An analysis of Annex 1 - Section 8.61.

 

Annex 1 requires the user to perform an air removal test.  Usually this would be the daily steam penetration test, such as the disposable daily Bowie & Dick test pack.  Now for the first time the user is asked to consider, as an alternative the use of an air detecting system.  On this, I will be clear in my position that it should not be an either/or decision.  I would expect the user to perform both the steam penetration test and consider the use of an air detecting system.  The air detecting system alone may not be sufficiently sensitive to detect potentially significant levels of air or non-condensable gases if these are trapped within the load, so a more direct test of air removal from the load is therefore recommended. This is why it remains a requirement within BS EN 285:2015 to perform a steam penetration test.  Let us now look at the wording of the section: -

 

8.61 There should be adequate assurance of air removal prior to and during sterilisation when the sterilisation process includes air purging (e.g. porous autoclave loads, lyophilizer chambers). For autoclaves, this should include an air removal test cycle (normally performed on a daily basis) or the use of an air detector system. Loads to be sterilised should be designed to support effective air removal and be free draining to prevent the build-up of condensate.

 

I have found over the years inherent confusion as to how to set up an air detector.  Indeed many shy away from installing one, especially on the continent.  However, a correctly installed and validated air detector provides increased assurance in the integrity of each sterilisation cycle ran through a steriliser and is therefore a powerful tool within the steriliser control strategy and is strongly recommended by the author.  It is one of several tests available in the test persons toolkit and cannot be relied upon in isolation to provide assurance of a satisfactory cycle. 

 

In the UK it has been a requirement since 1969, as per British Standard BS 3970-1:1966 (amended 1969) to have an air detecting system [ADS] installed.  No test methodology was provided in this early standard as to how to test the ADS, other than to request guidance from the manufacturer.  However, in 1990 the standard was revised (BS 3970-3:1990) and for the first time a test methodology and acceptance criteria were provided.

 

Before the ADS could be tested certain pre-requisite tests had to be completed.  Namely, an air leakage test, to confirm the chamber integrity was <1mmHg/min[i] (equating to the current  <1.3mbar/min) and thermometric performance testing on a small load and full load, (comprising of a standard test pack, prescribed by Bowie & Dick in their recommended steam penetration test).

 

The acceptance criteria for the performance testing did not include equilibration time, (this did not come about until the publication of BS EN285:1997).  The accepted measure of steam penetration was a depression between the test pack and the active chamber discharge of not more than 2⁰C, (The original Bowie and Dick Autoclave Tape Test agreed with thermocouple readings to within 2’C [ii]).[1]

 

The test method for the air detector testing was to determine the level or air leakage (induced by the test person via a metering device into the chamber) that would correspond to the failure point of 2⁰C.  If the level of induced air leakage reached 10mbar/min and the depression between the active chamber discharge and the test pack remained <2⁰C then the corresponding air detector reading would be noted at that point and no further testing would be required.  

 

The air detector performance test criteria as we understand it today, (a depression of <2⁰C) is therefore a legacy of the testing in BS 3970-3:1990 and the test method remains largely the same, now with an end point of 9-11mbar/min, should the depression remain within <2⁰C.

 

However, some may be aware of the Health Technical Memorandum [HTM] 2010, (replacing HTM10 in 1997) which I have talked about in previous Technical Notes.   The test method instructed the test person to induce an air leak of 9.5 ±1 mbar/min and confirm the temperature depression measured between the active chamber discharge and the pack.  If it was found to be >2⁰C then the induced air leak is reduced until the pass fail threshold of <2⁰C is seen.  If the induced air leak was found to fall to the region of 3-4mbar/min and depression between the active chamber discharge and the test pack was found to remain >2⁰C then the user was instructed to take advice from the manufacturer.  So, in other words the reverse of what BS 3970 said and BS EN285:2015 currently states.  The reason I feel is obvious, firstly - brevity of testing and secondly - robustness of the air removal cycle design and steam penetration. 

 

In the first matter, the test person does not need to run multiple cycles to determine the fail point.  If the test can pass with a 9-10mbar/min air leak then the requirements of the air detector performance test are immediately met.  In the second matter, knowledge that the cycle can tolerate such a high air leakage gives confidence in the robustness of the cycle design.  Especially when performed on the recognised standard towel pack, which is known to present a significant challenge to any steriliser above and beyond most production loading patterns[iii].

 

The air detector function test can then be completed which verifies that the required setting of the air detector remains valid and a fault is indicated as expected.

 

In Summary

The contemporary requirement of the air detector performance test to achieve a ∆T of <2⁰C has its origins in the grandfather standard BS 3970-3:1990, where the measurement of steam penetration, not as equilibration time of 15/30seconds, but as a temperature depression between the active chamber discharge (Drain) and the load on <2⁰C.

 

Compliance with the small-load, full-load and air detector tests ensures that the penetration of steam is effectively instantaneous and when accompanied with the air leakage and automatic control test/steam penetration test provide a powerful toolkit of tests that demonstrate the robustness and reliability of the steriliser. 

 

Whilst the existing standard maintains the instruction to control the rate of air leakage so that, at the start of the plateau period, the lowest temperature measured within the test load is not more than 2⁰C, (BS EN 285:2015 clause 19.2.2.9) and that it may be necessary to conduct a number of tests in order to establish the air leakage required, I would strongly recommend the test person to proceed firstly with an air leakage 9-11mbar/min as per clause 19.2.2.11 of the standard, for obvious reasons.

 

The thermometric and air detector tests for the small and full loads prescribed in BS EN 285:2015 are performed on standard towel packs, (a recognised industry standard challenge to a sterilizer and its performance).  It should be noted however that it is common to repeat these tests on actual production load patterns, once known, in order to ensure the sensitivity of the air detector system is valid.  Indeed, some regulatory bodies expect to see evidence of this testing.

 

[1] Interestingly, as a side note, it was also a requirement for the heating phase to be not more than 3 minutes and for the overall cycle time to be not more than 47 minutes for a 121⁰C cycle and 35 minutes for a 134⁰C cycle, (an extra 20 minutes was allowed for extended drying). 

 

References

[i] Darmady, Drewett & Hughes   

Survey on pre-vacuum high-pressure steam sterilizers J. clin. Path (1964), 17, 126

[ii] Bowie, Kelsey & Thompson     

The Bowie & Dick Autoclave Tape Test  The Lancet (1963), 586, March 16

[iii] Newson                                   

Relevance of the Bowie and Dick Test Today – Presentation by Brownes (Steris) 2006