Pre-analytical Considerations: Specimen Collection, Handling, and Rejection
This domain tests whether you can decide that a specimen is worth testing and whether your handling preserves the analyte. Most microbiology errors happen before the instrument ever sees the sample.
Collection rules that change results
- Blood cultures: 2–3 sets from separate venipunctures, drawn before antimicrobials; 20–30 mL of blood per adult episode (8–10 mL per bottle). Volume drives sensitivity more than timing does.
- CSF: send an uncentrifuged aliquot to microbiology, hold at 35–37 °C, never refrigerate - Neisseria meningitidis and Haemophilus influenzae die in the cold.
- Urine: clean-catch midstream; culture within 2 hours or refrigerate/use boric acid. Reject 24-hour collections and Foley catheter tips.
- Anaerobes: aspirate or place a deep swab in anaerobic transport. Dry swabs and delay kill Bacteroides and Fusobacterium.
Rejection criteria
Reject unlabeled or mislabeled containers (label at the bedside), leaking cups, dry swabs, expired transport media, and sputum that is really saliva. Sputum adequacy is graded on the Gram stain: more than 25 PMNs and fewer than 10 squamous epithelial cells per low-power field (Murray–Washington) is acceptable; the reverse pattern means oral contamination.
Worked example
A sputum Gram stain shows 30 squamous epithelial cells and 3 PMNs per low-power field, with mixed gram-positive cocci and gram-negative rods. Reasoning: squamous cells are shed from the oropharynx, so the PMN-to-squamous ratio is inverted and culture would simply report oral flora. Do not report "normal respiratory flora" as if it were a result. Reject, document the reason, and request an induced sputum or BAL.
Traps that cost marks
Refrigerating CSF or genital swabs; accepting pooled sputum; reporting a culture on a rejected specimen while quietly ignoring the screen; forgetting that a swab carries a tiny fraction of the blood in a bottle.