How to validated a variable load for steam sterilizers

Dear all,

Let’s say I want to validate a load of 9 different items (item 1, 2, 3, 4, 5, 6, 7, 8 and 9). It is desired to have flexibility during routine sterilization (i.e., we would like to have the possibility to only sterilize a partial load (1-8 items at the same time)).

To cover this request:

  • I will validate the maximal load as a whole.
  • Which other load configurations/items must I validate?

I was reasoning as follows, but would like to have your feedback:

The difference between maximum and minimum load is:

  • low residual air: the chamber is stacked up with a lot of items
  • total load has high heat capacity: the heating up phase will take a bit longer as the items take up quite a bit of heat energy from the environment.


  • high residual air: the chamber is stacked up with only one item
  • total load has low heat capacity: the heating up phase will go fast as one items takes up only a little of heat energy from the environment.

Thus, I would say: if I validate also the smallest item with the lowest heat capacity, and for which the air removal is the highest (let’s say Item 3), I have covered the whole load, and every item 1, 2, 3, 4, 5, 6, 7, 8 or 9 can be sterilized individually or any combination of these together.

Is this correct?

Thanks for your feedback!

The approach chosen applied the item bracketing and load bracketing concepts presented by PDA TR1, 2007 (1). The item bracketing approach allows for the use of a worst case item(s) (i.e. greatest sterilization challenge for heat penetration, mass, air removal, and/or condensate removal) to qualify items that are a lesser sterilization challenge. The load bracketing is established by using predefined minimum and maximum loads. This approach defined the minimum load as comprising the single item presenting the greatest sterilization challenge, irrespective of mass, while the maximum load comprised an assembly of the ten items identified as worst case (relative to sterilization difficulty), with additional items used create a maximum load mass (i.e. a load mass greater than any anticipated for routine operations).

The methods used for this qualification comprised the following primary activities:

Listing of all equipment / commodities to be steam sterilized.
Evaluation of equipment and commodities to determine the most efficient and effective means (relative to air removal, steam penetration, and maintaining a sterile barrier) of pre-sterilization preparation.
Heat penetration evaluation tests to identify the worst case (i.e. most difficult to heat) load items.
Maximum Load (i.e. greatest mass) heat penetration / biological challenge studies using the worst case load items.
Minimum Load (irrespective of mass) heat penetration / biological challenge studies using the single worst case load item.

We call this approach described here as Hardest To Heat (HTH) studies.
I fully agree with what joksavs stated.

The maximum/minimum approach is acceptable and optimal (saves time and money).

The maximum load should be based on a combination of a few factors (for example):

1 - Mass (heaviest load is worst case) Materials of construction (metal heats up faster, and plastics slower - so plastics will a lower Fo)
2 - Geometry - items which could trap/occlude air could be worst case
3 - Geometry - items which might not let steam get there (filters, check valves, small tubing) might be hard to heat

Overall you maximum load should contain a combination of things - largest/heaviest, plastics, occluded items. This can be called a mock load, or a maximum combination load - it often isn’t an actual commercial load, just a combination of the worst parts of the various load (in your case 9 loads)

Now regarding minimum load - this really shouldn’t be hard to steam sterilize and get appropriate Fo’s (this is done for maximum loads). What you really need to be careful here is to not overprocess things (and damage them).

Items which might be included in a minimum load are (for example):
1 - Filters/housings
2 - Plastics which could warp/damage
3 - Media/broth/chemicals/liquids/finished product (for terminal sterilization).

After a minimum load, you should check that the stuff isn’t visually damaged, or do filter integrity testing, or do a chemical analysis of the media/broth/product, etc.