Block 10 Flashcards
What is the intention of a diagnostic test?
To confirm or refute your understanding and belief about diagnosis and/or treatment
Define sensitivity
True positive rate =
Number of true positives / number of people with the disease
Describe the outcomes of a diagnostic test with high sensitivity
Correctly classifies a high proportion of people who truly have the disease
Define specificity
True negative rate =
Number of true negatives / number of people without the disease
Describe the outcomes of a diagnostic test with high specificity
Correctly classifies a high proportion of people who truly do not have the disease
What is positive predictive value?
The chance a having the disease if your test is positive =
Number of true positives / Number of people who test positive
What is negative predictive value?
The chance of not having the disease if the test is negative =
Number of true negatives / Number of people who test negative
Which measures of diagnostic tests do not change depending on the population?
Specificity and Sensitivity
Which measures of diagnostic tests change depending on the population?
PPV and NPV
As prevalence rises… (effect on PPV and NPV)
NPV falls
PPV rises
As prevalence falls… (effect on PPV and NPV)
NPV rises
PPV falls
The larger the LR+ve…
the greater the chance of having disease if the text is positive
The smaller the LR-ve…
the smaller the chance of having disease if your test is negative
Define screening
systematic application of a test or inquiry
to individuals at sufficient risk of a specific disorder to warrant further investigation or direct preventative actions
amongst persons who have not sought medical attention on account of symptoms of that disorder
What type of prevention is screening (normally)?
Secondary - it aims to prevent disease progression
There are 11 national screening programmes, name them…
- AAA
- bowel cancer
- breast
- cervical
- diabetic eye
- fetal anomaly
- infectious diseases in pregnancy
- newborn and infant physical examination
- newborn blood spot
- newborn hearing
- sickle cell and thalassaemia
Why carry out screening?
- opportunities for primary prevention are limited
- opportunities for treatment are limited
- screening will give potential for earlier +/- more effective treatment
For screening to take place, the condition must:
- be an important health problem
- be adequately understood
- have a detectable risk factor
- have a latent period
- have been targeting by cost-effective primary prevention
- have agreed further management plans in place in the case of a positive test
For screening to take place, the test must:
- be simple, safe, precise, and validated
- have a known distribution of test values with a known and suitable cut-off agreed
- acceptable
For screening to take place, the treatment must:
- be effective
- have better outcomes if given earlier
- be optimised
- have sufficient policies in place to cover who should be offered it or not
For screening to take place, the programme must:
- be confirmed to be effective in an RCT
- acceptable to the professionals and public
- reap benefits that outweighs the harms
- be viable economically in relation to healthcare spending (opportunity cost)
- be well planned; quality assurance, adequate staffing, appropriate facilities
Explain length bias
- You are more likely to detect more slowly progressing disease
- Individuals with slowly progressing disease already have a better prognosis than those who present with symptoms/signs
- So if you only look at the disease you have detected by screening then the screening looks as though it’s performing better
Explain lead time bias
- Screening makes the length time the patient lives with disease longer because you’ve diagnosed it earlier
- But the patient doesn’t actually live longer - they just have a “lead-time” advantage (like a head start)
- Therefore you shouldn’t measure survival when apprising screening: instead measure deaths prevented
What branch of ethics does screening conform to?
Utilitarianism
