6. Screening Flashcards
What is screening?
The systematic attempt to detect an unrecognised condition by application of tests, examinations, or other procedures, which can be applied rapidly and cheaply to distinguish between apparently well persons who probably have a disease and those who probably don’t.
What are the three modes of disease detection?
Spontaneous presentation, opportunistic case finding, screening.
What are the criteria for screening considering the disease?
Must be an important health problem in numbers and severity, epidemiology and natural history well understood, early detectable stage, primary prevention must be considered where possible.
What are the criteria for screening considering the test?
Simle and safe, precise and valid, acceptable to the population, distribution of test values known, agreed cut off known, policy on who to investigate further known.
What are the criteria for screening considering the treatment?
Effective evidence based treatment available, early diagnosis must be advantageous, agreed policy on who to treat.
What are the criteria for screening considering the programme?
Other options considered, benefits > physical and psychological harm, facilities for diagnosis and treatment.
What are false positives in screening?
Well people who get referred for further investigation that can be invasive.
What are false negatives in screening?
Failure to refer people who do have the disease, false reassurance.
What is the sensitivity of a screening test?
The proportion of people with the disease who test positive. The detection rate, or how probable it is for a case to test positive.
How is sensitivity of a screening test calculated?
True positives/(true positives + false negatives)
What is the specificity of a screening test?
The proportion of people without the disease who test negative. The probability a non-case will test negative.
How is specificity of a screening test calculated?
True negative/(true negatives + false positives)
What is the positive predictive value (PPV) of a screening test?
The probability that someone who has tested positive actually has the disease.
What is PPV influenced by?
The prevalence of the disease, if it is a high prevalence, then the PPV will be higher.
How is PPV calculated?
True positives/(true positives + false positives)
What is the negative predictive value (NPV) of a screening test?
The proportion of people who are test negative and who actually don’t have the disease.
How is NPV calculated?
True negatives/(true negatives + false negatives)
What are the advantages of screening?
Early detection may improve outcome, true negatives are reassuring.
What are the disadvantages of screening?
False positives expose patients to invasive diagnostic tests, false negatives provide false reassurance and mean patients aren’t offered diagnostic tests, expensive which means money can’t be spent elsewhere.
What is a lead time bias in terms of screening?
Screened patients appear to survive longer but only because the date of diagnosis is shifted earlier. They live the same amount of time but longer knowing they have the disease which could negatively impact their quality of life.
What is length time bias in terms of screening?
Screening programmes are better for picking up slow-growing, unthreatening cases than more aggressive cases. These are more likely to have better outcomes anyway and may have never presented as a problem anyway.
What is selection bias in terms of screening?
Those who attend screening are more likely to have had better health related behaviours in the first place anyway, healthy worker bias.
What are some of the concerns with screening?
It alters the doctor-patient relationship as the patients may be healthy, the screening programmes are very complex, screening is limited so can cause harm.
What are some of the screening programmes in the UK?
Abdominal aortic aneurysm, bowel cancer, breast cancer, cervical cancer, diabetic retinopathy, Down’s syndrome, foetal abnormalities, PKU, sickle cell and thalassaemia.
