Disposable sterilization test pack for evaluating steam and ethylene oxide sterilization

Structure for evaluating the efficacy of sterilization apparatus using either steam or ethylene oxide as sterilant is disclosed. The structure includes an insert of porous material having a cavity formed therethrough for retaining a biological or chemical indicator. The porous material has homogeneous porosity throughout. The insert of porous material and indicator are enclosed within an outer covering which is relatively impervious to penetration of gases and which provides restricted pathways to air and sterilant. The outer covering is provided with removable portions which are removed when the structure is used to evaluate ethylene oxide sterilization. Removal of the portions enlarges the existing pathways for ingress and egress of gases which may be appropriate in certain efficacy testing.

TECHNICAL FIELD 
This invention relates generally to evaluating the efficacy of sterilizer 
apparatus. Specifically, this invention is directed to an improved 
sterilization test pack. 
BACKGROUND 
Sterilizer units are used in many industries to treat items with a 
sterilant, such as heat or chemicals, to sterilize and disinfect those 
items. To assure that a sterilizer apparatus is operating properly and 
that items placed within the sterilizer are being completely sterilized, 
it is necessary to evaluate the efficacy of the sterilizer apparatus on a 
periodic basis. 
Protocols for evaluating steam sterilization and ethylene oxide (EO) 
sterilization were developed several years ago by the Association for the 
Advancement of Medical Instrumentation (AAMI), and are well-known. For the 
routine monitoring of steam sterilization processes, the AAMI steam 
sterilization test protocol requires precise folding and stacking of a 
selected number of freshly laundered, reusable huck or absorbent surgical 
towels. A biological indicator is placed in the approximate geometric 
center of a packaging structure and a chemical indicator is placed 
adjacent to the biological indicator. The biological and chemical 
indicators are then placed in the center of the configuration of folded 
towels. The pack is completed by taping the folded towels in a manner to 
yield dimensions of approximately 6".times.6".times.9" and a density of 
approximately 11.3 pounds per cubic foot. 
For the routine monitoring of ethylene oxide sterilization processes, the 
AAMI EO sterilization test protocol specifies placement of a biological 
indicator within the barrel of a plastic syringe (tip guard removed) which 
is then placed within the folds of a freshly laundered, reusable huck or 
absorbent surgical towel which is in turn placed within a peel pouch or 
outer wrapper. 
In view of the exacting preparation necessary to construct a testing device 
under the AAMI protocols, and further in view of the variance in testing 
results which occur from inconsistent construction of testing devices 
using the AAMI protocol, efforts have been made to develop pre-formed, and 
preferably disposable, testing devices. Such testing devices are 
well-known in the field as "sterilization test packs." Studies have 
demonstrated that, in addition to the benefit of greater consistency in 
construction, these testing devices offer significant savings to the user 
compared with the labor costs involved with assembling the AAMI devices 
(Caporino, P., 1988 Journal of Healthcare Material Management, May/June, 
pp. 38-42). Test packs have been developed which use chemical indicators 
or biological indicators, or both, centrally positioned within the test 
pack materials. 
Test packs which use chemical indicators are exemplified by the devices 
disclosed in U.S. Pat. No. 4,486,387 to Augurt, issued Dec. 4, 1984; U.S. 
Pat. No. 4,596,696 to Scoville, Jr., issued Jun. 24, 1986; U.S. Pat. No. 
4,576,795 to Bruso, issued Mar. 18, 1986; U.S. Pat. No. 4,579,715 to 
Bruso, issued Apr. 1, 1986; and U.S. Pat. No. 4,692,307 to Bruso, issued 
Sep. 8, 1987. Test packs which use biological indicators are exemplified 
by the devices disclosed in U.S. Pat. No. 4,528,268 to Andersen, et al., 
issued Jul. 9, 1985; U.S. Pat. No. 4,591,566 to Smith, issued May 27, 
1986; U.S. Pat. No. 4,828,797 to Zwarun, et al., issued May 9, 1989; U.S. 
Pat. No. 4,839,291 to Welsh, et al., issued Jun. 13, 1989; and U.S. Pat. 
No. 4,914,034 to Welsh, et al., issued Apr. 3, 1990. 
Test packs have been developed which consist of separate layers of 
disparate materials held together by a porous overwrap or a box. 
Intermediate layers of material are formed with a cavity to house a 
biological indicator within. Examples of such test packs are disclosed in 
U.S. Pat. No. 4,636,472 to Bruso, issued Jan. 13, 1987; U.S. Pat. No. 
4,863,867 to Joyce, et al., issued Sep. 5, 1989; and U.S. Pat. No. 
4,918,003 to Macaro, et al., issued Apr. 17, 1990. Because these test pack 
designs necessitate the use of papers or other materials having various 
porosities (e.g., non-porous, semi-porous or highly porous, and 
combinations thereof), and require a particular stacking arrangement, they 
are difficult and costly to manufacture and assemble. Additionally, access 
to the biological indicator housed within the materials is more difficult 
with these designs. 
Thus, there remains a need for a test pack useful for evaluating both steam 
and EO sterilization efficacy which is inexpensively manufactured, easily 
assembled, and more conveniently used by those in the field. 
DISCLOSURE OF THE INVENTION 
According to the present invention, disposable structure for evaluating the 
efficacy of sterilization apparatus is provided which presents an 
effective challenge to entry of sterilant into the interior of the 
structure and provides ease of construction and use. The structure 
provides accurate evaluation of sterilizer efficacy with both steam and 
ethylene oxide sterilant use. The structure provides sterilizer efficacy 
evaluation equivalent to the protocols set by the Association for the 
Advancement of Medical Instrumentation for the routine monitoring of steam 
and ethylene oxide sterilization processes. 
The structure disclosed herein generally comprises an insert of porous 
material having a cavity formed therein which is sized to receive an 
indicator for determining sterilant penetration into the structure. The 
insert is positioned within an outer covering. The insert of porous 
material has homogeneous porosity and restricts the entry of sterilant 
into the structure. The porous material may be any material of suitable 
porosity which provides adequate restriction of sterilant. Such materials 
may include finer paper, sponge, sintered polymers and the like. The 
insert may be of one piece construction, such as a thickness of 
sponge-like material. More typically, the insert may comprise a stack of 
individual sheets of porous material, such as paper, each one having 
comparable porosity. Paper having a Gurley porosity ranging from about 25 
to about 45 seconds may be particularly suitable. 
The porous material is formed with a cavity positioned in the center for 
retaining an indicator capable of detecting sterilant penetration. The 
indicator retained in the cavity preferably may be one which is proficient 
at evaluating both steam sterilization and ethylene oxide sterilization, 
but may be of a type which is suitable for the evaluation of a single type 
of sterilant. The indicator may be either a chemical indicator or a 
biological indicator, or both. 
The porous material and indicator are retained within an outer covering 
which is relatively impermeable to air and sterilants. The outer covering 
encloses the porous material and indicator in a manner which provides 
restricted pathways for egress of air and ingress of sterilant. The outer 
covering may be made of any innately impervious material, such as plastic, 
or the outer covering may be made of a material which may be contacted 
with a substance which renders it relatively impervious. 
The outer covering is further provided with removable portions which 
enlarge the pre-existing pathways for sterilant entry when the structure 
is being used to evaluate sterilization using ethylene oxide. Such 
removable portions may be provided in the form of at least one removable 
end flap, or in the form of removable tabs. The outer covering is 
constructed so that various numbers and combinations of removable portions 
may be removed to provide varying degrees of permeability to the outer 
covering. The porous material insert and the outer covering having 
variable removable portions provides a tortuous pathway through which 
sterilant must pass in order to contact the indicator within the cavity. 
The use of a single portion of homogeneously porous material provides 
economical manufacture and ease of construction and assembly of the 
structure thereby presenting a significant improvement over similar 
structures in the field. The simplified structure of the porous material 
is made possible by the configuration of the impervious outer covering 
which provides limited access of sterilant to the interior of the 
structure.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
Structure of the Illustrated Embodiment 
As shown by FIG. 1, the disposable test pack structure 10 of the present 
invention generally includes an insert of porous material 12, an indicator 
14 for determining sterilant penetration, and an outer covering 16 of 
relatively impermeable material which encloses the porous material 12 and 
indicator 14. 
The porous material insert 12, as shown by FIG. 1, is a stack 18 of 
individual sheets of paper, each sheet 19 having a porosity comparable to 
each other sheet 19 such that the insert 12 as a whole has a homogeneous 
porosity. Each sheet 19 of the insert 12 is cut with a hole in the center. 
When the sheets 19 are stacked together, a cavity 20 is formed in the 
center of the porous material insert 12 by alignment of the holes, as 
shown by FIG. 2. 
An indicator 14 is positioned within the cavity 20 where access is 
restricted by the porous material insert 12. The indicator 14 may 
preferably be one which is proficient at evaluating both steam 
sterilization and ethylene oxide sterilization, but may be an indicator 14 
of the type which can evaluate only a single type of sterilant. Both a 
chemical indicator 22 and a biological indicator 24 are shown by FIG. 1; 
however, a single indicator may be used instead. 
The porous material insert 12 and indicator 14 are enclosed within an outer 
covering 16 which is relatively impermeable to penetration of gases. That 
is, the outer covering may be constructed of an innately impervious 
material such as plastic; or, alternatively, the material may be of a type 
which retains the quality of being impermeable during normal use in a 
sterilizer, but which may become less impermeable following extended 
exposure to heat or moisture. The outer covering 16, shown by FIG. 1, is a 
box 26 constructed of sturdy material, such as paperboard stock, coated 
with a material which renders the paperboard relatively impermeable to air 
and sterilant. Such coatings include plastic sheeting, resinous or waxy 
substances, or the like. 
The particular dimensions of the outer covering 16 may vary. Dimensions of 
one-half inch high by four and one-half inches long by two and one-half 
inches wide (0.5"h.times.4.5"1.times.2.5" w) are very suitable for the 
outer covering 16 when used in most testing applications and most 
sterilizers. Variation in the dimensions of the outer covering 16, and 
thus the amount of porous material 12 therewithin, will modify the ability 
of sterilant to enter into the test pack. 
The outer covering 16 is constructed to provide openings through which air 
may exit the interior of the test pack 10 and through which sterilant may 
enter to reach the inner cavity 18. In the embodiment shown by FIG. 1, the 
box 26 includes end flaps 28 and 30 which fold and insert into the box 26 
to obstruct the open ends 32 and 34 thereof. Thus, small gaps 36 formed at 
the insertion point of the end flaps 28 and 30 create restricted pathways 
for air and sterilant travel, as illustrated by FIG. 3. 
When the test pack 10 is used to evaluate steam sterilization, the end 
flaps 28 and 30 are folded and inserted into the structure as illustrated 
by FIG. 3. Sterilant enters into the outer covering 16 as indicated by the 
enlarged arrows. When the test pack 10 is used to evaluate ethylene oxide 
sterilization, the perforated tabs 40, shown by FIGS. 1 and 3, are removed 
to enlarge the existing entryway for passage of sterilant into the 
interior of the test pack 10. As illustrated by FIG. 4, removal of the 
tabs 40 exposes the edge 42 of the porous material insert 12 and provides 
increased entry of sterilant. 
The box 26 may be configured with foldable end flaps 28 and 30 at opposing 
ends of the box 26, as illustrated by FIG. 1. Alternatively, the box 26 
may be configured with a single foldable end flap. The box 26 is 
configured with a plurality of tabs 40, typically four, any or all of 
which may be removed to enlarge the existing entryway for passage of 
sterilant in ethylene oxide sterilization evaluation. 
In an alternative embodiment illustrated by FIGS. 5 and 6, the box 26 is 
constructed with tabs 44 which are connected to the outer edges 46 and 48 
of the box 26 and are foldable inwardly. When the test pack 10 is used in 
evaluating steam sterilization, the tabs 42 are folded in, and the end 
flaps 28 and 30 are folded and inserted as described previously. Air and 
steam must then travel a more tortuous route around the tabs 44 to exit or 
enter between the end flaps 28 and 30 and the tabs 44. When the test pack 
10 is used to evaluate ethylene oxide sterilization, the tabs 44 are 
removed prior to folding and inserting the end flaps 28 and 30 into the 
box 26. Thus, an enlarged opening 36, creating a less tortuous pathway, is 
provided for entry of sterilant into the test pack 10, as illustrated by 
FIG. 6. 
Test Pack Standards Established by AAMI 
The porosity of the filer paper or other porous material used in the 
invention should be such that the performance of the test pack is 
comparable to that of the corresponding AAMI test pack. Results of a study 
published by the AAMI regarding the development and qualification of the 
16-Towel Biological-Indicator Challenge Test Pack (herein referred to as 
the 16-Towel Test Pack) for routine monitoring of steam sterilization 
suggest that when processed under gravity steam sterilization conditions, 
biological indicators placed within the 16-Towel Test Pack should survive 
15-16 minutes exposure at 250.degree. F., and be completely inactivated 
after 25 minutes exposure to the same conditions (Table E2. Appendix E. 
AAMI SSSA-1988. "Good Hospital Practice: Steam Sterilization and Sterility 
Assurance. AAMI Standards and Recommended Practices." Volume 1: 
Sterilization, Association for the Advancement of Medical Instrumentation. 
3330 Washington Blvd. Suite 400. Arlington, Va. 22201). When processed 
under deep vacuum conditions at 270.degree. F., biological indicators 
placed within the 16-Towel pack should survive processing of 0 minutes 
exposure, the time required for the sterilizer to reach its temperature 
setpoint, and be completely inactivated after 4 minutes exposure to the 
same conditions (Table E3,- ibid). 
Similar data has been published recently by the AAMI regarding the 
resistance performance characteristics of the AAMI Routine Test Pack for 
ethylene oxide sterilization. The test pack used by the AAMI for EO 
sterilization comprised a biological indicator placed within a 20 
milliliter plastic syringe (with the plunger inserted and the needle and 
needle guard removed) which in turn was placed in a single 100-percent 
cotton surgical towel, folded in thirds lengthwise and then in thirds 
widthwise to produce nine layers of folds. The folded towel was then 
placed in a 7 inch by 13 inch paper/film pouch and was sealed. The AAMI 
test packs were tested in BIER (biological indicator-evaluator 
resistometer) ethylene oxide exposure vessels. The AAMI tests suggest that 
the mean kill time for biological indicators placed in EO test packs 
averages 41.6.+-.11.9 minutes when processed in an EO BIER vessel under 
conditions of 600 mg/L EO, 60.+-.10% relative humidity and 
54.degree..+-.1.degree. C. (Table A.1, Annex A. Biological Indicator Test 
Packs. ANSI/AAMI ST41-1992 Good Hospital Practice: Ethylene Oxide 
Sterilization and Sterility Assurance.). 
Test Data of a Typical Embodiment 
To determine the effect of the porosity of the filler paper on the 
performance of the present test pack invention under steam sterilization 
conditions, testing was conducted with filler paper of various porosities 
as described in Table I. Each test pack tested was assembled as 
illustrated in FIG. 1 with inserts comprised of filler paper sheets each 
having the same porosity, those porosities being noted in Table I. For 
each test pack, a number of sheets were placed in the stack to produce the 
dimensions of 0.5 inches high by 4.5 inches long by 2.5 inches wide. Thus, 
for sheets with a Gurley porosity of 25 to 45 seconds, approximately 65 
sheets were used; for sheets having a Gurley porosity of 10-20 seconds, 
approximately 110 sheets were used; and for sheets having a Gurley 
porosity of 60-70, approximately 127 sheets were used. A commercially 
available self-contained biological indicator was employed to determine 
the effect of the filler paper of different porosities on the resistance 
performance of the test pack of the present invention. The data in Table I 
is presented as the number of biological indicators which were positive 
for growth per number of test packs of the present invention tested. 
TABLE I 
______________________________________ 
Effect of Filler Paper of Varying Porosity 
On the Resistance Performance of Test Packs 
Processed by Steam Sterilization 
cycle Porosity* 
Temp type** exposure 10-20 sec 
25-45 sec 
60-70 sec 
______________________________________ 
121 C. gravity 12 min 10/10 10/10 10/10 
121 C. " 15 min 10/10 10/10 10/10 
121 C. " 17 min 10/10 10/10 10/10 
121 C. " 20 min 8/10 2/10 10/10 
121 C. " 22 min 4/10 0/10 10/10 
121 C. " 30 min 0/10 0/10 9/10 
121 C. " 35 min 0/10 0/10 0/10 
121 C. " 40 min 0/10 0/10 0/10 
132 C. deep 0 min 10/10 10/10 10/10 
132 C. vacuum 0.5 min 4/10 8/10 10/10 
132 C. 1 min 0/10 1/10 8/10 
132 C. 4 min 0/10 0/10 0/10 
______________________________________ 
*Porosity in Gurley seconds according to TAPPI method T460 os68 Air 
Resistance of Paper. 
**Cycle type: gravity = no prevacuum deep vacuum = single prevacuum to 28 
Hg. 
Testing conducted in a 26" .times. 62" .times. 72" steam sterilizer. 
To evaluate the effectiveness of filler papers of varying porosities on the 
resistance performance of the present test pack invention under ethylene 
oxide sterilization conditions, testing was conducted as indicated in 
Table II. The evaluated test packs were assembled as illustrated in FIG. 1 
with inserts having homogeneously porous filler papers of the porosities 
noted in Table II. Test packs constructed of filler paper having a Gurley 
porosity of 25-45 seconds contained approximately 65 sheets per stack; 
test packs constructed of filer paper having a Gurley porosity of 10-20 
seconds contained approximately 110 sheets per stack; and test packs 
constructed of filler paper having a Gurley porosity of 60-70 seconds 
contained approximately 127 sheets. Each test pack had the dimensions of 
0.5 inches high by 4.5 inches long by 2.5 inches wide. A commercially 
available self-contained biological indicator was employed to determine 
the effect of the filler paper of different porosities on the resistance 
performance of the test pack. The data in Table II is presented as the 
number of biological indicators positive for growth per number of test 
packs of the present invention tested. 
TABLE II 
______________________________________ 
Effect of Filler Paper of Varying Porosity 
On the Resistance Performance of Test Packs 
Processed by Ethylene oxide Sterilization 
cycle Porosity* 
temp type** exposure 10-20 sec 
25-45 sec 
60-70 sec 
______________________________________ 
55 C. 600 mg/L EO 
20 min 10/10 10/10 10/10 
" " 25 min 4/10 10/10 10/10 
" " 30 min 1/10 7/10 7/10 
" " 35 min 0/10 0/10 2/10 
" " 40 min 0/10 1/10 0/10 
" " 60 min 0/10 0/10 0/10 
______________________________________ 
*Porosity in Gurley seconds according to TAPPI method T460 os68 Air 
Resistance of Paper. 
**Testing conducted in a Joslyn EO BIER vessel with 600 mg/L EO and 50 
.+-. 10% relative humidity. 
Based on the data shown in Table I and Table II, filler papers with a 
Gurley porosity range of 25-45 seconds are preferred to allow acceptable 
resistance performance with both steam and ethylene oxide sterilization. 
Filler papers with Gurley porosity values exceeding 45 seconds may lead to 
survival of the biological indicator within the test pack in excess of 25 
minutes exposure under steam 121.degree. C. gravity sterilization 
conditions (Table I). Filler papers with Gurley porosity values less than 
25 seconds may lead to premature inactivation of the biological indicator 
under ethylene oxide sterilization processing conditions. 
The number of sheets in the test pack of the invention will affect the 
performance of the test pack. That is, too few sheets allow a greater 
amount of sterilant to enter thereby leading to early inactivation of the 
biological indicator, while too many sheets prevent entry of a sufficient 
amount of sterilant to inactivate the biological indicator. The effect of 
varying numbers of sheets in the test pack of the invention is 
demonstrated in Table III. 
The test packs used in developing the data shown in Table III were 
constructed as shown by FIG. 1 and had the approximate dimensions of 0.5 
inches high by 4.5 inches long by 2.5 inches wide. Each test pack was 
constructed using sheets having a Gurley porosity of 25-45 seconds, and 
the test packs contained either 45, 55, 65 or 70 sheets. Testing was done 
in a steam BIER sterilizer or EO BIER sterilizer as indicated. From the 
data in Table m, it has been determined that approximately 65 sheets of 
Gurley porosity 25-45 seconds provides an appropriate inactivation of the 
biological indicator. 
TABLE III 
______________________________________ 
Number of Filler Sheets Per Pack 
Condition Exposure 45 55 65 70 
______________________________________ 
121.1 C. 12 min. 10/10 10/10 10/10 10/10 
gravity 
(steam) 
121.1 C. 15 min. 7/10 9/10 10/10 7/10 
gravity 
(steam) 
121.1 C. 17 min. 0/10 0/10 5/10 0/10 
gravity 
(steam) 
121.1 C. 20 min. 0/10 0/10 1/10 0/10 
gravity 
(steam) 
121.1 C. 30 min. 0/10 0/10 0/10 0/10 
gravity 
(steam) 
132.2 C. 2 min. 8/10 6/10 10/10 9/10 
prevacuum 
(steam) 
132.2 C. 4 min. 3/10 0/10 7/10 2/10 
prevacuum 
(steam) 
132.2 C. 6 min. 0/10 0/10 0/10 0/10 
prevacuum 
(steam) 
132.2 C. 8 min. 0/10 0/10 0/10 0/10 
prevacuum 
(steam) 
600 mg/L EO 20 min. 5/10 10/10 10/10 10/10 
" 25 min. 3/10 4/10 10/10 10/10 
" 30 min. 0/10 0/10 10/10 10/10 
" 60 min. 0/10 0/10 0/10 0/10 
______________________________________ 
Removal of tabs 40 from box 26 allows more gas or sterilant into the test 
pack invention. Removal of one or more tabs may be appropriate in certain 
types of sterilization, such as EO sterilization. The relationship between 
sterilization efficacy and the removal of varying numbers of tabs 40 from 
the box 26 is demonstrated in Table IV. 
TABLE IV 
______________________________________ 
Effect of the Number of Tabs Removed On the 
Resistance Performance of Test Packs 
Processed by Ethylene Oxide Sterilization 
cycle 
temp type* exposure 0 1 2 3 4 
______________________________________ 
55c 600 20 min 10/10 10/10 
10/10 10/10 
10/10 
mg/L 
EO 
30 min 10/10 10/10 
10/10 5/10 7/10 
40 min 10/10 6/10 2/10 1/10 0/10 
50 min 4/10 0/10 0/10 0/10 0/10 
60 min 2/10 0/10 0/10 0/10 0/10 
______________________________________ 
*Testing conducted in a Joslyn EO BIER vessel at 50 .+-. 10% relative 
humidity. 
The data in Table IV is presented as the number of biological indicators 
positive for growth per number of test packs of the present invention 
tested. Test packs were assembled as illustrated in FIG. 1 with filler 
paper having a Gurley porosity of 25-45 seconds. A commercially available 
self-contained biological indicator was employed to determine the effect 
of the number of tabs removed on the resistance performance of the test 
pack. The data in Table IV demonstrates that inactivation of the 
biological indicator increases with the number of tabs removed. 
The construction of the present invention is greatly simplified over prior 
test pack constructions. The single insert of homogeneously porous 
material eliminates the costly and time-consuming assembly of multiple 
layers of dissimilar materials characteristic of prior art test packs. The 
single insert construction also provides easy access to the indicator 
after the sterilization cycle has been completed. 
Reference herein to specific details of the illustrated embodiments is not 
intended to limit the scope of the claims which recite those features 
regarded as important to the invention and are intended to define the 
embodiments illustrated and their equivalents.