(L23) Screening: a special type of prevention strategy

Dictionary definition of ‘screen’

To sift passing through a ‘screen’ (sieve)

Screening in health involves

a) applying a screening test on a relatively large population to identity

• Exposure to disease risk

• unrecognised disease

• complication of a disease

b) intervention/treatment

Screening women for alcohol intake to prevent breast cancer

Primary prevention

Breast cancer screening

Secondary prevention

Screening for bone density following chemotherapy for breast cancer

Tertiary prevention

Total population - eligible population (population without symptoms) - screening test (less expensive diagnostic test) - test negative =

Re-screen

• (after a specified time period)

Screening test - eligible population (population without symptoms) - test positive =

Diagnostic test

• Gold standard test

• Often invasive, expensive (why cannot apply to whole population)

Diagnostic test - disease positive

Intervention or treatment

Diagnostic test - disease negative

Re-screen

• (after a specified time period)

Screening criteria

1. Suitable disease

2. Suitable screening test

3. Suitable treatment

4. Suitable screening programme

Objective screening initiative

To improve health outcome (morbidity, mortality, and/or disability)

Suitable disease - an important public health problem

• Relatively common

• Relatively uncommon:

  Early detection and intervention — better outcome

Suitable disease - knowledge of the natural history of the disease

• Relationship between risk factor/s and condition is known

• Disease is detectable at an early stage (disease marker)

• Increased duration of pre-clinical phase (between early features of disease and clinical diagnosis) e.g. breast cancer, cervical cancer, diabetes - more time to screen and prevent disease

Suitable test

• Reliable - provides consistent results

• Safe

• Affordable

• Acceptable

• Accuracy - the ability of a test to indicate which individuals have the disease and which do not

• Sensitivity, specificity

Diagnostic test accuracy studies

• Tests the accuracy of the screening test before it is included in a screening programme

• The Gold standard (the diagnostic test) is in effect the ideal test

• Colon cancer - colonoscopy

• Screening test (or a less expensive diagnostic test)

  • colon cancer: faecal occult blood test (poo test)

True positives

Those who test positive for disease in the gold standard test and test positive for disease in the new screening test

False negatives

Those who test positive for disease in the gold standard test and test negative for disease in the new screening test

False positives

Those who test negative for disease in the gold standard test and test positive for disease in the new screening test

True negatives

Those who test negative for disease in the gold standard test and test negative for disease in the new screening test

Sensitivity

• the likelihood of a positive in those with the disease

• the ability of the test to identify correctly those who have the disease (a) from all individuals with the disease (a+c)

Sensitivity calculation

true positives/all with disease x 100

(percentage of those with disease that were actually detected to have disease in all cases)

Specificity

• The likelihood of a negative test in those without the disease

• The ability to identify correctly those who do not have the disease (d) from all individuals free from the disease

Specificity calculation

True negatives/all without disease x 100

(percentage of those detected to have no disease out of everyone with no disease

The sensitivity of a screening test is high if…

the proportion of true positives is high

The specificity is high if…

the proportion of true negatives is high

Sensitivity and specificity are a…

fixed characteristic of the test

Positive Predictive Value (PPV) calculation

True positive/All people with positive results (a+b) x 100

Negative Predictive Value (NPV) calculation

True negatives/All people with negative results (c+d) x 100

Positive Predictive Value (PPV)

• The proportion who really have the disease of all people who test positive

• The probability of having disease if the test is positive

Negative Predictive Value (NPV)

• The proportion who are actually free of the disease of all people who test negative

• The probability of not having the disease if the test is negative

PPV and NPV are…

not fixed characteristics of the test

PPV and NPV reflect…

both the test accuracy and prevalence of the disease

PPV = 5445/5490 × 100 = 99.2%

99% of this population who test positive will actually…

have HIV

NPV = 4455/4510 × 100 = 98.8%

~99% of this population who test negative will actually…

not have HIV

If prevalence is high

higher false negative test results

If prevalence is low

higher false positive test results

Suitable treatment

• Evidence of early treatment leading to better outcomes

• Effective, acceptable and accessible treatment

• Evidence-based policies covering who should be offered treatment and the appropriate treatment to be offered

Suitable screening programme

• Benefits must outweigh harm

• RCT evidence that screening programme will result in:

• Reduced mortality

• Increased survival

  • Lead time bias

  • Length time bias

• Adequate resourcing and agreed policy for testing, diagnosis. treatment and programme management

• Cost effective

• The healthcare system must be able to support all elements of the screening pathway

• Needs to reach all those who are likely to benefit from it (specific initiatives for particular population groups)

Lead time bias

Apparent increase in life expectancy or lead time due to early screening - if diagnosed earlier, indiviudal may seem to have lived longer

• Hence, if the screening programme is evaluated in terms of survival time, this may give a false impression of success

Length time bias

• Screening identifies 2 patients with rapidly progressive diseases and 5 patients with slowly progressive disease

• Calculating mean survival from screened patients gives an impression of longer average survival than occurs in the population

• Longer times of survival until death and shorter time between onset and death - time not accurately represented

Suitable disease - breast cancer

• NZ has one of the highest breast cancer rates in the world

• Most common cancer among NZ women

• 2nd most common cause of cancer deaths among NZ women

• Rates for Maori > non-Maori (125 new cases/100000 verses 27.4 new cases/100000)

• Rates are also high for Pasifika women

• Incidence increases with age many rises steeply from the age of 50

Suitable test - breast cancer

Screening mammogram

• Detects breast lumps before it can be felt

• Sensitivity 75-95%, specificity 90-95%

Suitable treatment - breast cancer

Surgical treatment and other forms of treatment

5-year survival rates - 95-100%

Suitable screening programme - breast cancer

BreastScreen Aotearoa

• Women with positive tests are offered a diagnostic test or referred for specialist treatment

• in NZ, screening reduces breast cancer mortality by 30% (50-65 yrs) and 45% (65-69 yrs)

• Goal of improving screening participation rates for Maori and Pasifika women

• 10 year survival rates

  • screen detected Maori was 5.2% higher than NZ European