Module 5 Flashcards
Natural History of Disease
Course of a disease from onset to resolution
Critical point (in course of disease)
Time during pathogenesis beyond which there are serious consequences
*If detection before critical point, then secondary prevention is possible
Clinical horizon
Time during pathogenesis when signs/symptoms occur
Screening test
Presumptive identification of unrecognized disease through tests, exams, or other procedures that can be applied rapidly and inexpensively to populations
*Positive screening tests are followed up by diagnostic tests to confirm actual disease
Primary Prevention
Avoiding illness altogether by preventing disease development (occurs during pre pathogenesis)
- General health promotion (primordial prevention)
- Disease-specific efforts
Active Primary Prevention
Necessitates behavior change on the part of the subject
Ex: vaccine, protective devices
Passive Primary Prevention
Does not require behavior change
Ex: fluoridation of public water, vitamin fortification of bread and milk products
Secondary Prevention
Aimed at reducing illness onset, duration, and further transmission (occurs during pathogenesis)
Ex: screening programs
Tertiary Prevention
Aims to: -Cure illness -Limit disability due to disease -Restoring optional functioning Ex: physical therapy for stroke, fitness for heart attack
Screening Concepts
Distinguish people with disease from those without
- Major strategy for secondary prevention
- Not diagnostic
- Can be applied to entire population or selected at risk
Screening test
Stage: before symptoms
Characteristics: less risky, less expensive, acceptable
Population: healthy
Diagnostic test
Stage: after symptoms
Characteristics: ordered by doctor, expensive, risky, time consuming, pain/discomfort, definitive diagnosis
Population: individuals with disease
Characteristics of a good screening test
Simple, rapid, inexpensive, safe acceptable (by target pop)
Evaluation of screening tests
Reliability: consistency of results
Validity: accuracy
Sensitivity: proportion with disease test +
Specificity: proportion w/o disease test -
Positive Predictive Value: prop test + w/ disease
Negative Predictive Value: prop test - w/o disease
Reliability (Precision)
The ability of a measuring instrument to give consistent results on repeated trials
Validity (Accuracy)
The ability of a measuring instrument to give a true measure
*Can be evaluated only if an accepted/independent method for confirming the test measurement exists (gold standard)
Relationship between reliability and validity
- Possible for a measure to be highly reliable but invalid
- Impossible for a measure to be valid but unreliable
Validity measures of screening tests
Sensitivity and Specificity
Sensitivity
The ability of the test to correctly identify those who have the disease
TP/(TP+FN) OR TP/all those with the disease
Specificity
The ability of the test to correctly identify those who do not have the disease
TN/(FP+TN) OR TN/all those without the disease
To improve sensitivity
Cut point for disease should be moved farther into the non-diseased range
To improve specificity
Cut point should be moved farther into the range typically associated with disease
Positive Predictive Value
The proportion of individuals screened positive by the test who actually have disease
TP/(TP+FP) OR TP/all those with a positive result
Negative Predictive Value
The proportion of individuals screened negative by the test who actually do not have the disease
TN/(FN+TN) OR TN/all those with a negative test result
Effects of prevalence on predictive value of screening
When prevalence falls:
-PPV falls
-NPV rises
Ex: flu testing when flu isn’t prevalent
Criteria for a good screening program
- High sensitivity and specificity
- Simplicity, low cost, safety, patient acceptability
- Serious disease
- Test detects disease at earlier stage
- Treatment if screen positive more effective
- High prevalence in population screened
- Follow-up diagnostic/treatment available
