Screening for Disease: Part Two (problem solving)
By:Dr.Yossra K.Al-RobaiaayAssistant professorFICMS (FM)Problem 2: 20,000 women screened for breast cancer using mammography. As a result, 530 had excision biopsy. 134 of the biopsies showed cancer. In addition, 66 women developed breast cancer but were negative by screening. Evaluate test validity?
Mammography
Biopsy +veBiopsy -ve
total
positive
134
396
530
negative
66
19404
19470
total
200
19800
20000
Sensitivity=TP/TP+FN*100
= 67%
Specificity= TN/TN+FP*100
= 98 %
Problem 3: Rose-bengal test was done to 200 persons suspected to have brucellosis. 24 persons were positive, & the real number of patients was 30. Knowing that the predictive value of the positive test was 67%, write down your comments on test validity?
ROSE BENGAL TEST
+ve-ve
total
positive
16
8
24
negative
14
162
176
total
30
170
200
Positive PV=TP/TP+FN
67/100=TP/24
TP=67*24/100 =16
Sensitivity=TP/TP+FN*100
= 16/30*100= 53.3%
Specificity= TN/TN+FP*100
=162/170*100
=95.3 %
Problem 4: The prevalence of neonatal hypothyroidism is 30/10000. Early diagnosis would prevent mental retardation due to cretinism. A screening test is available (T4 RIA) using cord blood, with sensitivity 90%, and specificity 80%. Find out the positive predictive value, and evaluate the test?
Screening test
+ve-ve
total
positive
27
1994
2021
negative
3
7976
7979
total
30
9970
10000
Positive pv=TP/TP+FN
=27/2021*100
=0.13%.
A screening test for the presence of HIV was carried on 300 high risk subjects (IV drug users). A more accurate but time consuming and expensive test (gold standard) was also carried out to assess the accuracy of the screening test. The following results were obtained: 102 persons were +ve by screening test, 92 were positive by gold standard and 196 with negative test are free of infection. Find the validity & positive predictive value?