Setting of Alert and Action Limits for Microbial Monitoring

Alert and Action Limits are critical components of a microbial monitoring program, ensuring timely interventions to maintain control and prevent contamination. Here’s a step-by-step guide to calculate these limits:

Alert Limit (AL):

1. Determine the average (μ) of historical microbial data (e.g., 20-30 data points).
2. Calculate the standard deviation (σ) of the data.
3. Set the Alert Limit (AL) at 2-3σ above/below the average (μ).

AL = μ ± (2-3σ)

Action Limit (AcL):

1. Determine the maximum acceptable limit for microbial growth (e.g., regulatory limits, industry standards).
2. Set the Action Limit (AcL) at 80-90% of this maximum limit.

AcL = 0.8-0.9 × Maximum Acceptable Limit

Example Calculation:

Historical data (CFU/mL):

| 10, 20, 15, 30, 25, 18, 22, 12, 28, 20 |

Average (μ) = 20.5
Standard Deviation (σ) = 6.3

Alert Limit (AL) = 20.5 ± (2 × 6.3) = 12.9-28.1 CFU/mL

Maximum Acceptable Limit = 100 CFU/mL (regulatory limit)
Action Limit (AcL) = 0.9 × 100 = 90 CFU/mL

Interpretation:

• Alert Limit (AL): 12.9-28.1 CFU/mL ( triggers investigation and potential corrective actions)
• Action Limit (AcL): 90 CFU/mL (triggers immediate corrective actions to prevent contamination)

Considerations:

1. Data distribution and normality.
2. Sampling frequency and location.
3. Microbial type and growth rates.
4. Industry standards and regulatory requirements.
5. Continuous monitoring and data review.

Read also:

• Why Use Less than One Instead of Zero CFU during Counting of Microbiology Samples?
• Data Integrity Consideration for Microbiological Methods

Resource Person: Mahesh Bhandarkote