Identification of Infectious Disease Processes
The infectious process
Infection preventionists must separate colonization, contamination, and true infection. Pathogenicity depends on organism virulence factors (adhesins, toxins, capsules, biofilm formation) and host defenses - neutropenia, mucosal barrier injury, indwelling devices, and immunosuppressive therapy all lower the infective dose. Know the chain of infection (agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host) and where each control interrupts it.
Diagnostics
Be fluent in what each test actually answers. Gram stain is rapid and guides empiric therapy but has poor sensitivity; culture remains the reference standard with 24-72 hour turnaround; antigen tests are fast but less sensitive; nucleic acid amplification is highly sensitive and can detect non-viable organisms, making it a poor test of cure. Screening cultures (MRSA nasal, CRE rectal) detect colonization, not infection.
Interpreting results
Results from non-sterile sites (sputum, wound swab, urine without symptoms) may be colonizing flora. Asymptomatic bacteriuria is not treated except in pregnancy or before urologic instrumentation. Apply sensitivity, specificity, and predictive values: positive predictive value falls sharply as prevalence falls.
Traps
Questions often hinge on timing - whether an organism present on admission counts as a healthcare-associated infection - and on whether multiple organisms from one site represent one event. Beware conflating a positive culture with infection, and expect items on notifiable conditions, biofilms on devices, prion disease, and resistance mechanisms (MRSA, VRE, ESBL, CRE, Candida auris).
Rule of thumb: A test with 99% sensitivity and 99% specificity used where prevalence is 1% yields a PPV near 50% - half of the positives are false, so test the patient you suspect, not the population.