Diagnostic Microbiology and Laboratory Stewardship
Core concepts
Understand culture vs molecular vs serologic testing. Blood cultures: two sets from separate sites before antibiotics. Sputum: good quality (many PMNs, few epithelial cells). Urine: avoid contamination; do not culture asymptomatic patients. CSF: cell count, glucose, protein, Gram stain, culture, and PCR (HSV, enterovirus).
Must-know tests
MALDI-TOF for rapid organism ID, PCR for respiratory viruses, antigen tests (Streptococcus, Legionella urine antigen, Cryptococcus antigen), and serology (syphilis, HIV, hepatitis). Antimicrobial susceptibility testing: MIC interpretation, breakpoints, and mechanisms (ESBL, MRSA mecA, vancomycin MIC creep).
Traps
False-positive blood cultures from contamination-interpret with clinical context. Urine antigen for Legionella only detects serogroup 1. Cryptococcal antigen is highly sensitive in CSF and serum. Don't order broad panels without clinical context-stewardship applies to diagnostics too.
Worked rule of thumb
When a blood culture grows coagulase-negative Staphylococcus in a single bottle, suspect contamination-repeat cultures before starting therapy unless the patient has a prosthetic device or is immunocompromised.