{
  "$type": "site.standard.document",
  "bskyPostRef": {
    "cid": "bafyreie5yxckgazbjpuh4oxpl4yx5wszaca37qvpx5tinl24tldevrffnm",
    "uri": "at://did:plc:wwyqal4cnqhuwyacdj7rqq3n/app.bsky.feed.post/3mj3rdka7per2"
  },
  "path": "/t/sterilization-assurance-level-sal-in-medical-device-sterilization-processes/28700#post_1",
  "publishedAt": "2026-04-09T16:49:54.000Z",
  "site": "https://discourse.datamethods.org",
  "textContent": "I worked for 4.5 years as a medical device sterilization technician.\n\nIn sterilization science we use a concept named Sterility Assurance Level (SAL) to ensure the safety of medical devices after sterilization. However, I believe that the definition of SAL is flawed.\n\nSince I’m no statistician and I don’t use statistics at the professional level (let alone at the level discussed in this site), I’m posting this question here to understand whether I’m right or wrong.\n\nI will state the problem only with the necessary mathematical assumptions. I will leave all things related to microbiology, sterilization technology, human factors and so many other things aside. This is because the definition of SAL is purely mathematical and so I whish to address its possible mathematical flaws (I can deal with all its other non-mathematical flaws).\n\nI can explain SAL in these terms:\n\n-- Consider a medical device contaminated with 10^6 microorganisms. The medical device is subjected to a sterilization method (like moist steam) that kills the microorganisms at a fixed rate.\n\n-- We define the decimal reduction time, or D-value, as the time necessary to kill 90 % of the surviving microorganisms.\n\n-- A table can show how the microorganism population evolves during sterilization:\n\nThe usual interpretation is this:\n\n-- According to the table, after 12 D-values there will be 10^-6 viable microorganisms in the medical device. So, if we consider 10^6 medical devices initially contaminated with 10^6 microorganisms each (making a total population of 10^12 microorganisms), and if they are all subjected to the same sterilization method, in the end there will be just 1 viable microorganism. In other words, there will be 1 contaminated medical device among the 10^6 medical devices.\n\n-- SAL is the probability of ending up with 1 contaminated device in a group of devices after a decontamination process which takes a specific time. In this case SAL is 10^-6 and indeed current medical device sterilization processes are tuned to achieve a SAL of 10^-6.\n\nAgain, I will only address the mathematical aspects (there is a lot more to consider that compound the problem and make SAL a flawed concept). My problem is purely mathematical and I’m asking for help to understand whether my reasoning is flawed or not:\n\n-- In the same way that I can throw a dice 6 times and never get a “5”, and I may throw it a lot of times and never get such result, if I define SAL as a probability, then I can end up with far more than 999.999 decontaminated devices and, at the same time, end up with a device severely contaminated with thousands of microorganisms. However, the assumptions in this discussion, as well as the results in the table, are not about a random experiment.\n\n-- Is it correct to consider a population of 10^12 microorganisms as the same as 10^6 populations of 10^6 microorganisms? I believe not. The assumption is that all populations die at a fixed rate. If we separate them, after 7 D-values all populations are dead. If we join them, the single population is all dead after 13 D-values. If we consider the separate populations as a single one, than we would find that each separate population would not decay exponentially or continuously, just that the sum of all decays would be a decreasing exponential. But this contradicts the initial assumption that the killing rate is constant for each individual population.\n\nI’m not an English-speaking native, so if the problem is not clearly started, please do let me know. And thanks in advance!",
  "title": "Sterilization Assurance Level (SAL) in medical device sterilization processes"
}