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BMS1042 Screening and Diagnosis

Terms in this set (14)

Diagnostic test
- A diagnostic test is a test which is used in someone presenting with symptoms where you are using the test to help you establish the correct diagnosis
○ Example: using a mammogram to investigate a woman who presents with a lump in her breast
Screening test
- A screening test is one which is used to detect disease in an apparently healthy population
○ Example: women aged 50‐69 are invited for screening mammography every 2 years

- Different types of screening:
○ mass
○ multi‐phasic
○ targeted
○ opportunistic
Sensitivity
proportion of subjects with disease who have a positive test result [a/(a+c)]
Specificity
proportion of subjects without disease who have a negative test [d/(b+d)]
Positive predictive value
the proportion of persons with a positive screening or diagnostic test who do have the disease (the proportion of "true positives" among all who test positive). [a/(a+b)]
Negative predictive value
the proportion of persons with a negative screening or diagnostic test who do not have the disease (the proportion of "true negatives" among all who test negative) [d/(c+d)]
Characteristics of a diagnostic or screening test
- Sensitivity and specificity are features of the test and do not change with the prevalence
- In contrast, PPV and NPV do change with prevalence so even where sensitivity and specificity are both high, if the prevalence is low, the PPV will be low
- You cannot assume that a test which works well in a diagnostic setting will work well for screening
- You cannot develop a test in one population and then start using the test in another population and expect it to behave in the same way
Requirements before the introduction of screening
- The disease is well defined
- Prevalence is known
- There is a long period between when disease can be first detected and when the disease will present clinically
- The disease is serious and there is effective treatment available
- The screening test is simple and safe
- The test result distinguishes clearly between those with and those without the disease
- Doing the screening test is cost‐effective
- The facilities needed to do both the screening test and deal with the positive results are available
- The path for dealing with a positive result is clear and is acceptable both to the people being screened and to the authorities doing the screening
- There is equity of access to screening
Requirements before the introduction of screening
- Screening detects a different spectrum of disease from the disease that presents clinically (length time bias)
- Length time bias makes treatment look more effective because the spectrum of disease being treated in people diagnosed by screening is less aggressive
- In the case of cancer, screening will detect some slow growing cancers that may never have presented clinically in the lifetime of the patient (overdiagnosis)
- Overdiagnosis is particularly problematic if the treatment is dangerous or disfiguring
Lead time
extra time during which you know you have the disease if it is diagnosed by screening rather than by clinical presentation. Because of lead time bias, survival will look longer in screened individuals even if the course of their disease is unaffected
Length time
screening tends to diagnose disease that is less aggressive than disease that presents clinically. Because of length time bias, some cases diagnosed by screening would never present clinically if they had not been detected by screening: overdiagnosis
Why does overdiagnosis matter?
- In the case of breast cancer, over‐diagnosis will result in a woman being diagnosed with, and treated for, a cancer that she would never have known about if she had not undergone screening.
- So a woman in the over‐diagnosed category will undergo:
○ surgery
○ radiotherapy
○ possibly adjuvant endocrine therapy &
○ live the rest of her life believing she has a potentially fatal disease, for a condition which would have never presented clinically in her lifetime
Screening and breast cancer
- screening leads to fewer women dying from breast cancer
- screening leads to finding some breast cancers that are not harmful (over-detection)
- screening leads to some false positive results and extra testing
Drivers of screening
Health care providers need to understand their own motivations for advocating screening including an intuitive belief in early diagnosis and faith in technology, the driver of defensive medical practice or even self‐ interest
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