Steam sterilization is a critical process in the manufacture of many pharmaceutical and medical device products. Because of its importance and wide usage it receives a great deal of attention from both practitioners and regulators. Despite this focus, there are aspects relating to load size that prove troublesome, and can cause difficulty during sterilization cycle development, and validation. Regulators will routinely query users on their means for validation of varying load sizes in both pre-approval and routing inspection. The regulatory expectation is that the user has validated fixed and invariable load patterns, and thus load sizes for all sterilization processes.¹,² This is expected for both parts (porous) and terminal (non-porous) loads. While defined loads has been used by some firms, their use constrains operational flexibility and many industrial users have sought more flexible, and efficient means. Depending upon the details of the individual sterilization processes, their ability to defend their practices relating to load size variation may not be adequate.
It has been common practice to utilize maximum loads as "worst case" demonstrations of sterilization lethality and assume that smaller size loads will be appropriately sterilized. The assumption is that the larger size, number, and mass of the maximum load represents a worst case and challenges the sterilization process sufficiently during validation that the minimum load (and all intermediate size loads) can be considered acceptable by default. Depending upon the manner in which the sterilizer is controlled during the process, this may or may not be true. Varying air removal, and/or differences in heating /cooling rate as is possible with varying load size, can result in process differences that can result in reduced cycle efficacy.
This article will address the effect of load size on both parts (porous load) and terminal sterilization processes. The specific means to address load size differ between the sterilization processes; however the general approach to follow is similar. Using the content provided herein, the practitioner can establish their sterilization processes for either porous or non-porous loads with confidence that they can meet regulatory expectations, while maximizing operational flexibility.
PARTS (POROUS) CYCLES
The maximum load should be studied initially in the validation effort. The maximum load can be an actual load, or one that is artificially created for the validation exercise. Depending upon the sterilizer usage there may be several different maximum loads. The maximum load(s) should be well documented as to component load item size, number and mass. The load items should have been previously evaluated to determine the ‘slowest to heat’ location when properly wrapped and oriented. This should be performed with temperature sensors introduced in a way that does not compromise the integrity of the wrapping materials. The maximum load(s) are then subjected to validation in replicate studies with thermocouples and biological indicators demonstrating that sufficient lethality has been delivered throughout the load.
A major consideration with porous loads is that air must be efficiently removed from the load. As the maximum load typically has many wrapped items, the rate at which air can be removed can be influenced by the load size. Air present within wrapped items must diffuse through the wrapping, whereas air removal from outside the load has no restrictions and is rapidly removed (see Figure 1A). When a minimum load is processed, the amount of internal air is reduced and it is possible that air within the load items may not be adequately removed when the targeted vacuum level is attained (see Figure 1B). The presence of air retained within the load item can result in the minimum load receiving less lethality than necessary for its reliable sterilization. The means to address this potential problem vary with the sterilizer control system and its sophistication. Note that in all of the scenarios the number of vacuum pulses would be the same and defined by what is necessary to meet any defined equilibration time requirements.
Steam sterilizer with pressure set points, vacuum and steam injection rate control
A sterilizer with this capability allows the rate of chamber pressure drawdown and steam injection rate during the vacuum pulses to be controlled. This is the optimal design, as controlled rates can be established such that the impact of load size is eliminated. Once confirmation of adequate air removal is established with the maximum load and verified with a minimum load the same vacuum draw down rate and steam injection rate are utilized used for all loads in the sterilizer regardless of size. The ramp rates should be set at no greater than 90% of the maximum possible for more reliable control. The pre-vacuum and steam pulse duration for the sterilizer will thus be reproducible allowing for complete air removal regardless of load size. The overall sterilization cycle times for all load sizes should be nearly identical (see Figures 2A and 2B). With this control approach, validation of the maximum load(s) would support smaller loads comprised of similar items. To ease regulatory questions, a single challenge run for the minimum with thermocouples and biological indicators should be performed.
Sterilizer with pressure set points and vacuum/pressure hold time control
With this type of control, the vacuum drawdown time can vary with load size, with maximum loads typically requiring the longest time to attain the vacuum setpoint. The maximum load validation would be performed with no added hold period in each vacuum pulse and proven acceptable with physical and biological data (see Figures 3A and 3B). Next, the time required to draw the vacuum to setpoint of the maximum load would be established as the hold time at full vacuum for each vacuum pulse (see Figure 3C). Even though the steam pressurization rate may be less important, a similar increment of hold time at the end of each steam injection is also used. This approach increases the full duration of the vacuum drawdown and steam pulses to that required for the maximum load, and assure that the minimum load has adequate time for air removal/steam penetration. The overall cycle time is extended only slightly because of the added hold times after each vacuum/steam pulse. The overall cycle times with this control scheme would be slightly longer than what is possible with vacuum/steam injection rate control. Nevertheless, even this less refined sterilizer control, validation of the maximum load(s) supports the sterilization of smaller loads comprised of similar items. A single run minimum load with thermocouples and biological indicators should be performed to confirm the process.
Sterilizer with vacuum/pressure set points only
Because neither the vacuum drawdown rate nor the vacuum hold time is controlled, this type of sterilizer control system is not amenable to the methods described above to assure adequate sterilization of minimum loads (see Figures 1A and 1B). Validation of this sterilizer design requires consideration of minimum and maximum loads in triplicate studies using thermocouples and biological indicators.
TERMINAL STERILIZATION CYCLES
The sterilization of liquid filled aqueous containers represents a more challenging situation in that not only is there a minimum lethality expectation; the materials in the load must also not be over-processed. The means to assure consistency across load size is often facilitated because terminal sterilizers are ordinarily equipped with more sophisticated control systems allowing for the regulation of heating and cooling rates. Before commencing the effort, the lethality required to sterilize the filled containers must be chosen from container mapping studies and information on the expected bioburden resistance and population. This calculation will typically include some added margin of safety to ease routine operation.
Replicate mapping studies with the maximum load are performed next to identify the minimum and maximum lethality locations (these are more commonly a region or zone than an individual container). During these studies the heating and cooling rates should be set at approximately 90% of the equipment’s full capability (see Figure 4A). This value is selected to ensure controllability while not extending the overall cycle appreciably.
The critical concern in terminal sterilization is that the amount of heat to sterilize the filled containers varies with load size. The appropriate process dwell for the cycle must be established by adjusting for any lag time between the sterilization dwell control location (assumed to be somewhere in the chamber) and the ‘slowest to heat’ zone within the load. The lag time is greatest for the maximum load due to its greater mass but will vary based on load size (and to some extent upon the container size/product heat capacity as well). Were the chamber heating / cooling rates allowed to vary freely for different load sizes, the dwell period, which is usually established from a probe outside the load, may start prematurely and sufficient lethality might not be delivered to smaller loads (see Figure 4B). A similar situation occurs during cooling of the load and has a similar effect.The smaller the load, the less affect it has on the overall heating of the chamber and, thus, on the initiation of the dwell period. However, it should not be assumed that the parameters used for the maximum load are directly applicable to the minimum load. Both extremes should be confirmed, with the lag between chamber and load temperatures being determined experimentally. Because terminal sterilization processes must also consider the maximum conditions for their impact of product quality attributes, the comparative evaluation must consider high F0 locations as well. If the maximum and minimum lethality ranges are equivalent the control strategy is acceptable. Validation studies would then be performed for both maximum and minimum loads, in triplicate, using temperature probes and bioindicators. Confirmation of sterilization performance for intermediate load sizes would not be required as the extreme load sizes have been proven successful using an identical sterilization process.
The methods outlined should enable the practitioner to efficiently and confidently validate maximum and minimum loads undergoing steam sterilization with a fuller understanding of the control elements that will assure consistency of efficacy. The control suggestions indicated allow for greater flexibility in operation while maintaining the required lethality and product stability which are essential to regulatory compliance
 FDA, Guideline - Guidance for Industry for the Submission of Documentation for Sterilization Process Validation in Applications for Human & Veterinary Drug Products, 1994.
 EMA, Annex 1, Sterile Medicinal Products, 2008.
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