Surveillance and Epidemiologic Investigation
What surveillance is for
Surveillance is the systematic, ongoing collection, analysis, and dissemination of data for action. Distinguish active (trained staff seek cases) from passive (relying on submitted reports), prospective from retrospective, total-house from targeted (high-risk units, procedures, organisms), and process from outcome measures. Use standardized case definitions - NHSN for CLABSI, CAUTI, VAE, SSI, MRSA bacteremia, and C. difficile - applied exactly, including the patient-on-admission window and repeat-infection rules.
Rates and metrics
Compare rates, never counts. Device-associated rates use device days as the denominator; SSI rates use procedure counts. The standardized infection ratio (SIR) compares observed to predicted events after risk adjustment, so an SIR below 1 means fewer infections than expected - a complex hospital can have more infections yet a favorable SIR. The device utilization ratio (device days ÷ patient days) tells you whether a falling rate reflects better care or simply less device use.
Outbreak investigation
Work in order: confirm the problem and verify the diagnosis; write a case definition (person, place, time); find cases; draw the epidemic curve (a single sharp peak within one incubation period suggests a point source; successive peaks suggest person-to-person spread); generate hypotheses; test them (cohort study with attack rates and relative risk, or case-control study with odds ratio); implement control; communicate; evaluate.
Traps
Watch for shifting denominators, lab-identified events, prevalence mistaken for incidence, and denominators that change because a unit closed. Validate data - under-reporting is the norm, and a surveillance system's sensitivity should be measured, not assumed.
Rule of thumb: Attack rate = (cases ÷ population at risk) × 100; divide the attack rate in exposed by that in unexposed to get the relative risk.