Limits or thresholds for ATP testing can be set in a number of different ways. This document is designed to give some background information, advice and guidance about setting thresholds.
Threshold limits are used to interpret results from the test. We advise:
- Use 2 limits: a lower and an upper limit
- In all cases, the upper limit is twice the lower limit
- If the test value is below the lower limit, the interpretation is PASS
- If the test value is above the upper limit, the interpretation is FAIL
- In case the test value is in between both limits, the interpretation is CAUTION. In the latter case, it is best to repeat the test with a new sample from the same area to confirm. If it is again in the CAUTION range, the tested area is not clean and should be re-cleaned
Industry accepted critical limits:
< 10 RLU = Pass: surface was thoroughly cleaned
10 – 30 = Caution: surface may require targeted re-cleaning, especially for higher risk surfaces
30+ = Fail: cleaning process failure. Sanitation should be repeated and samples retested until a Pass score is achieved
Options to be considered when setting limits:
1. Use an action threshold based on the average swab blank
Many users take an average swab blank by testing several clean swabs. This is normally multiplied by 3 and then used as the action threshold. The blank is tested for every new batch of reagents or on a weekly / monthly basis. For instance, if the average swab blank is 3, the lower action threshold is 9 and the upper threshold is 18.
This is a very simple way to set thresholds for hygiene tests.
There may be some small but noticeable change in the blank swab values, particularly when the values are multiplied by three. Also in case of rough surface types (rubber, wood, meat cutting blocks), it may be difficult to get results below the lower threshold limit.
2. Set the thresholds by measuring dirty and thoroughly clean surfaces
Some sites set their initial thresholds on the basis of what level of cleanliness can be achieved at each test location. Each critical area or piece of equipment is tested after it has been thoroughly cleaned or even cleaned twice. This gives an indication of what RLU values are achievable for each location when it is thoroughly cleaned. The working thresholds are can then be set slightly higher than this value, depending on how critical the area is.
The process of setting thresholds is relatively simple and it is the most correct way.
Large sites may have a very large number of testing locations which each require cleaning and testing.
3. Set the thresholds by monitoring the typical values from the site over a period of several weeks
One approach used to set thresholds is to monitor the critical control points and document the RLU values which you get over a period of several weeks. This provides an indication of the typical levels of cleanliness which are achievable for any particular location. Thresholds are then set and monitored as necessary.
This is a relatively simple way of setting thresholds and it gives the user a chance to get used to typical test values over a short period of time.
It will take a few weeks of testing before enough information on typical RLU values are obtained.
4. Start with recommended RLU threshold values provided by Hygiena International and adjust at a later stage based on own results
This approach will allow the user to start interpreting the results immediately and take action on the RLU results obtained. Even though the recommended values are guidelines, they are appropriate to interpret results before final validation is complete.
Recommended RLU threshold values depend on the surface that is being sampled:
|Control Ultrasnap||< 5||6-10||>10|
The user can start interpreting results immediately and threshold values adjust later when appropriate.
It is not the most correct way of validation and customers may have to adjust threshold values at a later stage.
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