The specimen must represent the site of suspected infection. Sampling a colonized surface instead of the infected compartment produces a result that cannot answer the identification question.
Sterile sites versus colonized surfaces
Normally sterile sites
- Blood, cerebrospinal fluid, joint fluid, deep tissue, lower respiratory tract beyond the oropharynx
- Any growth is significant and usually identifies a pathogen directly
- Fewer organisms are present, so collection volume matters
Colonized surfaces
- Skin, throat, nasal passages, gut, vaginal mucosa, wound surfaces
- Growth is expected and mixed; the pathogen must be distinguished from residents
- The result is interpreted only alongside the clinical findings
Why the wrong site fails
A swab of a draining wound samples the surface flora and the drainage, not the tissue where the infection is progressing. A urine sample collected from a catheter bag reflects organisms that have colonized the tubing. In both cases the laboratory may grow something, but that growth describes a different compartment from the one causing the illness, so it cannot establish the causative organism.
Applying the match
Suspected bacterial meningitis calls for cerebrospinal fluid, not a throat swab, because the throat is colonized and the meninges are not. Suspected bloodstream infection calls for blood cultures drawn from separate venipuncture sites, not a swab of an intravenous line hub. Suspected deep soft-tissue infection calls for tissue or aspirated fluid from the infected area, not a surface swab of the overlying skin.