Screening for Disease: Part One
By:Dr.Yossra K.Al-RobaiaayAssistant professorFICMS (FM)What does “screening” mean?
Diagnostic and screening tests have major differences...why?What is the goal of screening test?
What are the purposes of screening ?What do we screen for?
What makes a disease or risk factor an appropriate target for screening?What are the characteristics of a good screening test?
What is a gold standard?What is the meaning of validity ,sensitivity & specificity ?
Predictive values?
•• Screening test
• Gold standard Test
• Disease Disease Free
• Total
•• Positive
•
• Negative
• (a) True Positive
• (b) False Positive
• (a+b) Total Test positive
• (c) False Negative
• (d) True Negative• (c+d) Total Test Negative
• Total
• (a+c)Total Disease positive
• (b+d) Total Disease Negative
• (a+b+c+d) Grand Total
Accuracy of the test?
Reliability of the test?
Relation between prevalence & predictive values?1.Differentiate between doing test for a patient, and screening?
2.Why do we screen for disease?3. How can we screen for low-prevalence diseases?
4. Some tests may give variable results on re-testing, why?5. The screening test should be both reliable & valid. Why?
Answer: Unreliable tests are confusing even if valid. Invalid tests are meaning less even if reliable (explain)6. Which do you prefer, a sensitive, or a specific test?
7. What are the drawbacks of using a test with low sensitivity?
8. What are the consequences of using a test with low specificity?
9. Do you agree that specificity is the ability of a screening test to be positive only if the disease is present?
10. How can we find out the prevalence of a disease in a community using a screening test survey?
Answer:
•
• Screening test• Gold standard Test
• Disease Disease Free
• Total
•• Positive
•
• Negative
• (a) True Positive
• (b) False Positive
• (a+b) Total Test positive
• (c) False Negative
• (d) True Negative
• (c+d) Total Test Negative
• Total
• (a+c)Total Disease positive
• (b+d) Total Disease Negative
• (a+b+c+d) Grand Total
• 11. If the screening criterion to detect glaucoma is set at 22mm Hg, what happens to the sensitivity & specificity of the test?
• 12. If the screening criterion to detect glaucoma is set at 26mm Hg, how do the sensitivity & specificity change?
•
• 13. What is the relation between sensitivity & specificity?
Answer: It is Reciprocal relation. (Explain).• 14. Where do you put the criterion if this screening test is to be used?