Q: What is the sterility assurance level (SAL) when performing an ethylene oxide (EO) Overkill Method qualification using the half cycle approach where 10 biological indicators (BIs) were used, tested, and there was 0 positives obtained (complete kill). Many people would say that the minimum SAL in this example would be 10-6, but it is actually greater than that.

A: If you tested 10 BIs, each with a population of 106, and got 0 positives, there would be a SAL of at least 10-1 (no more than 1 positive out of 10). This means that there would be at least a 7 log reduction (106 to 10-1 = 7 logs). To take this a step further, if you doubled the time to calculate the theoretical SAL in a full cycle run, you would actually have a minimum of a 14 log reduction (7 logs result from the half cycle + an extrapolated additional 7 logs to reach the full cycle time = 14 logs). This would then result in an SAL that was actually in excess of 10-8 (10E6 – 14 logs = 10-8). If you were to do the same qualification run as described above using 100 BIs instead of 10, and got 0 positives, you would actually be getting a minimum 8 log reduction in the half cycle, with a minimum 16 log reduction in a full cycle, resulting in a SAL of at least 10-10.