Screening is defined as the search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals.
The intention of screening is to identify disease in a community early, thus enabling earlier intervention and management in the hope to reduce mortality and suffering from a disease.
Case Scenario 1:
A mammography screening test for breast cancer was performed on 500 females. Screening test was positive in 100 individuals out of which only 35 females were positive for disease by Fine needle aspiration cytology. 250 females were true negative.
 Construct 2 x 2 table for the above information
 Label a, b, c & d cells.
 Calculate validity of the screening test & interpret your results in words.
Case Scenario 2:
A screening test was applied on smokers and nonsmokers to diagnose lung cancer in 1000 individuals. Out of 1000 individuals, 100 were smokers, out of whom 75 were diagnosed with lung cancer on Lung Biopsy. 900 were nonsmokers out of whom 125 were diagnosed with lung cancer.
 Construct a 2 * 2 table by the above information
 Label a, b, c and d.
 Calculate Yield of the screening test & interpret your results in words.
Case Scenario 3:
Screening Test Results 
Patients with Disease As confirmed by gold standard method Diseases Not Diseased 

Positive 
45 
20 
Negative 
98 
737 
 Calculate prevalence of the disease
 Calculate Accuracy & validity of the screening test & interpret your result in words.
Case Scenario 4:
In Village XYZ of Rawalpindi whose population is 1000, diabetes prevalence is 2 %. A screening test was applied on all population. Screening test was applied with sensitivity of 90 % and specificity of 95 %.
 Construct a 2 X 2 table with the above information.
 Calculate positive predictive value and negative predictive value & interpret your results in words.
Learning Objectives:
 Iceberg Phenomenon of disease.
 Time lag, lead time and screening time concepts.
 Screening aims and objectives
 Uses and types of Screening
 Screening test principles and its criteria (Wilson’s criteria)
 Shall be able to construct a 2 X 2 Table and be able to evaluate a screening test by finding out its
 Validity = Sensitivity & specificity,
 Yield = Positive predictive value and negative predictive values.
 Accuracy and Prevalence of the disease
 Shall be able to interpret the results in words.
Case scenario 1:
a) Construct 2X2 table & label a,b,c,d cells

Disease 
No Disease 
Total 
Test +ve 
35 (True +ve) 
65 (False +ve) 
100 
Test ve 
150 (False –ve) 
250 (True –ve) 
400

Total 
185 
315 
500 
b)Calculate validity of the test & interpret your results
Sensitivity = a/(a+c) x 100
= 35/185 x 100
= 18.91%
Specificity= d/(b+d) x 100
= 250/315 x 100
= 79.36%
Accuracy= (a+d)/(a+b+c+d) x 100
= (35+250)/500 x 100
= 85%
 The ability to detect true +ve is 18.91%
 The ability to detect true ve is 79.36%
 Accuracy of the screening test is 85%
Case scenario 2:
a) Construct 2X2 table & label a,b,c,d cells
Disease 
No Disease 
Total 

Test +ve 
75 (True +ve) 
25 (False +ve) 
100 
Test ve 
125 (False –ve) 
775 (True –ve) 
900

Total 
200 
800 
1000 
b) Calculate yield of screening test & interpret your results
Positive Predictive Value = a/(a+b) x 100
= 75/100 x 100
= 75%
Negative Predictive Value = d/(d+b) x 100
= 775/900 x 100
= 86.11%
 The test is able to predict that 75% of persons with a +ve test will have this disease
 The test is able to predict that 86.11% of persons with a ve test will not have this disease
Case scenario 3:
a) Calculate prevalence of the disease
Prevalence=Total diseased persons
=45+98
=143
b) Calculate accuracy & validity of the test & interpret your results
Sensitivity = 45/143 x 100
= 31.46%
Specificity= 737/757 x 100
= 97.35%
Accuracy= 45+737/900 x 100
= 86.88%
 The ability to detect true +ve is 31.46%
 The ability to detect true ve is 97.35%
 Accuracy of the screening test is 86.88%
Case scenario 4:
a) Construct 2X2 table
Disease 
No Disease 
Total 

Test +ve 
18 
49 
67 
Test ve 
2 
931 
933 
Total 
20 
980 
1000 
b) Calculate yield of screening test & interpret your results
PPV = 18/67 x 100
= 26%
NPV= 931/933 x 100
= 99.78%
 The test is able to predict that 26% of persons with a +ve test will have this disease
 The test is able to predict that 99.78% of persons with a ve test will not have this disease