Chapter 5(from in class packet) Flashcards
why do we screen for disease?
to detect disease before there are signs or symptoms in order to REDUCE morbidity and mortality rates
What is screening?
the application of a disease-detection test to people who are as yes asymptomatic
What is the purpose of screening?
to classify individuals with respect to their likelihood of having a particular disease
Can screening tests also be used as diagnostic tests?
YES
Diagnosis involves what?
involves CONFIRMATION of PRESENCE or ABSENCE of disease in someone suspected or at risk of disease
Screening is generally done among what type of individuals?
individuals who ARE NOT SUSPECTED of having disease
What are some popular screening tests?
- mammography
- colonoscopy
- fecal occult
- pap smear
- PPD
- A1C
What are the requirements for a Screening Test?
- should be relatively SENSITIVE and SPECIFIC
- should be simple and INEXPENSIVE
- should be VERY SAFE
- must be ACCEPTABLE to subjects and providers
People who are being screened are?
- generally NOT patients
- NOT sick
- AREN’T expecting treatment
What are the General Principles of Screening (by WHO)?
- the condition should be an IMPORTANT health problem
- there should be a TREATMENT for the condition
- FACILITIES for diagnosis and treatment should be available
- there should be a LATENT STAGE of the disease
- there should be a TEST or EXAMINATION for the condition
- the test should be ACCEPTABLE to the population
- the natural HISTORY of disease should be ADEQUATELY UNDERSTOOD
- there should be an AGREED POLICY on whom to treat
- the total cost of finding a case should be ECONOMICALLY BALANCED in realtion to medical expenditure as a whole
- Case-finding should a CONTINUOUS PROCESS not just a “once and for all” project
How good is the test?
should be VALID and RELIABLE
What is the term that asks, “How well does the test measure what it is supposed to measure?”
validity
Reliability is what?
REPEATABILITY
What is the term that asks, “How well does the test do in different populations?”
reliability
What is seen in the “perfect world”?
TRUE POSITIVES AND TRUE NEGATIVES
What is seen in the “real world”?
FALSE POSITIVES AND FALSE NEGATIVES
People who are NOT diseased but have a POSITIVE test
FALSE POSITIVE
People who really HAVE disease, but have a NEGATIVE test
FALSE NEGATIVE
ability of the test to identify those who are diseased in the screened population
sensitivity
the ability of the test to identify those who do not have the disease
specificity
What is the overall accuracy of the test?
TP+TN/ TP+TN+FP+FN
Sensitivity should be INCREASED when?
- the penalty associated with missing a case is high
- the disease can be spread
- subsequent diagnostic evaluation are associated with minimal cost and risk
Specificity should be INCREASED when?
- the costs or risks associated with further diagnostic techniques are substantial
- minimize false positives
True or False. Sensitivity and specificity are able to predict the performance of the screening test in the population.
FALSE because sensitivity and specificity are NOT able to predict
the probability that a person actually has the disease given a positive test
positive predictive value (PPV)
the probability that a person do not have the disease given a negative test
negative predictive value (NPV)
What is a predictive value?
measures whether or not an individual actually has the disease, given the results of a screening test
What are the 3 things predictive value is affected by?
- specificity
- prevalence of preclinical disease
- sensitivity
during sequential screening, if you take all the people who screened positive on the first test and give them a second test (of higher sensitivity and specificity) what happens to the net?
- net sensitivity decreases
- net specificity increases
true or false. the ppv is maximized when used in “high risk” populations since the prevalence of the pre-clinical disease is higher than in the general population
true
screening the general population for a relatively infrequent disease can be what?
- wasteful of resources and may yield few previously undetected cases
a test must be _____ before it can be valid
reliable
an invalid test can demonstrate ________ ________
high reliability
what are the three sources of variation?
- instrument variation
- inter- observer variation
- intra-subject variation
the need for calibration or standardization
instrument variation
changes in repeated bp measurements over TIME
intra-subject variation
inconsistency of interpretation by two or more diagnosticians
inter-observer variations
what is the deviation of the results from the truth?
bias
what is the interval between “diagnosis” of disease at screening and when it would have been detected from clinical symptoms?
lead time bias
to detect disease before there are signs or symptoms in order to reduce morbidity and mortality from disease
lead time bias
do you increase life expectancy for lead time bias?
no, you just cause the disease early
the average length of time a person lives after the diagnosis with the disease or condition
survival time
What kind of bias is this: “volunteers may be healthier than people who don’t volunteer”?
self selection bias
What kind of bias is this: “persons who screen positive who are really disease free can be erroneously diagnosed with a disease, resulting in a more favorable long term outcome. results in the appearance of effective screening”?
over diagnosis bias
