PHC 6001 Screening in Public Health Practice

PVP calculation

# of individuals with pre-clinical disease who test positive / # of individuals who test positive x 100 -2x2 table: a/a+b

PVN calculation

# of individuals without pre-clinical disease who test negative / # of individuals who test negative x 100 2x2 table: d/c+d

volunteer bias

- Are the people who are screened similar to those who are not screened -some people who are generally healthier volunteer for screening and this can affect the outcome -some people who are at higher risk of disease volunteer for screening and this can also affect the outcome

which diseases can be screened?

- more effective if it is: cost effective, doesn't hurt too much and isn't too risky, acceptable to the population, valid and reliable example: colonoscopy

Length Bias

-Do people who are screened have a better natural prognosis than those who aren't regardless of therapy? -screening tests could be just detecting the more survivable form. -patient might have had the same outcome without the screening test

What is screening?

-allows for early detection of disease -followed by clinical diagnosis and treatment -useful if early detection will improve treatment outcomes

why is prognosis important?

-establish disease control and treatment priorities -allow the patient to understand what to expect from disease -allow comparison of the disease before and after treatments/interventions

Lead Time Bias

-if a screening program seems to increase the 5-year survival of the people who were screened but in reality didn't, then this is lead time bias -doesn't happen as often with diseases that have a long

secondary type of prevention

-intervention in a person who has a disease, but is asymptomatic (preclinical). screening programs, glucose testing.

primary type of prevention

-intervention with healthy person before pathological onset of disease -goal of preventing them from ever getting disease

tertiary type of prevention

-reduces the impact of disease in someone who is clinically ill, rehab and quick treatment. relies on available treatment

Five-year survival rate

-the percentage of people diagnosed who are alive five years after diagnosis or 5 years after the treatment has begun -often used in cancer diagnosis

False Positives

-when a test indicates someone has a disease but they actually don't. -unnecessary treatment. may be a stigma that could last beyond the corrected test result.

Natural History of Disease: Pre-clinical phase

1. Biological Onset of Disease 2. Pathologic evidence of disease if sought 3. signs and symptoms of disease

what are the 2 rates that express prognosis?

1. case-fatality rate 2. five-year survival rate

Natural History of Disease: Clinical Phase

1. signs and symptoms 2. seek medical care 3. diagnosis 4. treatment

difference between PVP/PVN and Sensitivity and Specificity

The screening test itself is what determines the sensitivity and specificity, but its the prevalence of disease in a population that affected the predictive value (+) and (-).

If prevalence remains equal and you increase the sensitivity, what happens to PVP?

increase PVP

what happens to PVP when you increase the prevalence?

increase PVP

sensitivity calculation

number of individuals with pre-clinical disease WHO TEST POSITIVE / number of individuals with pre-clinical disease x 100 - 2x2 table: TP/TP+FN a/a+c

specificity calculation

number of individuals without pre-clinical disease who test negative / number of individuals without pre-clinical disease x100 -identifying true negatives -2x2 table: TN/TN+FP d/d+b

case fatality rate

number of people who die from a disease/number of people who have the disease (outbreak situations) -usually applied to acute diseases/outbreaks -typically no statement of time -different from mortality rate

why does specificity affect PVP more than sensitivity

specificity deals with people who don't have disease. Sensitivity deals with people who do have disease. Most people WILL NOT have the disease you are screening for, so specificity will influence PVP more.

Predictive Value Negative (PVN)

tells us what proportion of patients who screen negatively TRULY DON'T have disease

Predictive Value Positive (PVP)

tells us what proportion of patients who screen positive TRULY HAVE disease

What is sensitivity?

the probability that the test will correctly identify a person with a disease as being positive -identifying true positives

Specificity

the probability that the test will correctly identify a person without disease as being negative

false negatives

when the test indicates someone does not have the disease but they actually do - failure to treat the disease. opt to not change lifestyle and not make plans they wouldve made if they knew they had the disease -potential spread of disease

reliability

you can administer the test moer than once and it will give the same result for an individual