Steam Quality: Are you testing it?

Introduction
AAMI guidelines state - Steam quality, purity and quantity can be affected by the design, use, and maintenance of the overall steam system. Steam systems should be designed to ensure that the steam delivered to the sterilizer is saturated steam having a steam quality of 97% to 100%. Steam of poor quality can contribute to wet packs and to sub-optimal steam sterilization cycles that might or might not be identified by biological monitoring. In certain circumstances, house steam from hospital steam boiler systems might not be acceptable for sterile processing due to the design of the overall system and the type and method of using boiler feed water treatment chemicals.

Steam provides moisture that allows the coagulation of cell wall proteins and supplies the energy that heats the components and maintains their temperature, the combination of moisture and temperature results in sterilization. The higher the temperature, the shorter the sterilization time required. For proper sterilization, steam requires a number of attributes described herein. For these reasons, steam quality testing should be applied to steam sterilizers whenever steam is delivered from another source.‚ The intent of this article is to provide the reader with an appreciation for the potentially complex nature of what is often assumed to be a simple heating process.

Wet Steam
The correct steam quality combined with proper packaging and sterilizer loading techniques, should ensure that dry loads are consistently achieved. When items are processed, steam condenses and gives up latent heat. Should surplus condensate settle on the item either as a result of water entrained in steam or by condensate dripping from another item, it may not dry. In practice, as steam condenses, much of the water generated will drain and reduce the need for the latent heat contribution from the item. The drying time will be dependent upon the location of the condensate, it's surface contact area and the specific heat of the item. That is to say, large quantities of condensate in contact with a small surface area of an item will be slow to evaporate and insulators will dry more slowly than good conductors of heat. To test for wet steam, you can weigh a towel pack before and after sterilization and cooling. There should be no more than a 3% increase in the weight of the pack and no visible evidence of excessive moisture.

Superheated Steam
Superheated steam is steam that is at an elevated temperature for it's saturation pressure and is usually generated as the result of pressure drops through either pressure-reducing values or orifices. The impact of the pressure drop is to modify the pressure of the steam while it's energy content remains the same. The excess energy for the pressure present will result in any excess moisture turning to steam. If the steam is already dry saturated, or if excess energy is still present after converting moisture to steam, an increase in temperature will be evident. Once steam is dry saturated only a small amount of energy is needed to create high temperatures. Superheated steam acts as hot air and at the temperatures present will have little or no sterilizing effect. The excessive temperatures can also cause damage to items and packaging. To test for superheated steam, a small load test is performed where a thermocouple is placed 2 inches above a standard linen pack. According to European standard EN285, the temperature measured by the thermocouple at the start of the sterilization stage should not exceed the control temperature by more than 5ºC and should reduce to less than 2ºC in one minute.

Non-Condensable Gases
Non-condensable gases (NCGs) are gases liberated by steam when it condenses. The source of such gases is usually the steam generator feed water, and the impact of such gases is that they modify the steam from being pure water vapor to a mixture of steam and gas, so that they are an unwanted contaminant. If an item is hollow or porous, these gases will be forced to the center and can prevent direct contact or insulate the item. Such conditions are identical to inadequate air removal, where small amounts of air remain. European standard EN285 limits NCGs to 3.5% and is expressed in terms of ml of gas collected per 100 ml of condensate. To test for NCGs , dryness or superheat, a kit is available from SPSmedical. Portable and easy to use, this proprietary kit comes complete with fittings, instructions for use and SOP.

Conclusion
As indicated herein, wet steam, superheated steam and non-condensable gases all have the ability to adversely affect the sterilization cycle and therefore the sterility of porous loads. The extent to which the process will be affected will be dependent on the extent of the problem and the type of load. Good practice indicates that we should be aware of the condition of the steam used for sterilization of equipment or porous loads. Periodic testing of steam quality is needed to detect any transient or seasonal problems that may exist. Properly designed, constructed and maintained steam distribution systems are critical to meet the demand for proper sterilization.

References
AAMI ST 46. Steam sterilization and sterility assurance in healthcare facilities. Revised and approved March 2002.
ISO 11134: (1994) Sterilization of health care products - Requirements for validation and routine control - Industrial moist heat sterilization.
EN285 Sterilization - Steam sterilizers - Large Sterilizers (1996) - Section 24 Steam quality tests.
K. Shuttleworth. The Derivation of United Kingdom Physical Steam Quality Test Limits. PDA Letter, December 1999.