3M Attest Technical Information

Technical Information

STEAM

3M™ Attest™ Steam

Chemical Integrator

Dynamics of Steam Sterilization

Steam sterilization has been used for over 100 years. Decades of research have shown that the efficacy of a steam sterilization process is the function of three basic parameters: time, temperature and the presence of saturated steam. All three are critical process variables for effective steam sterilization.

The importance of saturated steam is demonstrated when dry heat sterilization is compared with steam sterilization. The use of steam allows faster sterilization than dry heat. For example, dry heat sterilization requires a sterilization time of 60 minutes at 320°F (160°C), while steam sterilization at the same temperature would take less than a minute.1 Clearly, steam quickens the kill time of living organisms by many orders of magnitude and is generally preferable to dry heat.

2

3M™ Attest™ Steam Chemical Integrator

Technical Information

Once a saturated steam environment is obtained, the independent variables of time and temperature can be determined by the following formula:2

In order to show the high sensitivity of kill time to temperature, the above formula can be solved for 247°F (119°C).

t = Fo × 10(250-T)/Z

Where

t = time for 100% kill at temperature T T = processing temperature (0°F)

Fo = kill time for Geobacillus stearothermophilus with a z-value of

18°F (10°C) and D-value of 1 minute at 250˚F (121˚C)

z = rise in temperature required to increase the rate of kill by a factor of 10 (usually about 18˚F (10˚C))

Interpretation of this formula shows that the relationship of processing time (t) versus temperature

(T) can be plotted as a logarithmic function. Expressed differently, it means that a small fluctuation in the temperature results in a large change in the actual processing time required for 100% kill. Figure 1 shows the thermal death time at different temperatures for

1 million live spores of Geobacillus stearothermophilus.3

This curve can be expressed mathematically by the following formula which shows that it takes 12 minutes to kill 1 million living spores in a 250°F (121°C) steam sterilization cycle.

t = (12)10(250-T)/18

Where

Fo = 12 min for G. stearothermophilus

z = 18°F (10°C) for G. stearothermophilus

t = (12)10(250-247)/18

t = (12)10(0.167) = (12)(1.47) t = 17.6 minutes

In theory, therefore, if the inside temperature of a sterilizer were actually operating at 247°F (119°C) instead of 250°F (121°C), a time of 17.6 minutes would be required to kill the 1 million spores of G. stearothermophilus at 247°F (119°C) versus the 12 minutes needed to kill the spores at 250°F (121°C).

This interdependence of time and temperature (in saturated steam) is an important relationship which should be understood by all personnel responsible for providing sterility assurance for steam sterilized items. Consider the ramifications if a sterilizer was inadvertently set at a processing temperature at

247°F (119°C) instead of 250°F (121°C). Or, if the load was processed at 247°F (119°C) as a result of a minor malfunction of the sterilizer (e.g., air pocket or small air leak), a slight calibration error or a natural drift in the temperature monitoring system, incorrect loading or packaging.

Because even small decreases in temperature during steam sterilization may significantly increase the time necessary for assurance of sterility, an accurate means of monitoring internal sterilizer and pack conditions are essential.

3M™ Attest™ Steam Chemical Integrator

Technical Information

3

Integrating Indicator vs. Biological Death Curve

100.00

Three Typical Stated Values for 3M™ Attest™ Steam Chemical Integrators

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<![if ! IE]> <![endif]>Time	1.0
			Theoretical G. stearothermophilus
			Thermal Death Time
	0.1
	118		122	126	130	134	138

Temp (°C)

Figure 1. Graph comparing three typical Stated Values of 3M™ Attest™ Steam Chemical Integrators with the theoretical death curve of Geobacillus stearothermophilus spores.

Pack and Load Control

The dynamics of steam demonstrate the need for accurate monitoring of internal sterilization conditions. Pack control is the use of chemical indicators for the internal monitoring of packs, trays, containers, and peel pouches. Internal chemical indicators should be used inside each type of packaging to address the potential for interference with proper steam sterilization conditions in all of these types of packaging.4,5,6

Several problems can occur in the packaging and loading of individual packs that can inhibit air removal and steam penetration which leads to a lower temperature. Packing problems include:

•Incorrect packaging or container system chosen for the cycle parameters;

•Incorrect preparation of the container for use

(i.e., filters and valves or inappropriate bottom tray);

•Placing a folded peel pouch inside another peel pouch;

•Placing a peel pouch inside of an instrument tray or container system (if not recommended by the manufacturer);

•Preparing textile packs that are too dense to sterilize in the cycle parameters chosen;

•Over loading the individual packaging or container system chosen (an over weight package).

Loading problems include:

•Stacking container systems (if not recommended by the manufacturer);

•Laying peel pouches flat or on top of each other instead of on edge;

•Improperly placing peel pouches on edge (plastic sides not facing all in one direction);

•Turning instrument trays on edge;

•Laying fabric packs or basins flat;

•Placing packages too close to each other impeding air removal and sterilant penetration around and through the load;

•Rigid containers systems loaded above wrapped or pouched items.

Malfunctioning equipment can also result in insufficient sterilization conditions inside of packaging as the result of:

•Incomplete air removal;

•Inadequate cycle temperature;