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Recommendations by the Quality Task Group
[14] Thermal Washer-Disinfectors - Investigating the Disinfection Effect with Thermologgers

For routine investigation of the effect of disinfection in washer-disinfectors with a disinfection step, thermoelectric procedures using temperature sensors are described in Standard prEN 15883-1 "Validation of washer-disinfectors", since these can be easily conducted, evaluated and documented on site. A PC and appropriate software as well as one or several thermologgers spanning at least the range 10 °C - 100 °C are required. The standard describes in detail the requirements to be met by the measuring instruments as well as execution of measurements for the various test steps. They supplement any existing in-process control of the washer-disinfectors because they measure independently and can be compared with the data of the in-process control, to the extent that the latter can be read.

The A₀ value concept
In STANDARD prEN DIN ISO 15 883-1, Annex A, the term A₀ value has been introduced as a yardstick for the lethality effect against microorganisms in moist heat processes. For a moist heat disinfection process, a particular temperature over a particular time can be expected to have a predictable lethal effect against a population of microorganisms with a particular resistance. If particularly resistant test organisms are selected and it is assumed that they are present in numbers in excess of that likely to be encountered in the real product, it is possible to define in a standard the required temperatures and exposure times. On compliance with these values, it can then be assumed that automated processing methods guarantee the required lethality effect against microorganisms. Since the temperature is conveyed by means of the water acting upon or flowing through to the items being cleaned, it must be ensured that all inner and outer surfaces are reached. The TEMPERATURE MEASUREMENTS are therefore not effected in the washer-disinfector drain but between the load. The standard furthermore requires that also the trays and walls of the chamber(s) furnish proof of the required temperatures for the corresponding time period.

"A" is defined as the equivalent time in seconds at 80 °C to produce a given disinfection effect. The D value is the time needed at a particular temperature to reduce the microbial count to 10% (by 1 log₁₀ level). The z value is the increase in temperature needed to reduce the D value to 1/10.

If the defined temperature is 80 °C and the z value is 10, the term A₀ is used. The A0 VALUE of a moist heat disinfection process is the lethality expressed in terms of the equivalent time in seconds at a temperature of 80 °C delivered by that process to the product with reference to microorganisms possessing a z value of 10. This is the case for many microorganisms.

For disinfection processes aimed at bacteria, including mycobacteria, fungi and heat-sensitive viruses, an A₀ VALUE OF 600 is defined, corresponding to a hold time of 600 s = 10 min at 80 °C. The A₀ value of 600 can also be achieved at 90 °C with a hold time of 1 min. Table 1 shows conversion of the A₀ value of 600 to other temperatures according to Machmerth.

If efficacy is to be ensured also against HEAT-RESISTANT VIRUSES, e.g. hepatitis B, an A₀ value of 3000 must be chosen, corresponding to a temperature of 90 °C and a hold time of 5 min. It is recommended to select in general the A₀ value of 3000 for programmes used to process surgical instruments.

For the TYPE TEST for washer-disinfectors based on standard prEN ISO 15883-1 extensive temperature measurements are needed, which in some cases are repeated during the operational and performance qualification after installation of the appliance on site using loads defined by the operator.

For the routine tests, the operator defines in a QUALITY ASSURANCE CONCEPT which machines are to be checked and how often and in what positions the measurements are to be conducted.

Unlike in the case of microbiological methods with biological indictors as used for periodic testing, the results can be read at once. But they do not provide any information on the preceding IMPECCABLE CLEANING of products, which constitutes a precondition for reliable disinfection. Hence supplementary investigations for cleanliness must be carried out.

While recording the temperature course for the entire process with thermologgers, important information is also gathered on the exposure time and on the temperature range traversed in the cleaning step.

Table 1 Conversion of the A₀ values to other temperatures (after Machmerth)

Figure 1: Temperature curve of a Vario programme