What is prognosis?
An assessment of the future course and outcome of a patient’s disease, based on knowledge of the course of disease in other patients together with the general health, age, and sex of the patient
What are prognostic factors?
Certain patient characteristics can predict outcomes more accurately
What are risk factors?
Patient characteristics associated with the development of the disease in the first place.
Why is prognosis important? (2)
- Helps in treatment decision making
- Helps in discussing the opinions of patients or relatives concerning management
What are the types of prognosis questions? (3)
- Which outcomes could happen? (qualitative aspect)
- How likely are the outcomes to happen? (quantitative aspect)
- Over what period will the outcomes happen? (temporal aspect)
What are the most appropriate studies to measure prognosis? (2)
- Cohort
- Case-control
Why are RCTs inappropriate to measure prognosis?
It is not appropriate to randomise patients to different prognostic factors
How are cohort studies used in selecting the study population? (2)
- Recruit individuals with the disease who are exposed to the possible prognostic factor
- Compare group with disease with group without disease
What type of bias can cohort studies produce?
Selection bias
How do we collect data in cohort studies? (2)
- Baseline data/Record of exposure
- Follow-up data
What is needed in the follow up of cohort studies? (4)
- Needs to be long enough to identify outcome of interest
- Need to maximise completeness
- If too many participants lost at follow-up, results could be biased
- To reduce potential for bias
What is needed in the interpretation of cohort studies? (2)
- Provides evidence of association does not cause
- Need to be aware of possible bias
What are the strengths of cohort studies? (6)
- Useful for investigating the natural history and incidence of a disease
- Useful for investigating multiple outcomes
- Useful in the study of rare exposures
- Allows calculation of useful estimates (including incidence rates, relative risks, and confidence intervals)
- Historical cohort studies are effective in the investigation of diseases that have a long development time
- Reduces risk of survivor bias (a failure to consider those without the outcomes of interest)
What are the weaknesses of cohort studies? (4)
- Can be expensive and time consuming
- Attrition bias -Loss to follow up can be a difficulty. There could be significant differences in the loss to follow between both groups.
- Susceptible to selection bias
- Recall bias is very minimal in cohort studies, and many results can be studied at the same time.
What is attrition bias?
Systematic differences between people who leave the study and those who continue.
What is selection bias?
Enrolling an exposed person if they have the outcome of interest, the measure of association will be biased
What are the strengths of case control studies? (5)
- Effective in the investigation of diseases that have a long development time
- Useful in the study of rare diseases
- Allows calculation of useful estimates (including incidence rates, odds ratios, and confidence intervals)
- Can study multiple risk factors/exposures
- Most efficient use of resources (money, time and effort)
What are the weaknesses of case control studies? (3)
- If the frequency of the exposure is low, case-control studies quickly become inefficient
- Can be difficult to choose an appropriate control group
- Susceptible to selection and recall bias
How can prognostic evidence be presented? (3)
- Median survival
- Percentage of survival/specified outcome at a particular point in time
- Survival curves
What is a survival curve?
Depict at each point in time the proportion (expressed as a percentage) of the original sample who have not yet died/experienced the outcome
What is a median survival?
The length of follow-up by which 50% of study patients have died/experienced the outcome (e.g. at least half of all patients with this disease survive 5 years)
