Lecture 5 Screening Flashcards
What is primary prevention?
Prevents the onset of disease, which may be accomplished through the removal of a risk factor
What is secondary prevention?
Reduce the impact of a disease that may or may not be a asymptomatic
What is tertiary prevention?
Reduction or prevention of health deterioration once disease is present
Define Screening
The practice of investigation apparently healthy individuals with the object of detecting unrecognised disease or its precursors in order that measures can be taken to prevent or delay the development of disease or improve prognosis
What is a diagnostic test?
Is used to determine the presence or absence of a disease when a subject shows signs or symptoms of the disease
When is the Detectable pre-clinical phase?
The time interval between possible detection by screening and later detection after symptoms
What is the pre-clinical phase in screening?
The time between the Biological Onset and the when Symptoms develop
What is the clinical phase in screening?
The time between when Symptoms develop and Death(outcome) occurs
Why is Screening useful?
Early diagnosis of disease where early intervention improves prognosis
Identification of high risk individuals where intervention improves prognosis
Identification of those posing a risk to others
What are the different approaches to Screening?
Mass
Targeted
Systematic
Opportunistic
What is Mass approach in screening?
Applies to whole population (usually defines by age or gender)
What is the Targeted approach in screening?
Select sub groups though to be at increased risk
Example: TB contacts
What is the Systematic approach in Screening?
The population is called for screening using a register
Example: Brest cancer, cervical cancer:
What Is the Opportunistic approach in Screening?
The population is approached when they make contact for another reason
Example: HIV in GUM clinics
What are the disadvantages of Screening?
Where delaying treatment is no less effective than early treatment
When the prevalence of the condition is low
When the screening test is not accurate
What are the characteristics of a good Screening test?
Inexpensive Easy to administer Minimal discomfort Reliable (consistent outcome) Valid (distinguished diseased & non-diseased people )
What are the sources of unreliability (inconsistency)?
Biological variability
Instrument variability
Inter-observer variation
What determines the validity of a test?
Sensitivity & specificity
What is Sensitivity?
The ability of the test to identify correctly those who have the disease
What is specificity?
The ability of the test to identify correctly those who do not have the disease
What is the formula for Sensitivity?
Sensitivity= True Positive/ (True Positive + False Negative)
X 100 for percentage probability
(Left column in contingency table)
What is the formula for Specificity?
Specificity= True Negative/ (True Negative + False Positive)
X100 for percentage of probability
(Right column in contingency table)
Define Positive predictive value (PPV)
The proportion of patients who test positive who actually have the disease
TRUE POSITIVES
Define Negative predictive value (NPV)
The proportion of patients who test negative who are actually free of the disease
(DO NOT HAVE DISEASE)
TRUE NEGATIVES
What is the formula for Positive Predictive Value (PPV)?
PPV=True Positive/ (True Positive + False positive)
FIRST ROW IN CONTINGENCY TABLE
What is the formula for Negative Predictive value (NOV)?
NPV= True Negative/(True Negative+ False Negative)
SECOND ROW IN CONTINGENCY TABLE
What do Predictive values depend on?
The prevalence within the given population
What are the two main cons of Screening?
False positives , people who test positive even if they don’t have the disease
False negative, people who test negative but the do have the disease
What are the factors that influence screening?
Referral/Volunteer Bias
Selection Bias
