Diagnosis by Lab Results (Trawick) Flashcards
Sensitivity
big net
if it’s negative, you can be pretty sure they don’t have it
100% means the test correctly identfies every person who has the target disorder
SnOut– sensitive rules out
Specificity
Very picky.
If it’s positive, they really do have it.
SpIn- Specific rules in
100% means The test correctly identifies every person who does not have the target disorder
Western Blot
is very specific for HIV - you can have the confidence to tell them they have it
ELISA test
is very sensitive for HIV - initial screening, when it’s negative you can reassure them conclusively
Using Elisa vs Western blot
If 100 people were to be tested for HIV, one would start by testing them with ELISA, a test that is very sensitive but not very specific.
Then, all the people who had positive ELISA tests would be retested with a Western blot, which is so specific that it would almost never falsely show HIV.
The people with false-positive results could then be reassured that they almost certainly do not have HIV, because the very specific Western blot can accurately tell the difference.
Validity
is a measure of the test’s ability to indicate which individuals have the disease and which do not
Reliability
is another term for consistency. If the test is administered repeatedly , does it yield the same results.
Incidence
Incidence is calculated as the number of new cases of a disease or condition in a specified time period (usually a year) divided by the size of the population under consideration who were initially disease free.
For example, the incidence of meningitis in the US in 2010 could be calculated by finding the number of new meningitis cases registered during 2010 and dividing that number by the population of the US who didn’t have meningitis.
As this incidence rate would be very small again we tend to consider number of cases per 100,000 people.
Incidence is a direct measure of disease risk—higher incidence implies higher risk
Prevalence
Prevalence is the actual number of cases alive with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence).
Period prevalence provides the better measure of the disease load since it includes all new cases and all deaths between two dates, whereas point prevalence only counts those alive on a particular date.
Prevalence is frequently reported as the number of cases as a fraction of the total population at risk.
highly contagious, rapidly fatal disease can have the following:
high incidence, low prevalence
A chronic, incurable disease can have the following:
high or low incidence, high prevalence
A short-duration, curable condition can have the following:
high incidence, low prevalence
Test Categories: screening vs diagnostic tests
A screening test identifies an asymptomatic individual who may have a particular disease
A diagnostic test is used to confirm the presence of a disease when a subject shows signs or symptoms of the disease
Typically a diagnostic test is performed after a screening test has yielded a positive results.
Common Screening Tests
Pap smear Mammography Fasting blood cholesterol Fasting blood sugar Blood pressure Fecal occult blood Ocular pressure PKU test TSH
Four legitimate reasons for ordering a laboratory test
Diagnosis (To rule in or rule out a diagnosis)
Monitoring (To evaluate the effect of therapy)
Screening (To discover disease not clinically apparent)
Research (To understand the pathophysiology of a particular disease)
The tests do not make your diagnosis!
