Understanding acceptance criteria in temperature mapping

Also read: Temperature mapping: Tips, frameworks, and pitfalls

What are acceptance criteria in temperature mapping?

Setting clear acceptance criteria is a foundational step in any GxP-compliant mapping project. They define the exact range of temperatures your equipment or facility must maintain to be considered fit for storing your sensitive products. In other words, you can think of them as the pass/fail rules for your temperature mapping study. These criteria are a critical tool to ensure your products remain safe and effective.

Also see: 7 updates from USP <1079.2>: How to evaluate temperature excursions with MKT

Why do you need clear acceptance criteria?

Your acceptance criteria form the backbone of your temperature mapping protocol. They must be defined before you begin any testing and be based on a deep understanding of the products you plan to store. Without them, it is impossible to analyze your mapping results or make a final conclusion about your facility's performance.

A well-defined set of criteria demonstrates to auditors that your qualification strategy is sound and that you take full responsibility for the work performed, even if you use a third party for testing.

How do you define standard temperature ranges?

While your specific product needs should always be the final authority, several standard ranges serve as a common starting point in the industry:

• Cold storage: 2°C to 8°C
• Frozen storage: Below -20°C
• ULT (ultra-low temperature) storage: Below -82°C, though this can be product-dependent
• Ambient storage: 15°C to 25°C

These ranges define the minimum and maximum temperatures required in most situations, but you should always tailor them to your specific storage requirements.

Also read: What are the WHO’s guidelines for temperature mapping?

How should you handle temperature excursions?

In the real world, it's often impractical to keep temperatures strictly within a defined range, especially in areas near doors or gates that are opened during normal operations. This is where defining allowable excursions becomes useful.

An excursion is a brief, defined period where the temperature is allowed to deviate from the primary range without causing a non-conformity. For example, a valid criterion could be:

Temperatures must be maintained within 2°C to 8°C, with excursions of 2°C for a maximum of 30 minutes.

This approach is more realistic because the thermal mass of packaged goods prevents their internal temperature from changing instantly with short-term fluctuations in air temperature. When you define an allowable excursion, you must provide a rationale based on the thermal mass of the products you store. You can strengthen this rationale with tests that show the actual impact of an excursion by placing a sensor inside a real or simulated product.

Also see: Temperature excursion investigation guidelines

How does measurement uncertainty affect your acceptance limits?

Every data logger has a degree of measurement uncertainty, often referred to as its Maximum Permissible Error (MPE). You must account for this uncertainty when analyzing your data. This is done by narrowing your acceptance range for the recorded data to ensure that even with the potential error, the true temperature remains within your required limits.

The formula is straightforward:

• Lower Acceptance Limit = Lower Limit + MPE
• Higher Accptance Limit = Higher Limit - MPE

For example, if your required range is 2.0°C to 8.0°C and your data loggers have an MPE of 0.5°C, your acceptance criteria for the recorded data become 2.5°C to 7.5°C. Any reading outside this adjusted range is considered a deviation.

Is there another way to calculate the limits?

There is some discussion in the industry about an alternative way to calculate acceptance limits. This argument suggests that because limits like "2°C to 8°C" are stated without decimals, the true range could be interpreted as 1.51°C to 8.49°C, which would round to 2°C and 8°C.

Using this logic with a 0.5°C MPE, the adjusted lower limit would be 2.01°C (1.51°C + 0.5°C) and the upper limit would be 7.99°C (8.49°C - 0.5°C), creating a wider acceptable range for the recorded data. While some auditors and inspection bodies accept this approach, it is not universally recognized.

Also read: Continuous temperature mapping: A framework to eliminate re-mapping