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BTS CPI study materials
BTS CPI study resources

BTS CPI: Surveillance, Outbreak Investigation, and Epidemiologic Methods

BTS CPI · Work through your exam, one topic at a time.

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Surveillance, Outbreak Investigation, and Epidemiologic Methods

Surveillance and outbreak investigation questions test whether you can define a case, calculate a rate correctly, and defend an intervention. Expect NHSN definitions, device-associated rates, the standardized infection ratio, epidemic curves, and outbreak steps.

Core rules

  • Surveillance is systematic collection, analysis, interpretation, and dissemination of health data. Active surveillance uses trained staff reviewing records; passive relies on reporting.
  • Use a written case definition (clinical, laboratory, and time/person/place criteria). NHSN definitions are procedure- and device-specific; changing definitions changes rates.
  • Rates: incidence = new cases / population at risk over time; attack rate = cases / population at risk in an outbreak; device-associated rate = infections / device-days × 1,000.
  • The standardized infection ratio (SIR) = observed infections / predicted infections from a risk-adjusted model. SIR >1 means more infections than predicted; SIR <1 means fewer. Report a 95% confidence interval; if it includes 1, the result may not be statistically significant.
  • Outbreak steps: detect and confirm, verify diagnosis, define a case, search for cases, describe by person/place/time, form a hypothesis, test with analytic study, implement control, communicate, and evaluate.
  • Epidemic curve shows time distribution; a point source has one peak, a propagating outbreak has successive peaks, and a continuous common source has a plateau.
  • Analytic studies: cohort gives relative risk; case-control gives odds ratio. Calculate attack rates in exposed versus unexposed.

Worked example

A unit has 6 CLABSIs and 2,000 central-line days; predicted is 3.0. SIR = 6/3.0 = 2.0. The rate is 6/2,000 × 1,000 = 3.0 per 1,000 line-days. The SIR suggests twice as many infections as predicted. Next, check the 95% CI and whether the increase is diffuse or clustered; if clustered, do a line-insertion/access review and a case-control study. Do not conclude that the SIR alone proves a practice failure-risk adjustment, case finding, and definition changes matter.

Traps

Do not use the number of infections as a rate without a denominator. Do not compare SIRs across facilities without considering risk adjustment and reporting differences. Do not call an increase an outbreak without a case definition and epidemiologic link.