Lecture 23 Flashcards
(19 cards)
Screening as a prevention strategy: primary
Screening for alcohol intake to prevent breast cancer
Screening as a prevention strategy: secondary
Screening for breast cancer
Screening as a prevention strategy: tertiary
Screening for bone density following chemotherapy for breast cancer
Screening criteria + objective
Suitable:
1. Disease
2. Screening test
3. Treatment
4. Screening programme
Objective:
To improve health outcome (morbidity, mortality and/or disability)
Suitable disease
- An important public health problem
- relatively common/uncommon (early detection and intervention –> better outcome) - Knowledge of the natural history of the disease
- Relationship between risk factors and condition is known
- disease is detectable at an early stage (disease marker)
- Increased duration of pre-clinical phase
Suitable test
- Reliable
- Safe
- Simple
- Affordable
- Acceptable
- Accuracy - the ability of a test to indicate which individuals have the disease and which do not
Sensitivity, specificity
Sensitivity
The likelihood of a positive test in those with the disease
The ability of the test to identify correctly those who have the disease (a) from all the individual with the disease (a+c)
Sensitivity = (a/a+c) x 100
= (true positives/all w disease) x 100
Specificity
The likelihood of a negative test in those without the disease
The ability of the test to identify correctly those who do not have the disease (d) from all individuals free from the disease (b+d)
Specificity = (d/b+d) x 100
= (true negatives/all without disease)
x 100
Diagnostic test accuracy studies
Evaluation test accuracy
The SENSITIVITY of a screening test is HIGH if the proportion of TRUE VALUES is HIGH
The SPECIFICITY is HIGH if the proportion of TRUE NEGATIVES is HIGH
Specificity and sensitivity are a fixed characteristic of the test
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
PPV = (a/a+b) x 100
= (true positives/all who test positive) x100
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
NPV = (c/c+d) x 100
= (true negatives/all who test negative)
x100
PPV and NPV
They are not fixed characteristics of the test
Reflect both the test accuracy and prevalence of the disease
If the prevalence is high
Higher false negative test results
If the 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:
1. Reduced mortality
2. Increased survival time
- 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
Length time bias
Breast cancer and screening criteria
- Suitable disease
- NZ has one of the highest breast cancer rates in the world amongst NZ women
- Rates for Maori > non-Maori - Suitable test
- screening mammogram detects lumps before it can be felt
- 5 year survival rates –> 95-100% - Suitable treatment
- Surgical treatment and other forms of treatment - Suitable screening programme
- BreastScreen Aotearoa
- Women who test positive are offered a diagnostic test or reffered for specialist treatment
- Goal of improving screening participation rates for Maori and Pasifika women
- 10 year survival rates
- Screen detected Maori was 5.3% higher than NZ European
