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.
Possible Causes:
- Contaminated media
- Improper sterilization
- Environmental breach
- Operator handling error
What Is a Negative Control?
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 β No microbial growth
If growth occurs, the laboratory must determine why.
What it means for your results
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.
How to Respond When It Happens
- Invalidate the batch or run: Don’t report results that came out of a run with a compromised control.
- Investigate the root cause: Check sterilization logs, reagent lot numbers, technique, equipment maintenance records, and environmental monitoring data.
- Re-sterilize and re-verify: any suspect media or reagents before reuse.
- Repeat the test: with fresh, verified negative controls once the source has been identified and corrected.
- Document the deviation: In regulated environments, this isn’t option it’s usually a formal requirement.
