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Log reduction and filter validation according to ASTM F838

In order to reduce the number of germs in a liquid or on solid surfaces, these must be filtered, cleaned or disinfected. To achieve comparable, reproducible results, we must be able to measure the resulting bacteria reduction.

Contents:

  • Log reduction table
  • Log reduction in water hygiene
  • Filter validation

Log levels: explanation and calculation

The bacteria reduction is usually specified in log levels. One log level describes a decrease in germs by a power of ten.

Therefore, 1 log10 level means a 90% reduction in bacteria. This means that, if the original population was 100 (10x10), only 10 germs survive.

A 99% reduction (2 log10 levels) corresponds approximately to the effect of soaps.

In the non-medical sector, disinfection is defined as a decrease in all germs by a factor greater than or equal to log 4 or 99.99%. In this case, a maximum of 1 out of 10,000 germs can survive.

The restrictions are higher in the medical sector. According to the requirements of the ASTM (American Society for Testing and Materials) and the FDA (Food and Drug Administration), sterile filtration can only be considered to take place from Log 7 upwards.

Log reduction table

The following table shows examples of the log levels, whereby the basis for the reduction is always 100,000,000 microorganisms.

Log reductionReduction in %Reduction from 100,000,000 microorganisms to
19010,000,000
2991,000,000
399,9100,000
499,9910,000
599,9991,000
699,9999100
799,9999910
899,9999991


Log reduction in water hygien

The categorisation using log levels is also used in water hygiene.

Membrane filters, which are class I medical devices, are increasingly being used for the filtration of drinking water, particularly in the medical sector. With a pore size of just 0.2 μm, they offer reliable protection against water-borne germs. According to the definition of "sterile filtration", they therefore lead to a bacteria reduction of at least 7 log levels of the test germ Brevundimonas diminuta (Brev. diminuta) per square centimetre of filter surface, i.e. to a germ reduction of 99.99999%.

The bacteria Brev. diminuta is the smallest water-associated germ and is therefore used as a test bacterium in accordance with legal specifications. If the filter can stop this bacterium, it can be assumed that the filters are also capable of reliably retaining larger pathogens such as legionella or pseudomonads.

Filter validation: what to take into account

Like other processes in the pharmaceutical industry, validation is an important step to prove that filters fulfil their intended purpose. To this end, various aspects are analysed, such as bacterial retention (in accordance with ASTM F838), filter integrity, release of particles from the membrane, chemical compatibility, etc.

Bacterial retention according to ASTM F838

The bacterial retention test according to ASTM F838 is basically a load test. A standardised procedure is used to check by how many log levels the filter can reduce the number of bacteria. A defined bacteria solution is pressed through the filter, which has been installed in a test apparatus in advance.

All components of the test apparatus must be sterile and therefore free from other germs. The results of the tests must be precisely documented.

Testing the filter service life

Another aspect that plays a major role in filter validation is the service life, i.e.: the time from filter installation to filter removal. It is paramount to test how long the filter can retain the bacteria, how resistant it is to chemical or physical factors and how reliably retrograde, i.e. ascending contamination can be avoided.

Final test of the filters in field studies

In addition to the tests that take place under laboratory conditions, it is important to check how they work in practice. This is crucial since other factors such as retrograde contamination, user behaviour, water quality and the specific hygiene environment play a decisive role.

 

"Filters are tested in field studies"

 

To this end, the filters are tested in field studies by using them in everyday clinical practice (e.g. on a transplant ward) over a longer period of time. The water is then tested weekly for legionella, pseudomonads and other pathogens. To ensure that the sterile filters are really safe within the recommended service life (up to 124 days, depending on the filter), longer service lives are generally also tested in these field studies.