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Risk estimation of terminally sterilized items that don't meet the required sterility assurance level 

When airborne microbes penetrate the sterile barrier system and contaminate the sterile product, the minimum infective dose of the heterogeneous microbiological species is the key factor for possible medical consequences, because a rather small number of entering microbes can be expected. The minimum infective dose is among others dependent on the susceptibility of the host (e.g. age, health status), the virulence of the pathogen and the route of exposure. Because the most typical airborne microorganisms such as fungi or aerobic bacilli, e.g. Bacillus subtilis are organisms of low virulence, further specific circumstances must be present to increase the risk of serious health care-associated infections.
 
Gunaratne et al. (2006) associate the recontamination of 43 of 679 disposable plastic syringes with Aspergillus fumigatus with sub-optimal storage conditions during the 6 months after the tsunami in Sri Lanka. They concluded that the use of the syringes with these storage conditions was one of the most likely causes for Aspergillus fumigatus meningitis in 5 women. The infections followed spinal anaesthesia for caesarean section in two hospitals. Patients with intramuscular and intravenous injections didn’t show any Aspergillus infection. The specific type of administration and entry into an area of low immunological resistance was an additional condition for the observed infections.
 
It must also be kept in mind that already few airborne micro-organisms can be the starting point for an outbreak of hospital acquired infections if they are enabled to multiply and if a contaminated stock solution is used for a batch of hundreds or thousands single doses. Outbreaks of this type of hospital acquired infection are frequently traced back to micrbiological species which are not commonly known for patient to patient transmissions in hospitals.
 
Kainer et al. (2012) described a fungal (Exserohilum rostratum) meningitis outbreak in a clinic (clinic A) where vials of contaminated methylprednisolone were administered that were prepared by a single compounding pharmacy. Mostly epidural injections were applied, 58 of 656 patients who were treated with methylprednisolone, developed infections. A significant increase of the infection rate was found for patients who received older vials (vial age >50 days) compared with those who were treated with more recent vials. This indicates that fungal growth was present in the vials during the storage at room temperature and that rather high microbial doses caused the infections.
 
A HAI outbreak or a single HAI followed by transmission of pathogens through sterilized products when it was compromised by ingress of microbes through the porous packaging material can not be traced back easily to this recontamination because other routes of transmission have to be considered and can not be ruled out as causative factors. Therefore, the attainment of the product’s sterility at the SAL of 1:1,000,000 up the point of use should be verified by suitable methods such as the consideration of the filtration efficiency and the estimation of recontamination by airborne microbes during storage and transport (see also: Dunkelberg H. Maintenance of Sterility: Base of Aseptic Procedures. Dtsch Arztebl Int 2016;113:737).
References
Gunaratne PS, Wijeyaratne CN, Chandrasiri P, et al. An outbreak of Aspergillus meningitis following spinal anaesthesia for caesarean section in Sri Lanka: a post-tsunami effect? Ceylon Med. J.2006;51:137-142
Kainer MA, Reagan DR, Nguyen DB, et al. Fungal infections associated with contaminated methylprednisolone in Tennessee. N Engl J Med 2012;367:2194-203