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BioMicrobial load (cfu/g or cfu/ml) can be expressed as log10. So, if you have 100,000 microbes that is 5 log, 10,000 microbes is 4 log, 1,000 is 3 log, 100 microbes is 2 log and 10 microbes is 1 log. Now, if you went from 100,000 microbes cfu/g to 10,000 microbes cfu/g that would be a 1 log reduction (5  4 log). If you went from 100,000 to 32,000 that would be a 0.5 log reduction (5  4.5 log) and so on.

"Log reduction" is a mathematical term (as is "log increase") used to show the relative number of live microbes eliminated from a surface by disinfecting or cleaning.
For example, a "5log reduction" means lowering the number of microorganisms by 100,000fold, that is, if a surface has 100,000 pathogenic microbes on it, a 5log reduction would reduce the number of microorganisms to one.
Log Reductions
1 log reduction means the number of germs is 10 times smaller
2 log reduction means the number of germs is 100 times smaller
3 log reduction means the number of germs is 1000 times smaller
4 log reduction means the number of germs is 10,000 times smaller
5 log reduction means the number of germs is 100,000 times smaller
6 log reduction means the number of germs is 1,000,000 times smaller
7 log reduction means the number of germs is 10,000,000 times smaller

Log Reduction ValueA Measurement of Retention Efficiency
There are three common measurements of retention efficiency, Percent Efficiency, Beta Ratio and Log Reduction Value (LRV). This session of One Minute Filtration will focus on Log Reduction Value.
Log Reduction Value is a ration of the log to the base 10 of the challenge concentration divided by the filtrate concentration. It is most often used when there is a high concentration of a single challenge material, such as bacteria or latex beads. In the Pharmaceutical Industry the definition of a Sterilizing Grade Filter is one in which has a LRV of 7 and a sterile filtrate when challenged in accordance with the HIMA (Health Industry Manufacturing Association) test protocol.
LRV = Log10 Challenge/Filtrate
In the case of the Sterilizing Grade Filter, it is challenged with 107 bacteria of a prescribed type (Brevundimonas diminuta ATCC19146) per square centimeter and there is zero in the filtrate. Since it is not appropriate to divide by zero (0), a 1 is substituted and the terminology of “sterile filtrate” is added.
LRV = Log10107/1 = Log10107 = >7/cm2 and a sterile filtrate
Another example commonly used for measuring the efficiency of filters in the Microelectronics industry is a challenge of a particular size latex bead. In this case a filter might be challenged with 106 0.05 µm latex beads with 102 in the filtrate. In this case the LRV would be 4 per filter.
LRV = Log10106/102 = Log10104 = 4/Filter

