Screening and Diagnosis Part 1 Flashcards
A good screening test is one that (3 examples)
A good screening test is one that:
- Can detect the disease early in asymptomatic phase
- For which effective treatment is more effective in the asymptomatic phase
- For which early detection and treatment can positively impact the course of the disease
Screening vs Diagnostic Testing
Screening vs Diagnostic Testing
Screening: Process of identifying those who are at sufficiently high risk of a specific disorder which warrants further investigation and/or action
Diagnostic: Confirmation of a disease in someone with symptoms or in someone who had a positive screening test
Sensitivity (Definition and Formula)
Sensitivity: Percentage of people with the disease who will have a positive test
Formula: Test+/Disease+ = TP/(TP+FN)
Specificity (Definition and Formula)
Specificity: Percentage of people without the disease who will test negative
Formula: Test-/Disease- = TN/(TN+FP)
Prevalence (Definition and Formula)
Prevalence: Number of patients with a disease per total population
Formula: (# of patients with the disease)/(total population)
Positive Predictive Value (PPV)
Definition and Formula
Positive Predictive Value: Percentage of people with a positive test who have the disease
Formula: (True Positives)/(All Positives)
Negative Predictive Value (NPV)
Definition and Formula
Negative Predictive Value: Percentage of people with a negative test who do not have the disease
Formula: (True Negatives)/(All Negatives)
How does the PPV and NPV relate to prevalence?
- The lower the prevalence the lower the PPV; thus, many positive test results will be false positives when the prevalence is low
- At low prevalence the NPV is higher than the PPV (it is easier to exclude a disease with a negative a negative test)
- As the prevalence of the disease increases, so does the PPV
