When a negative control shows microbial growth, the entire test is considered invalid. This means the results cannot be trusted because contamination has occurred somewhere in the testing environment, reagents, equipment, or handling process.
๐๐จ๐ฌ๐ฌ๐ข๐๐ฅ๐ ๐๐๐ฎ๐ฌ๐๐ฌ:
- Contaminated media
- Improper sterilization
- Environmental breach
- Operator handling error
๐๐ก๐๐ญ ๐๐ฌ ๐ ๐๐๐ ๐๐ญ๐ข๐ฏ๐ ๐๐จ๐ง๐ญ๐ซ๐จ๐ฅ?
A negative control is a control sample designed to demonstrate that microbial growth has not been introduced by the test system or procedure.
Depending on the test, a negative control may contain:
- Sterile diluent
- Sterile media
- A blank solution
- Other appropriate control material
The exact control used depends on the validated test method.
Expected result:
Negative control โ No microbial growth
If growth occurs, the laboratory must determine why.
๐๐ก๐๐ญ ๐ข๐ญ ๐ฆ๐๐๐ง๐ฌ ๐๐จ๐ซ ๐ฒ๐จ๐ฎ๐ซ ๐ซ๐๐ฌ๐ฎ๐ฅ๐ญ๐ฌ
If the negative control shows growth, you generally cannot trust the positive results or test sample results from that same run. Any “positive” signal in your actual samples could just be the same contamination showing up there too, so you can’t distinguish a true positive from background contamination.
๐๐จ๐ฐ ๐ญ๐จ ๐๐๐ฌ๐ฉ๐จ๐ง๐ ๐๐ก๐๐ง ๐๐ญ ๐๐๐ฉ๐ฉ๐๐ง๐ฌ
- ๐๐ง๐ฏ๐๐ฅ๐ข๐๐๐ญ๐ ๐ญ๐ก๐ ๐๐๐ญ๐๐ก ๐จ๐ซ ๐ซ๐ฎ๐ง: Don’t report results that came out of a run with a compromised control.
- ๐๐ง๐ฏ๐๐ฌ๐ญ๐ข๐ ๐๐ญ๐ ๐ญ๐ก๐ ๐ซ๐จ๐จ๐ญ ๐๐๐ฎ๐ฌ๐: Check sterilization logs, reagent lot numbers, technique, equipment maintenance records, and environmental monitoring data.
- ๐๐-๐ฌ๐ญ๐๐ซ๐ข๐ฅ๐ข๐ณ๐ ๐๐ง๐ ๐ซ๐-๐ฏ๐๐ซ๐ข๐๐ฒ: any suspect media or reagents before reuse.
- ๐๐๐ฉ๐๐๐ญ ๐ญ๐ก๐ ๐ญ๐๐ฌ๐ญ: with fresh, verified negative controls once the source has been identified and corrected.
- ๐๐จ๐๐ฎ๐ฆ๐๐ง๐ญ ๐ญ๐ก๐ ๐๐๐ฏ๐ข๐๐ญ๐ข๐จ๐ง: In regulated environments, this isn’t option it’s usually a formal requirement.
