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Epidemiology > Screening Chpt5 > Flashcards

Screening Chpt5 Flashcards

0
Q

Are diagnosis and screening the same thing?

why?

A

Diagnosis does not equal screening

Screening test-often used to diagnose disease generally done to individuals who are not suspected of having disease

Diagnoses-confirmation of the presence or absence of disease in someone who suspected to be at risk for disease

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1
Q

Define screening and explain the purpose of screening

A

Using a test to detect disease in people who show no symptoms of the disease

The purpose-to classify individuals into categories of likelihood of having a particular disease

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2
Q

List some popular screening tests

A

Cholesterol-CVD

Mammography-breast cancer

Pap smear-uterine cancer

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3
Q

List in order the natural history of disease to from beginning to end

Explain which parts of that timeline are included in the screening

A

Stage of susceptibility

Exposure

Stage of subclinical diseases (pathological changes)

Onset of symptoms

Stages of clinical disease (usual time of diagnosis)

Stage of recovery disability or death

Screening can be done;

During stage of susceptibility

Stage of subclinical disease

And the beginning part of stage of clinical disease

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4
Q

List the requirements for a screening test

A

Should be relatively sensitive and specific

Should be simple and inexpensive

Should be very safe

Must be applicable to subjects and providers

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5
Q

What ethical issues arise when discussing screening?

A

People who are being screened are generally not patients …..not sick and are expecting treatment

Screening may not benefit the individual, may have a negative impact on the individual

May divert resources away from treatment, how are the resources equitably distributed especially in developing countries

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6
Q

List that in general principles of screening from the world health organization

A

should be important health problems being addressed

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 for examination for the condition

The test should be acceptable to the population

The natural history of the 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 relation to medical expense as a whole

Case findings should be a continuous process, not just a once and for all project

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7
Q

How is the quality of a screening test measured

A

By assessing validity and reliability

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8
Q

Define validity

A

Validity is how well does the test measure what it supposed to measure

The extent to which the test distinguishes between people without the disease high validity requires high sensitivity and specificity

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9
Q

Define reliability

A

Reliability is how well does the test due in different populations?

repeatability

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10
Q

Define false positive

Why are false positives a problem?

A

People who are not diseased but have a positive test

This is a problem because;

Additional and possibly more invasive screening

Increased burden on healthcare system-money

Psychological impact

Other stigma

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11
Q

Define false negative?

Why are false negatives a problem?

A

Faults negative-people who really have disease but have a negative test

Problems with false negative:

depend on the severity of the disease

Faults reassurances

We missed an opportunity to change the course of the disease

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12
Q

When setting up a 2 x 2 table as testing screening test what are the titles that should go along the top along the left-hand side and then fill in each box?

A

Column labels from left to right

Positive for disease negative for disease

Row titles from top to bottom:

Positive test result negative test results

First box true positive alongside faults positive bottom row false-negative negative alongside true negative

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13
Q

Define sensitivity and give the formula

A

Sensitivity-ability of the test to identify those who are diseased in the screening process

(remember validity of the test is determined by sensitivity and specificity)

Formula= TP/(TP + FN)

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14
Q

The probability of testing positive if the diseases truly present is another definition for what?

A

Sensitivity

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15
Q

Define specificity and give the formula

A

Specificity-the ability of the TEST to identify those who do NOT have the disease

Specificity = TN/(TN + FP)

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16
Q

What is the formula for overall accuracy of a test?

A

TP + TN/TP + TN + FP + FN

BUT remember this is less useful than sensitivity and specificity

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17
Q

The probability that a screening test will be negative if the disease is truly absent defines what?

A

Specificity

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18
Q

Screening by mammogram will correctly classify 96% of all non-breast cancer patients as being disease-free

What does this measure?

A

Specificity

19
Q

Screening by mammograms will identify 77% of all true breast cancer cases

What does this measure?

A

Sensitivity

20
Q

Because sensitivity and specificity can’t always be 100% at the same time ….one of those factors must take precedence in varying circumstances

When should sensitivity be more important and when should specificity be more important?

A

Sensitivity should be increased when the penalty associated with missing a case is high… When the diseases spread quickly and easily… When subsequent diagnostic evaluations are associated with minimal cost and risk

Specificity should be increased when the cost or risk associated with further diagnostic test techniques are substantial… Minimize false positives… Positive screen requires a biopsy to be performed

21
Q

Sensitivity and specificity are not able to predict the performance of the screening test in a population

Therefore what value do we use to assess the performance of the test in a population?

A

Positive and negative predictive value

22
Q

Define positive predictive value

A

The probability that a person actually has the disease given a positive test

23
Q

Define negative predictive value

A

The probability that a person is disease-free given a negative test

24
Q

What does predictive value measure?

and what is predictive value affected by?

A

Predictive value is measured whether or not individual actually has the disease, given the results of the screening test

Predictive value is affected by specificity, prevalence of pre-clinical disease, sensitivity

25
Q

Give the formula for positive predictive value

A

PPV = TP/TP + FP

IT’S THE PROBABILITY THAT A PERSON ACTUALLY HAS THE DISEASE WHEN THEY RECEIVE A POSITIVE TEST RESULT

26
Q

Get the formula through negative predictive value

A

NPV = TN/FN + TN

PROBABILITY THAT A PERSON IS DISEASE-FREE WHEN RECEIVING A NEGATIVE TEST RESULTS

27
Q

Which predictive value is the following sample;

Among persons who test -99% disease-free

A

Negative predictive value

28
Q

Which predictive value is the following example:

Among persons who test positive, 40% are found to have heart disease

(in other words given a positive test there’s only a 40% chance of having the disease)

A

Positive predictive value

29
Q

the formula for percent agreement

A

A + D / A+B+C+D all multiplied by 100

The denominator should be total everyone

30
Q

The formula for percent agreement expected by chance alone ?

A

In a 2 x 2

A + D/the total sum of all

31
Q

Sequential screening

If you take all the people who screen positive on the first test and give them a second test (of higher sensitivity and specificity) what happens to those values??

A

The net sensitivity decreases

The net specificity increases

32
Q

What is the relationship of disease prevalence to predictive value

A

As the disease prevalence increases the positive predictive value increases

33
Q

What is the relationship of specificity to positive predictive value

A

As the specificity of a test increases the positive predictive value increases

34
Q

When is the positive predictive value maximized? Why?

A

PPV is maximized when used in high risk populations since the prevalence of pre-clinical disease is higher than in the general population

Screening the general population for a relatively infrequent disease can be very wasteful of resources and may you a few undetected cases

35
Q

Why is reliability of screening test important

A

The extent to which the screening test will produce the same or similar results each time it is administered is crucial

I test must be reliable before it can be valid

However and in valid test can demonstrate high reliability

36
Q

What are the sources of variation for reliability of a test

A

Instrument variation-need for calibration or standardization

Intrasubject variation-changes in repeated blood pressure measurements overtime

Inter-observe her variation-inconsistency of interpretation by two or more diagnostic workers

37
Q

Define bias in relation to a screening program

A

The deviation of the results from the truth

38
Q

Define survival time

A

The average length of time a person lives after the diagnosis with the disease or condition

A measure of prognosis

39
Q

What is lead time bias?

A

Leadtime by S-the interval between diagnosis of the disease at the screening and when it would have been detected by clinical symptoms

Survival me appear to increase among screen detected cases simply because diagnosis was made earlier in the course of the disease

40
Q

Explain self selection bias

A

Volunteers may be healthier than people who don’t volunteer

Worried well-increased participation due to family history or lifestyle characteristics

41
Q

Explain overdiagnosis bias

A

Person to screen positive who are really disease-free (FP) can be erroneously diagnosed with the disease resulting in a more favorable long-term outcome. Result in the appearance of effective screening

42
Q

What is the formula for inter-observer variation a.k.a. percent agreement

A

Percent agreement = a + F + K + P/all readings (Times 100)

Diagonal line from top left corner to bottom right corner all of the agreements

43
Q

What are the risks of screening?

A

Faults positives:

Anxiety

Fear future test

Money of future testing

False negatives:

Delayed intervention

Disregard of symptoms could lead to delayed diagnosis

44
Q

What does the Kappa statistic question?

what is the formula?

A

The Kappa statistic-are the readers better than chance?

Numerator:

% agreement observed - % agreement expected by chance alone

Denominator:

100% - % agreement expected by chance alone

Epidemiology (6 decks)

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