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


It has been noted that a fairly large number of facilities use spore strips as their biological indicator for the monitoring of biowaste bags to be steam sterilized. These waste bags include regulated medical waste (RMW) treatment, biological waste and a variety of lab, clinic or media prep waste bags. The waste contents could be made up of various discarded agar slants, petri dishes, tubes of TSB or other such common lab waste that is likely to contain various strains of bacterial cells, including spore-forming gram-negative bacillus, E. coli, fungi, mold and/or yeast cultures. Isolate and contaminant cultures, body fluid wipes and bloody swabs and gauze can also be included with RMW and thrown into the biobags. Waste accumulates from the intensive care units (ICUs), pathology labs and surgical suites. The number and types of possible pathogens is very large. Included in this potpourri may also be antibiotic discard, cleaning tissues or cloths used to sanitize surface areas, or clean laminar-flow hoods that contain leftover sporicidal or antimicrobial spray, alcohols or disinfectants that may have been used in cleaning processes.

The waste bags usually contain an abundance of petri dishes, etc. that will liquefy under sterilization conditions. Agar slants, tubes of TSB or other growth media and numerous items containing collected or cultured contaminants will soon be free-flowing within the bag as temperatures reach the agar melting point and plates and tubes open up under pressure and temperature.

Other contributing medical/bio sites would include environmental/water-testing labs, tattoo facilities, dental offices, veterinary clinics and a huge number of public health labs. All of the aforementioned facilities are accumulating biowaste and it must be sterilized prior to disposal. Most of these waste bags sit in collection areas until they are sent off to be autoclaved, either on-site or off-site. Many sites use biological indicators (BIs) in the form of spore strips to measure the cycle’s effectiveness. Could just the presence of this hodgepodge of waste-bag items discarded into the bags have a localized affect upon the spore strips being used to monitor a sterilization cycle?

Prior to providing information pertaining to the question above, let’s look at why spore strips are being used as BIs for waste-bag cycles. The most common form of BI used as an indicator for monitoring sterilization cycles involving steam is the spore strip. Spore strips are fairly easy to use, they are the least expensive BIs to purchase and most individuals using BIs are familiar with the use of spore strips. Of the regulatory standards that require the use of BIs for monitoring medical or biological waste, they specifically mention and allow for the use of spore strips in autoclaves. When one is preparing for environmental laboratory accreditation, autoclaves must be monitored with spore strips or suspensions. Autoclave loads would also encompass lab/biowaste loads.1 Numerous agencies, including many state department of health agencies, follow the four proposed sterility assurance levels for microbial inactivation set by the U.S. State and Territorial Association on Alternative Treatment Technologies (STAATT) for the treatment of medical/biowaste. RMW/biowaste requires a Level 3 minimum for treatment. With Level 3, the use of a minimum challenge of log-4 population is used.

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