Study Design for Food Poisoning Investigation
A case-control study is the most appropriate design to investigate the cause of food poisoning after a trip, as it allows rapid identification of the contaminated food by comparing exposures between affected and healthy individuals. 1
Why Case-Control Design is Optimal
Practical Advantages for Outbreak Investigation
Case-control studies are specifically designed for foodborne outbreak investigations, where participants are selected based on their illness status (cases vs. controls) and then exposures to different foods are assessed retrospectively 2, 3
This design can be conducted rapidly and inexpensively, which is critical during an active outbreak when timely identification of the contaminated food source is needed to prevent additional cases 2
Real-world validation demonstrates effectiveness: In France, 74% of foodborne outbreak investigations using case-control studies successfully identified the responsible food with confirmation, compared to only 17% of investigations without this design 3
The Dallas County sodium azide poisoning investigation exemplifies this approach, where a case-control study identified iced tea as the contaminated vehicle with an odds ratio of 65 (95% CI: 2.4-3,292) 1
Statistical Power for Identifying Food Vehicles
Case-control studies efficiently calculate odds ratios to quantify the association between specific food exposures and illness, allowing investigators to rank suspected foods by strength of association 2, 3
Multiple food items can be evaluated simultaneously, with logistic regression used to identify which foods remain statistically significant after controlling for confounding 4
Dose-response relationships can be assessed by examining whether increasing consumption of a suspected food correlates with higher attack rates 5
Why Other Designs Are Inappropriate
Cohort Studies Are Impractical
Cohort studies require prospective follow-up of exposed and unexposed groups over time to determine who develops the outcome 6
This design cannot be applied retrospectively to an outbreak that has already occurred, as you cannot go back in time to identify and follow travelers before they consumed the contaminated food 2, 6
Cohort studies are resource-intensive with high risk of participant dropout, making them unsuitable for urgent outbreak investigations 6
Randomized Controlled Trials Are Unethical
RCTs would require deliberately exposing participants to potentially contaminated food, which is ethically unacceptable 2
The outbreak has already occurred, making randomization impossible in this retrospective investigation scenario 2
Common Pitfalls to Avoid
Insufficient sample size reduces statistical power: At least 10 investigations (16%) in the French survey lacked power due to small populations, though they could still identify the most likely food source 3
Selection of appropriate controls is critical: Controls must come from the same "study base" as cases (i.e., people who attended the same trip/meal but did not become ill) to avoid selection bias 2
Recall bias can occur when cases remember exposures differently than controls, particularly if significant time has elapsed since the exposure 2
Methodological errors in 8 French investigations (13%) led to results inconsistent with bacteriological findings, emphasizing the need for proper study design and execution 3
Integration with laboratory testing is essential: Case-control findings should be confirmed with microbiological analysis of suspected foods and clinical specimens 3, 5, 4