What are the formulas for the following:

1. Sensitivity
2. Specificity
3. PPV
4. NPV

1. True positive / (true positive + false negative)
2. True negative / (true negative + false positive)
3. True positive / (true positive + false positive)
4. True negative / (true negative + false negative)

Note: PPV and NPV change with prevalence of disease, sensitivity and specificity are test-specific

What are the formulas for the following:

1. Positive likelihood ratio
2. Negative likelihood ratio

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Note: The negative is on top for negative likelihood. Sensitivity is always on top because its too sensitive to be put down

The ratio of the (1) chance of a test being positive given that you have a disease, over (2) the chance of a test being positive if you DON'T have the disease.

• If a ratio is high (>10), a positive test will significantly increase the likelihood that the individual has the condition.
• If the ratio is low (<0.1) a negative test has almost certainly disproven the individual has the condition.
• A ratio of 1 means the test is worthless; the probability the patient has the condition is simply the prevalence of their population (i.e. the pre-test probability = the post-test probability)

What is the formula for relative risk reduction? Absolute risk reduction? Number needed to treat (NNT)?

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1. (1-Y/X) x 100%
2. Y - X; the reduction in risk associated with a treatment as compared to a placebo (or previous standard)
3. 1/absolute risk or 1/(Y-X)

1. Primary prevention avoids the development of a disease. Most population-based health promotion activities are primary preventive measures (e.g. HPV vaccine)
2. Secondary prevention activities are aimed at early disease detection, thereby increasing opportunities for interventions to prevent progression of the disease and emergence of symptoms (e.g. Pap smear)
3. Tertiary prevention reduces the negative impact of an already established disease by restoring function and reducing disease-related complications.

• Disease has major social impact
• There is an effective treatment if the disease is discovered
• The test is accurate
• Early detection improves outcome as compared to detection at time of clinical presentation
• Screening is feasible and acceptable

Patients die at same age, but duration of survival with the disease is increased due to early detection

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Overestimation of survival duration among screen-detected cases due to the relative excess of slowly progressing cases. These are disproportionally identified by screening because the probability of detection is directly proportional to the length of time during which they are detectable (thus inversely proportional to the rate of progression)

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A.— The USPSTF strongly recommends that clinicians provide [the service] to eligible patients. The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms.

B.— The USPSTF recommends that clinicians provide [this service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms.

C.— The USPSTF makes no recommendation for or against routine provision of [the service]. The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation.

D.— The USPSTF recommends against routinely providing [the service] to asymptomatic patients. The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits.

I.— The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service]. Evidence that the [service] is effective is lacking, of poor quality, or conflicting and the balance of benefits and harms cannot be determined.

• Grade I for men <75 y/o (insufficient quality evidence)
• Grade D for men >75 y/o (harms outweigh the benefit)

• Biennial screening mammography for women 50-74 = Grade B
• Biennial screening mammography for women <50 y/o = Grade C
• Screening mammography for women >75 y/o = Grade I
• Breast self-examination = Grade D

Grade D

The probability is high that the bladder cancer that is detected by the test will be non-invasive, and put the patient at future risks due to unnecessary follow-up care

• Diagnostic testing usually addresses symptoms a patient is currently experiencing
• Screening asks asymptomatic individuals to undergo procedures requiring time, money, anxiety and possibly discomfort

• Condition has a significant effect on quality and duration of life
• Incidence of condition justify costs
• Condition has asymptomatic period during whihc detection and treatment reduces morbidity and mortality

• Screening for cervical cancer in women who have been sexually active and have a cervix.
• Screening for blood pressure in adults 18+ y/o