Midterm 2 Flashcards
What is the ultimate goal in Epidemiology?
To control and prevent disease and to improve health.
Why are there no tertiary prevention screening program?
Tertiary is to keep the patient comfortable. There is no cure or disease cannot be treated.
What is the purpose of screening?
To reverse, halt, or slow down the disease process by detecting it as early as possible in the natural history of that disease.
What is the assumption in screening?
Early detection will lead to treatment which will be more effective than later detection.
Give two examples of successful screening programs.
- Breast cancer.
- Prostate cancer.
- Cervical cancer.
- HIV screening.
- Tuberculosis screening.
What is the difference between screening test and diagnostic test?
Screening test is used to detect early disease or risk factors for disease in large population. Diagnostic test is to establish the presence or absence of a disease.
What are two measures of Validity?
Sensitivity (SE): the ability of a test to correctly identify those with a disease.
Specificity (SP): the ability of a test to correctly identify those do not have a disease. SE and SP are independent of each other.
How do you calculate Sensitivity?
How do you calculate Specificity?
DISEASE NO DISEASE
**POSITIVE A B **A + B
**NEGATIVE C D ** C + D
A + C B + D
SENSITIVITY (SE) = A/(A + C)
How good is the test in correctly identifying those with the disease?
SPECIFICITY (SP) = D/(B+D)
How good is the test in correctly identifying those without the disease?
A certain screening test has a sensitivity of 90% and a specificity of 75%. Interpret these values.
The test was accurate in identifying 90% of the disease and 75% of the non-disease.
What are true positives and true negatives?
True positive: (Box A) sick people correctly identified as sick.
True negative: (Box D) healthy people correctly identified as healthy.
What are false positives and false negatives?
- False positives are healthy people incorrectly identified as sick.
- False negatives are sick people incorrectly identified as healthy.
What is Overlapping Distributions
- Most screening test do not produce “either or” results
- There is a set of vallues where persons with and without the disease overlap
- Point where healthy and sick connect
Cutoff Point
Specific value for where a screening test is considered to be positive (and also negative) for disease
How do we determine the best cutoff point to decide positive or negative screening results?
By finding the compromise between SE and SP. Choice of cutoff point results in misclassifying some diseased cases as non-diseased and vice versa
Predictive Value Positive
Percent of all people who test positive that truly have the disease
Predictive Value Negative
Percent of all people who test negative that truly do not have the disease
How do you calculate predictive value positive and predictive value negative?
DISEASE NO DISEASE
POSITIVE A B A + B
NEGATIVE C D C + D
Predictive Value Positive (PVP): A/(A+B)
Predicitive Value Negative (PVN): D/(C+D)
How do you interpret predictive value positive and predictie value negative?
A patient who tests positive has a (69.6%) probability of having the disease.
A patient who tests negarive has a (98.8%) probability of not having the disease.
How does prevalence affect the predictive values?
The predictive values are dependent on prevalence.
What are the criteria for a successful screening program?
- Disease should be an important health problem
- Accepted treatment for patients with disease
- Available treatment for diagnosis and treatment
- Effective intervention
Define Prognosis
A prediction of the probable course and outcome of a disease.
The likelihood of recovery from a disease.
When does preclinical phase begin?
Biologic onset of disease
What is the middle of the pre-clinical phase?
Pathologic evidence of disease
When does the clinical phase begin in the natural history of disease?
Signs and symptoms of disease
What is the middle to end of the clinical phase?
- Medical care sought
- Diagnostic
- Treatment
Where can we get survival data?
CDC, WHO, State and National Public Health Department
What are the 5 ways of expressing prognosis?
- Case-fatality rate
- Five-year survival rate
- Observed survival rate
- Median survival time
- Relative survival rate
Of the 5 methods (expressing prognosis), which method is the most accurate?
Relative Survival Rate
Which prognosis method can we observe survival using life tables?
Observed Survival Rate
Which prognosis quantitative expression use for acute diseases and less useful for chronic diseases?
Case-fatality rate
State the problems for the 5-year survival rate.
- Difference in screening times
- Difference in stage of diseases when screening begins
- 5 years is arbitrary
- Can be a problem when death occurs less than 5 years
Which prognosis method is an age adustment for survival rates?
Relative Survival Rate
Which prognosis method is a measure of virulence?
Case-Fatality Rate
Which prognosis method allows for calculation of only half the study population?
Medial Survival Rate
Give the formula for case-fatality rate.
# people who die from disease
people with disease
What does a case-fatility rate of 80% mean?
Of those who are infected with the disease, 80% will die from the disease.
How do you calculate five-year survival?
# of people survived the disease after 5 years
The total # of people treated for the disease initially
Is it possible to calculate two-year survival?
Observed Survival Rate
What does a five-year survival rate of 90% mean?
After 5 years, 90% of those treated survived.
What prognosis method improves upon the limitation of five-year survival?
Observed Survival Rate
How do we calculate cumulative probabilities?
An Outcome
The total # of outcomes
What can we construct using cumulative probabilities?
Survival Curve
Give two reasons why median survival time is preferred over mean survival time?
- Less effected by Outliers
- Only need to observe half the deaths
Define and give the formula for the relative survival rate?
Observed survival in diseased Victims
Expected survival if victims had no disease
How does it (relative survival rate) improve upon observed survival rates?
- More accurate than observed survival rates
- Age-adjustment
- Estimates the chance of surviving the effects of cancer
Are survival data usually representative of the general population? Why or why not?
Yes because it takes the sample of the general population.
Control of Disease
Reduction of incidence, prevalence, morbidity and mortality to locally accepted level.
Elimination of disease
Reduction of disease incidence to zero in a defined geogrphic area.
i.e.: Neonatal Tetamus.
Elimination of infection
Reduction of infection incidence to zero in a defined geographic area.
i.e.: Measles and poliomyelitis.
Eradication
Permanent reduction of the worldwide incidence of infection to zero.
Extinction
The specific infectious agent no longer exists in nature or laboratory.
What is the small pox eradication strategy?
- Mass vaccination campaign in each country, using vaccine of ensured potentiacy that would reach 80 >= of population.
- Development of a system to detect and contain cases and outbreak.
How was the small pox symptoms described?
- Severe pain in large and small joints
- Cough, shaking, listlessness, and langour
- Postules are red, yellow and white
- Burning pain
- Body studded with rice
What are the small pox control strategies?
- Small Pox Hospitals (Japan 982 AD)
- Variolation (10th century AD)
- Quarantine (1650’s)
- Home isolation in VA (1667)
- Jenner vaccination and mass vaccination with Vaccinia
- Survaillance and containment
What are the observation variolation inoculation with small pox pus?
- Pot marked persons are never infected
- Persons innoculated with small pox pus or dried scabs usually had milder disease
- Case fatality rate still 2%
- Can transmit disease to others during illness
State the history of small pox vaccination.
- Growth of virus in flank of calf in Italy (1805)
- Publicity of vaccine production at a medical congress (1864)
- Most of Europe small pox free (After WWI)
- Transmission interrupted in Europe and North America (After WWII)
- Stable freeze-dried vaccine perfected by Collier (1940)
State the History of Small pox eradication.
- Pan American Sanitary Organization decides to undertake eredication hemisphere-wide (1950)
- World Health Assembly adopts goal to eradicate small pox. (1959)
- World Health Assembly decides to intensifiy eradication and provide more funds (1966)
What are the principle indicators of eradicability?
- Humans essential for life cycle
- Practical diagnostic tools
- Effective intervention capable of interrupting transmission
What are the small pox eredication strategy?
- Mass vaccination for each country, using vaccine of ensured potency that would reach 80 >= of the population
- Development of a system to detect and contain cases and outbreak
What are the assumptions of small pox prior to the eradication programs?
- Highly contagious
- Vaccine- induced immunity is short lived
- High vaccination coverage needed to meet herd immunity threashold
What was learned about small pox transmission during the eradication program?
- Common transmission: airbone by droplets
- face to face contact
- greater transmission by prolonged contact
- Rare transmission: airbone over long distance
- more frequently seen in hospitals where coughs were present
- fomites
- No carrier state
- No evidence of transmission by food or water
What was learned about small pox during the eradication program?
- Vaccines can provide protection for several years, but full protection decreases over time.
- Vaccination soon after exposure can provide some degree of protection
- Transmission does not occur before onset of symptoms
- Survellience and targed vaccination could significantly decrease transmission during outbreak
What are the factors influencing small pox spread?
- Lower temperature/ humidity, higher viability
- Intensity and duration of contact
- Length of contagious period
- Coughing and sneezing
What is the most efficient strategy?
Surveillience and containment
- Search for cases
- Containment of spread by vaccinating primary and their contacts
How long is the incubation period?
12 days
What are the stages of the small pox rash?
- Macules
- Papules
- Vesicles
- Pustules
- Scabs
