold is the point that gives maximum correct
classification. Three criteria are used to find optimal threshold
point from ROC curve. These three criteria are
1. Points on curve closest to the (0, 1)
2. Youden index and
3. Minimize cost criterion
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50.  First two methods give equal weight to sensitivity and
specificity and impose no ethical, cost, and no
prevalence constraints.
 The third criterion considers cost which mainly includes
financial cost for correct and false diagnosis, cost of
discomfort to person caused by treatment, and cost of
further investigation when needed. This method is rarely
used in medical literature because it is difficult to
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51.  If sn and sp denote sensitivity and specificity,
respectively, the distance between the point (0, 1) and
any point on the ROC curve is
d = √[(1 – sn)2 + (1 – sp)2].
 To obtain the optimal cut-off point to discriminate the
disease with non-disease subject, calculate this distance
for each observed cut-off point, and locate the point
where the distance is minimum.
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52. The Youden's index
 Youden index that maximizes the vertical distance from line of
equality to the point [x, y].
 The x-axis represents (1- specificity) and y-axis represents
sensitivity.
 In other words, the Youden index J is the point on the ROC curve
which is farthest from line of equality (diagonal line).
 Y= Sensitivity +Specificity-1
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54.  Conceptually, the Youden index is the vertical distance between
the 45 degree line and the point on the ROC curve.
 Youden Index = Sensitivity +Specificity-1
 Higher values of the Youden index are better than lower values
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55. Cost consideration
 This approach is based on an analysis of the costs of the four
possible outcomes of a diagnostic test: true positive (TP), true
negative (TN), false positive (FP), and false negative (FN).
 If the cost of each of these outcomes is known. The average
overall cost C of performing a test at a given cutoff is given by
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56.  Here, C0 is the fixed cost of performing the test
 CTP is the cost associated with a true positive,
 P(TP) is the proportion of TP’s in the population, and so on.
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57. References
 Park K. Park’s textbook of preventive and social medicine. 22nd
edition. Jabalpur(India):Banarsidas Bhanot; 2012. p127-134.
 Gordis L. Text book of epidemiology. 4th ed. Saunders:
Elsevier;2009. p.85-108
 Hajian-Tilaki K. Receiver Operating Characteristic (ROC) Curve
Analysis for Medical Diagnostic Test Evaluation . Caspian
Journal of Internal Medicine. 2013;4(2):627-635.
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58.  Kanchanaraksha S. Evaluation of diagnostic and screening tests:
validity and reliability, 1st edition. John Hopkins Bloomberg
school of public health. 2011. p.1-124.
 Kumar R, Indrayan A. Receiver Operating Characteristic (ROC)
Curve for Medical Researchers. Indian Pediatrics.2011;48:277-
87.
 Hanley JA, McNeil BJ. The meaning and use of the area under a
receiver operating characteristic (ROC) curve. Radiology
1982;143:29-36.
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