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Presented at CRC course held at King Fahad Medical City, Riyadh on 26 January 2020. 09:00 am - 10:00 am by Professor Omar Hasan Kasule Sr. MB ChB (MUK), MPH (Harvard), DrPH (Harvard), Chairman of the KFMC IRB
LEARNING OBJECTIVES:
• Definition and types of case-control studies.
• Design, strengths, and weaknesses of case-control studies.
• Selection of cases and controls.
KEYWORDS AND TERMS
• Case: selection, incident, prevalent
• Case-base
• Case-cohort
• Case-control
• Case-only
• Control, community control. Dead, friend, hospital, neighbor, relative
• Exposure odds ratio
• Matching, 1:1
• Matching, 1: many
• Matching, overmatch
• Population base
BASICS OF THE CASE-CONTROL STUDY
• Popular because of its low cost, rapid results, and flexibility.
• Uses small numbers of subjects.
• Is used for disease (rare and non-rare) as well as non-disease situations.
• Can be exploratory or definitive.
• Is retrospective information on exposure (cause of disease).
VARIANTS OF THE CASE-CONTROL DESIGN
• Case-base,
• Case-cohort,
• Case-only, crossover designs.
CASE-BASE DESIGN
• Cases are all diseased individuals in the population.
• Controls are a random sample of disease-free individuals in the same base population.
CASE-COHORT DESIGN (NESTED)
• Sampling from a cohort (closed or open).
• Cases and controls are selected from the same cohort.
• Blood and other biological specimens collected from the cohort at the start can be analyzed for exposure information when cases of disease appear.
CASE-ONLY DESIGN
• Used in genetic studies in which the control exposure distribution can be worked out theoretically.
• No need to select controls because information about them is known theoretically.
CROSSOVER DESIGN
• Used for sporadic exposures.
• The same individual can serve as a case or as control several times without any prejudice to the study.
BASIC 2x2 TABLE FOR CASE-BASE STUDIES
• The marginal totals, a+b, and c+d, are fixed by design before data collection thus prevalence cannot be computed.
• The basic statistical parameter is the odds ratio = ad/bc.
• The source population for cases and controls must be the same.
CASE SELECTION
• Cases are sourced from clinical records, hospital discharge records, disease registries, data from surveillance programs, employment records, and death certificates.
• Cases are either all cases of a disease or a sample thereof.
• Only incident cases (new cases) are selected.
SELECTION OF CONTROLS
• Controls must be from the same population base as the cases and must be like cases in everything except having the disease being studied.
• Information comparability between the case series and the control series must be assured.
• Hospital, community, neighborhood, friend, dead, and relative controls are used.
• There is little gain in efficiency beyond a 1:2 case-control ratio unless control data is obtained at no cost.
PREVENTION OF CONFOUNDING
• Stratification,
• Matching,
• Restriction.
SOURCE OF EXPOSURE INFORMATION
• Interviews,
• Hospital records,
• Pharmacy records,
• Vital records,
• Disease registry,
• Employment records,
• Environmental data,
• Genetic determinants,
• Biomarkers,
• Physical measurements,
• Laboratory measurements.
STRENGTHS OF THE CASE-CONTROL DESIGN
• Computation of the OR
• Low cost,
• Short duration, convenience for subjects because they are contacted/interviewed only once.
WEAKNESSES OF THE CASE-CONTROL DESIGN
• OR is an approximation,
• Prevalence cannot be computed because marginal totals are fixed in advance,
• the time sequence between exposure and disease outcome is not clear,
• vulnerability to bias (misclassification, selection, and confounding).