Historically Low Temperature Steam Formaldehyde (LTSF) sterilization processes have been tested with blood-contaminated biological indicators (BIs) Geobacillus stearothermophilus according to the standard DIN 58948-14:1987. However practical experience demonstrated using BIs with blood contamination increased the resistance, but does not produce reproducible results and therefore cannot be used.

Therefore, the replacing European and International standard EN ISO 11138-5 recommends BIs without blood contamination. The resistance of the indicators should be determined in a Resistometer with a gas atmosphere created by a formalin solution, characteristic for LTSF sterilization processes. A publication of the gke application laboratory demonstrated that this kind of resistance determination can cause big failures (see G