GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 75 of 500
Check each positive and negative control against its expected result, and keep the log on this device.
Try sample values — fills the form with the worked example below.
Control log
Saved on this device only. Calculate a result first, then add it to the log.
| Date / time | Control | Observed, CFU | Expected range, CFU | Result |
|---|
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Control result
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Observed count (CFU)
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Expected range (CFU)
Equation: Positive control passes when expected low ≤ observed ≤ expected high. Negative control passes when observed = 0.
Method: Control-strain practice under ISO 11133 (preparation, production, storage and performance testing of culture media) and ISO 7218 (general requirements for microbiological examinations): a positive control must show the expected growth and a negative control must show no growth before sample results are accepted.
Assumptions: the control strain, the medium and the incubation conditions match the method; the expected range is the laboratory's own validated range for that control.
Limitations: a passing control does not prove the sample results — it only shows the medium and the procedure worked for that run. A failed control invalidates the whole run, and the expected range must come from the laboratory's own data, not from this tool.
Rounding: counts are whole numbers and are never rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Control-strain practice under ISO 11133 (preparation, production, storage and performance testing of culture media) and ISO 7218 (general requirements for microbiological examinations): a positive control must show the expected growth and a negative control must show no growth before sample results are accepted.
Assumptions: the control strain, the medium and the incubation conditions match the method; the expected range is the laboratory's own validated range for that control.
Limitations: a passing control does not prove the sample results — it only shows the medium and the procedure worked for that run. A failed control invalidates the whole run, and the expected range must come from the laboratory's own data, not from this tool.
Rounding: counts are whole numbers and are never rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Positive control, observed 85 CFU, expected range 30–300 CFU. 85 lies inside 30–300, so the control passes and the run can be accepted.
Enter values above to see results.
Run a positive and a negative control with every batch of media and every test run. If a control fails, stop, investigate, and do not report the sample results from that run. Tool 75 of 500 — GIFSQ
