Screening and Prevention Flashcards
What is screening?
The investigation of asymptomatic people in order to classify them as likely or unlikely to have the disease. People who appear likely to have the disease (via the test.) are investigated further to arrive at a final diagnosis. Those found to have disease are treated.
What are the prerequisites for screening (WHO 1968)?
Screening depends on having a disease which is suitable, on a test which is sufficiently inexpensive and valid, and on a programme which is possible to administer and not prohibitively expensive. Wilson and Junger defined the following as essential prerequisites for screening in 1968 for the WHO: the condition should be an important public health problem; there should be a recognised pre-symptomatic or latent phase; there should be a suitable test or examination; there should be an accepted treatment for the disease; facilities for diagnosis and treatment should be available; the test should be acceptable to the target population; the natural history of the disease should be understood; there should be an agreed policy on who to treat as patients; the cost should be economically balanced in relation to the cost of medical care as a whole; case-funding should be a continuous programme and not a one-off project.
What diseases are suitable for screening?
Diseases should be relatively common and have severe consequences. They must pass through a pre-symptomatic phase during which the disease is undiagnosed but detectable. Early treatment must offer some advantage over later treatment. Screening should have evidence of net benefit.
What is lead time bias?
Successful screening will detect disease in its presymptomatic phase. The period between detection and death could therefore be longer simply because we have observed the process for longer without actually increasing the length of time that the patient would have survived had the disease been detected at the onset of symptoms. By bringing forward the day on which the diagnosis is made, the length of time between diagnosis and death will be increased by the lead time. Such lead time bias must be accounted for when comparing survival between those screened and unscreened.
What is length bias?
Length bias reflects the fact that diseases which lend themselves to be identified by screening are more likely to be indolent and less aggressive conditions. More aggressive disease is less likely to be detected by screening because it is likely to develop fully between successive routine screening points. Survival following screen detected disease may be lengthened by the relatively less aggressive nature of the disease process.
What measures are used to characterise screening tests?
Sensitivity, Specificity, Positive predictive value, Negative predictive value.
Define sensitivity.
Proportion with condition who test positive.
Formula: (a / [a+c])
Define specificity.
Proportion without condition who test negative.
Formula: (d / [b+d])
Define positive predictive value (PPV).
Likelihood of having the condition if identified as unhealthy.
Formula: (a / [a+b])
Define negative predictive value (NPV).
Likelihood of being healthy having been identified as healthy.
Formula: (d / [c+d])
What are the 3 NHS population-based screening programmes?
Breast cancer, Bowel cancer, Cervical cancer.
What is the performance of mammography in the NHS Breast Screening Programme?
Sensitivity in women aged over 50 ranges from 68% to 90%, with most trials achieving about 85%. In women aged 40-49, the sensitivity is lower, ranging from 62% to 76%. Specificity ranges from 82% to 97%. Positive predictive value for the NHS Breast Screening Programme ranges from 6% to 8% for first screens and from 12% to 14% for subsequent screens.
What is the NHS Breast Screening Programme (NHSBSP)?
NHSBSP provides free breast screening every 3 years for all women in the UK aged 50 and over. Around 2 million women are screened in the UK each year. Women aged between 50 and 70 are routinely invited every 3 years; women over age 70 must request an appointment. It is estimated that the NHSBSP saves around 1300 lives each year in the UK.
What are criticisms of the NHS Breast Screening Programme?
Ductal carcinoma in-situ (DCIS) registrations have increased substantially since the introduction of NHSBSP, as the condition is usually not palpable and mostly diagnosed by mammography. Critics have concerns that identifying DCIS leads to overdiagnosis of breast cancer, resulting in overtreatment.
What are the results of breast cancer screening?
About 1 in 7 (14%) of those ‘called back’ following screening have ‘cancer’. Of these, about 20% have DCIS. 30% of screen detected DCIS have a mastectomy. We need better molecular measures to predict which lesions cause problems.
What are the benefits vs harms of breast cancer screening?
There had been much debate that the information for informed consent for breast screening was biased. The information was changed after this review, and informed consent is now based on a fairer comparison of risks and benefits.
What factors relate to the uptake of NHS breast cancer screening?
Number of cars available in the household increases the odds of having a mammogram. There is a relationship with the type of housing a woman lives in. Breast cancer screening had no relation to educational attainment, socio-economic classification, ethnicity, or geographic region.
What is recommendation 13 regarding uptake and coverage?
High priority should be given to spreading the implementation of evidence-based initiatives to increase uptake, including text reminders for all screening programmes and social media campaigns.
What are the 3 potential screening tests available for prostate cancer?
Digital Rectal Examination (DRE), Transrectal Ultrasound (TRUS), Prostate Specific Antigen (PSA).
Why should screening for prostate cancer be implemented?
Prostate cancer is the most common cancer in men, and many cases are diagnosed when the disease is widespread and incurable. An accurate method of detecting disease at an earlier stage when there is a better chance of cure is highly desirable.
Why are there not official screening programmes for prostate cancer?
All potential tests have disadvantages. The PSA test, while the most acceptable, has serious problems, including false positives and negatives, and the natural history of the disease is poorly understood.
