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Monitoring Medical/Biowaste Bags With Spore Strips as Biological Indicators and ‘False’ Negatives


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With an exposure time of 4 minutes, the spore strips should have survived the cycle in all three waste bags. Even though all three waste bags were exposed to the same sub-lethal exposure time of 4 minutes at 121 degrees C, the spore strips in waste bags #1 and #2 did not show signs of growth while waste bag #3 without the tissues did grow and was not killed by the short exposure cycle. It was not the exposure cycle that killed the spore strips in bags #1 and #2 but the waste bag contents contributed to lethality.

Admittedly, additional tests should be run with various exposure times used and strip placement variation within the bags. However, I feel that this brief investigation allows for the ‘possibility’ that if one were using spore strips for monitoring waste bag sterilization, a false negative is within reason as a possible risk. A possible risk that even if ever so slight, simply should not be allowed to exist.

With typical waste sterilization cycles dealing with PreVac, exposure time and temperatures, most likely even a waste bag bioburden of 1012 or 1014 pathogens would likely be killed due to low moist heat resistance. However, if we are monitoring a cycle’s effectiveness with spore strips and a possibility exists where test results could be false, our confidence level for sterility assurance has been greatly lowered.

Over the past 15 years I have come into contact with numerous waste bag steam cycles where spore lethality just did not occur. Some were due to inadequate exposure time but by far a majority was due to inadequate air removal and thus poor steam penetration. Some cycles just did not include a PreVac phase and thus steam penetration was extremely difficult or impossible to obtain and some were mechanical vacuum failures. Time and temperature recordings were fine but the prevac was missing and TCs were measuring dry heat rather than moist heat within a bags content. ‘IF’ spore strips were used and ‘IF’ they were in contact with bactericidal or disinfectants in a very localized area in such a situation where adequate steam penetration did not occur and the strip should have survived, we could possibly have a failed cycle yet the spore strips would indicate adequate cycle parameters were met and a failed cycle not detected. The accompanying photo would show what an extreme biological indicator compromise situation could look like if the strips were allowed to come in contact with waste bag contents.

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