Screening Flashcards
What is screening?
Screening is the process to detect among healthy people disorders or risk factors of which they are unaware
What are the types of screening?
- Mass
- Multiple or multiphasic
- Targeted
- Case-finding or opportunistic
What is mass screening?
Screening applied to the whole population
What is multiple/multiphasic screening?
If more than one screening procedure is applied on the same occasion, this is multiple screening
What is targeted screening?
Screening tests can be applied to a group of specific exposure, this involves targeted screening and is often used in environmental and occupational health
What is case-finding/opportunistic screening?
restricted to patients who consult a health practitioner for another purpose
What factors make a diseases appropriate for screening?
- Important health problem
- High prevalence
- Natural history understood
- Long latent period
- Early detection improves prognosis
What are the factors that can be used to evaluate a screening program?
- Reliability
- Feasibility
- Validity
- Performance
- Effectiveness
What is reliability?
the consistency of results when the screening program is repeated on the same persons under the same conditions
What are the possible causes of variation in results when repeated screening is performed?
- Biological variation
- biological variation of the actual manifestation being measured
e.g. blood pressure which varies considerably for a given individual with time and under various circumstances - Program method
- the precision of the instrument used could result in varying results
e.g. standard mercury sphygmomanometer for blood pressure - Intraobserver variability
- differences in repeated measurements from the same screener - Interobserver variability
- inconsistencies attributed to differences in the way different screeners apply or interpret the screening program results.
What determines the feasibility of a screening programe?
- acceptability
- cost effectiveness
What determines the acceptability of a screening program?
- Quick
- Easy
- Safe
- minimum discomfort
- no side effects
The acceptability of the screening program can be measured by?
- the number of persons examined
- the proportion of the target population that is screened
What is the cost effectiveness of a screening program?
The cost of the screening program includes not only the cost related to the screening procedure itself but also those arising from subsequent diagnostic procedures, follow-up and intervention among those who test positive
> cost effectiveness in
1.Screening
2. Diagnosis
3. Follow-up
4. Intervention
What determines the validity of a screening program?
- Sensitivity
- Probability to test positive among truly affected - Specificity
- Probability to test negative among truly unaffected
Note: false negative are important to avoid especially in severe conditions while those picked wrongly as positive false positive are just an alarm
How do you measure the performance of a screening test?
measured by predictive values
1. PV+ : Probability to be affected among test positives
2. PV- : Probability to be unaffected among test negatives
3. PCC : Probability to be correctly classified
Note: the predictive values depend on the prevalence, the higher the prevalence the higher the PV+ and the lower the PV-
How do you determine the effectiveness of a screening program?
Outcome measures:
1. Morbidity
2. Disability
3. Mortality
Explain the role of bias in determining effectiveness of a screening program?
- Patient self-selection
- volunteers for screening who represent either a group with
i. better health, lower mortality and are more likely to adhere to prescribed interventions
ii. group of the “worried well” who are asymptomatic, at higher risk of mortality because of medical or family characteristics regardless of the screening - Lead time
- represents the amount of time by which the diagnosis has been advanced as a result of screening and can give a false impression of increased survival among screen-detected cases
- Cases progressing rapidly will gain less lead time than those who progress slowly. - Length
- The length of the preclinical phase may also affect the evaluation of a screening program as conditions with long preclinical phase are detected by screening at an earlier stage than those with short preclinical phase
Name study designs for screening?
- correlation studies
- analytical studies
- Randomized trials
What are correlation studies?
Correlation studies have been used to examine disease trends in relation to screening frequencies within a population or to compare the relationship between the frequencies of screening and disease rates for different populations
- Correlation studies are useful in suggesting a relationship between screening and decline in morbidity or mortality
Describe the disadvantages of correlation studies?
- since the information from these studies concerns populations rather than individuals, it is not possible to identify that those experiencing the benefit are the same persons who received screening
- such studies cannot allow for control of potential confounding risk factors
- the measure of screening frequency is usually an average value for the population, so it is impossible to determine an optimal screening strategy for an individual.
Use, strength and limitation of correlation studies?
- Use
Description of population - Strength
Suggest possibility of benefit - Limitation
Can’t test hypothesis
What are the types of analytical studies?
- case control
- case cohort
What is a case control analytical study?
In case-control design individuals with or without the disease are compared with respect to their past exposure to screening. However, as with any case-control study, the definition and selection of cases and controls are important for the validity of the findings.
What is a cohort analytical study?
Using a cohort design, a case-fatality rate of those who chose to be screened is compared with the comparable rate among those whose diagnoses were symptom-related
Use, advantages and limitation of analytical studies?
- Use:
Comparison of rates - Advantage:
Test hypothesis - Limitation:
- Selection
- Lead time
- length
What is a randomized trial?
a study in which the particpants are divided by chance into separate goups that compare different treatments or other interventions
Advantages of randomized trials?
- Self-selection bias is also controlled as individuals are allocated to either group at random after they have agreed to participate in the trial
- Lead time bias can be taken in account by adjusting for the average lead time when comparing screening versus symptom- detected individuals or preferably by comparing age-specific mortality rates for both groups
- Trials can also control the length bias by comparing the mortality experience of the groups after repeated screening
- controls potential confounders when the sample size is sufficiently large
Limitations of randomized trials?
the problems of costs, ethics and feasibility and most evidence on the effects of screening programs come from nonexperimental study designs
Use, strength and limitation of randomized trials?
- Use:
Comparison of rates - Strength:
Most valid test of hypothesis - Limitation:
Cost, ethics & feasibility
Evaluation of screening?
- sensitivity = TP/(TP+FN)
- specificity = TN/(TN+FP)
- prevalence = (TP+TN)/(TP+TN+FP+FN)
Objectives of screening?
- Reduce disease incidence
- Reduce morbidity, disability & mortality
Criteria for screening?
- Appropriate factor
- Beneficial program
