Diagnostic Testing, Susceptibility Interpretation, and Therapeutic Drug Monitoring
Core concepts
Know the diagnostic modalities: culture (blood, urine, sputum, sterile sites), Gram stain, rapid molecular assays (PCR, NAAT), antigen tests, serology, and biomarker tests (procalcitonin, CRP). Understand susceptibility testing: MIC, disk diffusion, broth microdilution, and automated systems. Interpret breakpoints (susceptible, susceptible-dose-dependent, intermediate, resistant) and know that breakpoints are organism-, drug-, and site-specific. Recognize special resistance phenotypes (ESBL, AmpC, CRE, MRSA, VRE) and reporting nuances (e.g., inducible clindamycin resistance with D-test). Therapeutic drug monitoring applies to drugs with narrow therapeutic indices (vancomycin, aminoglycosides, voriconazole, posaconazole); know timing of levels relative to dose and the target index (trough vs. AUC).
Traps
- Interpreting an MIC without the correct breakpoint table.
- Confusing screening tests with confirmatory tests.
- Drawing drug levels at the wrong time (e.g., peak vs. trough confusion).
- Assuming susceptibility in vitro guarantees clinical efficacy at the site of infection.
Rule of thumb
A result is only meaningful in context: match the test to the syndrome, the breakpoint to the organism and drug, and the drug level to the dosing interval.
Worked example: A vancomycin trough drawn 30 minutes after a dose is not interpretable as a trough; redraw at the correct pre-dose time per protocol.