Specimen Quality
Specimen quality determines whether a microbiology result can answer the clinical question.
Core idea
A technically correct test can still mislead if the specimen is contaminated, delayed, dried out, or collected from the wrong site.
Principle
The best specimen comes from the site of infection, is collected before therapy when possible, and reaches the lab in the right container and transport condition.
Specimen Quality Notes
| Specimen type | Better collection | Common problem |
|---|---|---|
| Blood culture | Multiple sets from separate venipuncture sites | Single-bottle growth may be contamination depending on organism |
| CSF | Prompt delivery, adequate volume, sterile container | Delay can reduce recovery and cell reliability |
| Sputum | Deep cough specimen with inflammation | Saliva-heavy specimens may need recollection |
| Wound | Deep aspirate or tissue when possible | Superficial swabs often collect colonizing flora |
| Urine | Clean-catch, catheterized, or suprapubic specimen as appropriate | Mixed growth can reflect contamination |
Basic workflow
- Confirm the specimen source and collection method.
- Check timing, transport, volume, and container.
- Assess direct smear quality when applicable.
- Reject or recollect when the result would be unreliable.
- Choose routine, anaerobic, fungal, AFB, molecular, or serologic testing based on the question.
How to read it
- Deep aspirates and tissue usually answer more than superficial swabs.
- Sterile-site specimens usually carry higher significance.
- Mixed flora from poor-quality specimens may reflect contamination.
High-yield trapCommon mistakes
- Treating every swab as equal.
- Ignoring transport needs for anaerobes.
- Reporting organism lists without specimen context.
ShortcutStudent shortcut
Before asking what grew, ask whether the specimen deserved to grow it.