Collection, Handling, and Processing of Specimens
Core concepts
Specimen quality drives result quality. Know collection timing (before antibiotics, at peak symptoms), site-appropriate technique (blood cultures from two sites, sputum from deep cough, clean-catch urine), and transport conditions (timing, temperature, preservatives). Understand rejection criteria (e.g., unlabeled specimens, delayed transport, saliva instead of sputum, duplicate specimens within a defined window).
Know the processing workflow: accessioning, direct examination (Gram stain, wet mount), culture on appropriate media, incubation conditions (aerobic, anaerobic, CO2), and identification/susceptibility. Understand blood culture principles: volume matters most, two to three sets, drawn before antibiotics. Know urine processing (colony count thresholds, mixed flora interpretation).
Traps to expect
- Assuming a single blood culture set is adequate - volume and number of sets drive sensitivity.
- Forgetting that anaerobic specimens (abscess, deep tissue) require anaerobic transport and media.
- Overlooking that sputum must be screened for squamous epithelial cells to confirm adequacy.
- Confusing transport media (preserves viability, does not support growth) with enrichment media.
Worked example / rule of thumb
A sputum Gram stain shows many squamous epithelial cells and few neutrophils. Rule of thumb: reject or request recollection - this is likely saliva, not lower respiratory specimen. The threshold often cited is >10 squamous epithelial cells per low-power field. Document rejection and notify the provider; do not culture a rejected specimen.