Diagnostic Microbiology and Laboratory Testing
Core concepts
Diagnostic testing starts with specimen quality. Blood culture yield depends on volume, not just timing. CSF should be sent for cell count, protein, glucose, Gram stain, culture, and PCR when viral meningitis is suspected. Urine in infants should be obtained by catheter or suprapubic aspiration; bag specimens are for screening and not culture. Rapid antigen testing for group A Streptococcus is specific but less sensitive than culture in children, so a negative rapid test may need culture. Serology is best for immunity or convalescent diagnosis; IgM testing is prone to false positives. Molecular panels detect nucleic acid, not viability, and can represent colonization.
Traps
- Coagulase-negative Staphylococcus in a single blood culture bottle often represents contamination.
- A negative CSF HSV PCR early in disease does not exclude HSV; repeat testing and treat if suspicion is high.
- Sputum and wound cultures may grow colonizers; interpret with Gram stain and clinical findings.
- Do not use ASO titers to diagnose acute streptococcal pharyngitis.
Worked example
A 2-month-old with fever and pyuria: obtain a catheterized urine specimen; a culture growing 50,000 CFU/mL or more of a single uropathogen supports urinary tract infection. A bag specimen with mixed flora is not diagnostic.
Rule of thumb: before ordering a test, ask whether the result will change management; if not, do not send it.