Infection Control Today - 08/2003: NO MAGIC NUMBER FOR INSTRUMENT-SET WEIGHT


By Becki Harter, CST, RCST. CRCST, FEL

Over the years, many articles have been written on the management of instrument-set weight in the central sterile/sterile processing (CS/SP) department. Regulatory standards and professional clinical organizations have published the no magic number concept for instrument set(s) in the sterilization process for years, giving the reader the idea that the weight of the instrument set(s) did not really matter in the overall process. I have struggled to make this concept work in clinical practice and found the battle too daunting to win. Instrument sets that exceed the recommended weight by manufacturers of sterilizers have created and continue to create hardships in CS/SPD such as (but not limited to):

  • Wet packs (condensation and re-condensation issues)
  • Event-related sterility issues (i.e., compromised packaging that is designed for the standard recommended weight)
  • Compromise of sterile technique because sets are too heavy to carry or present to the sterile field in the standard manner or practice Compromise SAL because sets are too heavy and have too many layers (greater than two) to provide efficacy in the sterilization process.
  • Personnel injury related to repetitive lifting over an extended period of time of sets that are too heavy (in excess of 30 to 50 pounds) Remember, personnel in the decontamination area will lift one set on average of three times. One of those times requires that personnel lift the set(s) from inside a sonic chamber in water that must drain from the set(s) after removal. In the prep and pack area the set will be lifted on average of five times before its final destination.

Now multiply that by all the sets that are processed in a shift and your personnel have the potential to lift the equivalence of 3,500 to 4,700 pounds in one shift.

I began to research the issue of weight and found that there is, indeed, no magic number but that there is a number supported by scientific data that supports SAL requirements for our patients safety. The recommended weight of 16 to 20 pounds comes from the physical properties and requirements to achieve SAL of 10 to the minus six in a sterilization process. I found that the healthcare environment largely misunderstands or does not know the importance of weight in the sterilization process. All forms of sterilization have specific weights by which SAL is measured and achieved. Whereas weight is not the only factor, it is a very important factor in a successful sterilization process. The idea that SAL can be achieved simply by placing items in a sterilizer and pressing a button and sterilization magically occurs no matter what is placed inside the sterilizer is dangerous and cannot be supported by scientific data to be true.

To understand the importance of a 16 to 20 pound weight limit for instrument set(s) we must go back to the beginning. Before gravity displacement, dynamic air removal or steam-flush pressure pulse steam sterilizers, dry heat, convection heat and ETO/EO or plasma sterilizers there was a sterilization unit that worked much like a pressure cooker. It had a valve to release pressure and a gauge to measure that pressure. Once the pressure had reached the set-upon point for a specific amount of time, the valve was released and the cycle was completed. But there was no real data to support terminal kill, just that a terminal kill process had occurred and an assumption or hope based on basic principle was that any viable microorganisms were killed and would not cause infection or harm in patients.

This practice has been abandoned because of scientific growth and understanding of microbial growth; what kills and what does not at a specific rate (F value) at a specific time (D value). The standard healthcare sterilization cycles we have today are based upon how long it takes at a specific temperature to kill microorganisms at a 10 to the minus sixth log that has been determined not to produce disease in the compromised tissue of patients.

Moisture requirements for instruments in steam loads are minimal, and only in few cases is moisture added to an instrument to facilitate the sterilization process (i.e., it is recommended that sterile water is added to cleaned/disinfected lumen instruments in gravity displacement sterilizers prior to sterilization by instrument manufacturers). As a standard, however, moisture in or on instruments prior to a steam sterilization process is not recommended because moisture can create a growth medium for microorganism and has the potential to form a biofilm on instruments. Moisture found in sets post-sterilization for wrapped sets or sets in closed containers is not recommended because of the possibility the set can become contaminated and become the breeding ground for microorganisms.

Remember, most microorganisms need three things to grow: warmth, moisture and a dark environment. All three of these requirements are met inside of a wrapped sterilization tray or in a closed container if standards of practice are not followed. But where does the weight come in and why does it matter?

Once time and temperature parameters were determined it had to be determined what kind of set load would best support the SAL. In that process it was found that a set containing 99 instruments that did not exceed 15 to 17 pounds best supported the SAL level and minimized the risk of wet packs related to set density and configuration. Autoclave manufacturers test our sterilizers with a 16-pound test pack and have based the standard healthcare cycles on that data. Whereas a set can weigh greater than 16 to 20 pounds and can be validated in a sterilization cycle, the sterilization cycles required to achieve SAL in a set weighing over 20 pounds often requires a cycle that falls outside of what is achievable in healthcare and falls outside the published standard healthcare cycles preset by autoclave manufacturers that follow the healthcare standard based upon scientific data to support a 10 to the minus sixth log of kill.

The argument has been submitted that cycles can be changed. This is true; they can be changed. But your autoclave manufacturer will not stand behind any cycle that it has not validated and if a healthcare system chooses to go outside of what the sterilizer manufacturer has recommended and pre-set, the healthcare system must validate that process according to AAMI ST8 guidelines.

CS/SPDs do not validate but verify parameters using biologicals such as Bowie Dick or Dart test and indicators/integrators. Healthcare device manufacturers validate medical devices and healthcare verifies that according to published standards of practice. If an SPD decides to operate and accept medical devices that fall outside the standard it must accept all liability and it is worth noting that your warranty on your sterilizer (if applicable) will be void. In addition, by operating outside the standard healthcare cycles to include standard weights, you could be causing your facility to violate federal, state and local regulations and jeopardize your facilities insurance coverage.

The standard cycles that we have in healthcare today are based on the SAL of 10 to the minus six. If instrument sets are greater than 20 pounds, they must be validated for SAL, which will be difficult to do within the published standard healthcare cycles because they are based on a maximum weight of 16 to 20 pounds. There is no doubt in this consultants mind that if we as a healthcare industry do not adhere to all standards concerning sterilization to include:

  • Time
  • Temperature
  • Moisture Weight
  • Load configuration
  • Set density and configuration (no more than two levels) Our patients safety has great potential to be compromised. Secondary to that, the cost of workers compensation cases will increase as weights are allowed to exceed the recommended 16 to 20-pound weight limit.

Guidelines for sterilization to include weight requirements/ recommendations are found in:

  • AAMI ST8
  • AAMI ST12
  • AAMI TIR 33
  • AAMI ST37
  • AAMI ST46
  • Sterilizer manufacturer recommendations for use following AAMI guidelines It is prudent to point out the necessity to adhere to the standards of practice.

Out of all the reasons we should, to include:

  • Scientific data that supports SAL
  • Repeatable, achievable results in healthcare
  • Standards of universal practice

There is one reason that stands out among them all, and that is our patients safety. Every manufacturer or healthcare professional, who has ever said that there is no magic number for weight for sterilization of instrument set(s), is correct. The number is not magic at all and it never has been. However, the way weight has been dealt with in the past has treated the sterilization process like a Houdini in that the assumption was made that weight of any proportion did not bear any negative consequence in the sterilization process. It has been assumed that we can place any proportion of weight and density in a sterilization process and abracadabra it comes out sterile! Not so. It is based upon scientific data that supports SAL and more critically, positive patient care outcomes.

What is the magic number? There isnt one. But what is the actual number? Its 16 to 20 pounds. Its not magic. It is a hat of scientific principles with a wand of supporting data on a table spread with the standards of practice with an audience of patients that depend on us for their safety.

Becki Harter, CST, RCST, CRCST, FEL, is president/CEO of Indianapolisbased Sterilization by Design and is the 2003 winner of the AAMI Becton Dickinson Career Achievement Award.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.