Specimen Collection and Transport
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Direct answer
Specimen collection and transport is the pre-analytical stage that decides whether a laboratory result can be trusted. Samples should be collected before antibiotics, in sterile containers, labelled properly and delivered promptly in the correct transport medium. A specimen that is delayed, leaking, unlabelled or contaminated is usually rejected.
What you must remember
- Before antibiotics: collect cultures before the first dose, since antibiotics lower the yield.
- Transport time: most specimens should reach the laboratory within about 2 hours, kept at room temperature.
- CSF: do not refrigerate for bacterial culture, because cold kills Neisseria meningitidis and Haemophilus influenzae.
- Urine: a midstream clean catch, where 10^5 CFU/mL is the classic threshold for significant bacteriuria.
- Sputum: an early-morning, deep-cough specimen, commonly accepted when neutrophils exceed about 25 and squamous cells are below 10 per low-power field.
- Stool: Cary-Blair medium for enteric pathogens when culture is delayed, with stool preferred over rectal swabs.
- Pus and aspirates: sent in anaerobic transport, not on an open swab, because anaerobes die on exposure to air.
- Rejection: unlabelled, leaking, wrongly contained or delayed specimens are rejected, with the reason recorded.
Specimen by specimen
The right collection rule depends on the site, because each specimen has its own risk of contamination and its own fragile organisms. The table lists the rule and transport needed for the specimens most often tested.
| Specimen | Collection rule | Transport |
|---|---|---|
| CSF | Sterile container, aseptic lumbar puncture | Room temperature, processed at once |
| Urine | Midstream clean catch | Within 2 hours, or refrigerated |
| Sputum | Early morning, deep cough | Prompt delivery in a sterile container |
| Stool | Fresh, collected before antibiotics | Cary-Blair medium for enteric culture |
| Pus or aspirate | Aspirated into a sterile syringe, air removed | Anaerobic transport device |
| Throat or genital swab | Swab taken from the lesion or site | Amies or Stuart's medium |
Most failures come from delay, cold storage or contamination with normal flora rather than from the test itself. Recording the time of collection and the transport conditions helps the laboratory judge whether a result is reliable. Specimens that fail these rules should be reported as unsuitable, with a request for recollection.
Where the exam tries to catch you
Pre-analytical questions test the rule and its reason, such as why cold or delay affects a particular organism.
- Trap: refrigerating CSF until it is processed Correct: cold kills fragile organisms such as Neisseria meningitidis and Haemophilus influenzae, so CSF goes to the laboratory at room temperature.
- Trap: 10^3 CFU/mL in urine confirms infection Correct: the classic threshold for significant bacteriuria is 10^5 CFU/mL in a midstream clean catch, and lower counts need clinical correlation.
- Trap: saliva is acceptable as a sputum specimen Correct: saliva carries squamous cells and normal oral flora, so specimens dominated by squamous cells are rejected.
Frequently asked questions
Why should CSF not be refrigerated before bacterial culture?
Cold temperature kills fragile organisms such as Neisseria meningitidis and Haemophilus influenzae. CSF should reach the laboratory at room temperature and be processed promptly for bacterial culture.
What is the significant colony count for a urine culture?
A count of 10^5 colony-forming units per mL of midstream clean catch urine is the classic threshold for significant bacteriuria. Lower counts may still matter in catheter samples or in symptomatic patients.
Which transport medium is used for stool when cholera is suspected?
Cary-Blair medium is commonly used for stool when culture is delayed, because it preserves enteric bacteria such as Vibrio cholerae. Stool should be collected before antibiotics and delivered promptly.
Why must sputum be collected early in the morning with a deep cough?
Early morning sputum contains the most secretions from the lower airway. A deep cough brings up lower respiratory material rather than saliva, which reduces contamination with oral flora.