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[Before the 3-week summer break, we discussed detection limit. To refresh your memory, we are resending the article with some readers' feedback before we go into quantitation limit next week.]

We went over the procedure of a quantitative measurement (counting fungal cells in swab samples). After you get the result of the enumeration back, how should you evaluate the data? Although it's not often discussed, understanding the quantitation limit (QL) of an analysis is critical in data interpretation. In order to talk about QL, we need to understand detection limit first.

Detection limit (DL) is the lowest quantity of a substance that can be distinguished from the absence of that substance. For microscopic direct exam, DL is one spore, yeast, or hyphal fragment. For culture method, DL is one colony. This is also called the "Instrument Detection Limit" (IDL) in analytical chemistry. In those two microbiological analyses mentioned above, the "instrument" is human eye. One cell and one colony is the lowest unit an eye can observe. Assuming 200-, 2000- and 20000-fold dilutions of a swab sample were made during sample preparation, one colony (IDL) in an agar plate containing 200-fold diluted sample means the "Method Detection Limit" (MDL) is 200 colony forming unit (CFU) per swab. Assuming a 2 in2 area were swabbed, the "Sample Detection Limit" (SDL) is 100 CFU per in2.

It can be demonstrated as the following calculations.
1 colony/agar plate
= 1 CFU/agar plate
= 1 CFU/(1/200 of the swab)
= 200 CFU/swab
= 200 CFU/(2 in2)
= 100 CFU/in2


If the agar plates for 200- and 2000-fold dilutions were overloaded by too many colonies, the SDL will become 10,000 CFU/in2. SDL is sample-specific and cannot be generically calculated. A simple way to calculate the SDL is to use "one" as the raw count in the analysis and calculation of a particular sample.

We will continue next week and discuss what quantitation limit is and why it is important to know it.

Click here for more details

Feedbacks from readers:
Detection limit (DL) is the lowest quantity of a substance that can be distinguished from the presence of the substance from the background. Analytical sensitivity is the lowest quantity of a substance that can be distinguished from the absence of that substance. In microbiolgical analysis, the background is 'free" of the substance, therefore, detection limit and analytical sensitivity would be the same. However, they are different in chemistry analysis since the background concentration is often not zero.

Technically speaking, DL should equal to 3 raw counts to have 95% confidence level. However, it's a convention to use 1 raw count as the detection limit for microbiological analysis.

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Host: Joe Hughes/Cliff Zlotnik

Dr. Martin D. Chapman, formerly Professor of Medicine and Microbiology at the University of Virginia and a member of the UVA Asthma and Allergic Diseases Center. Dr. Chapman is a Fellow of the American Academy of Allergy, Asthma and Immunology and has served as a consultant to the U.S. National Institutes of Health, the Environmental Protection Agency, and several biotechnology and environmental products companies. IB is organized as two affiliated companies, INDOOR Biotechnologies Inc. (IBI), located in Charlottesville, Virginia, USA, and INDOOR Biotechnologies Limited (IBL) based in Warminster, UK. IBI carries out research, development and sales in the Americas and IBL handles sales, distribution and business development in Europe, the Far East, Australia and New Zealand.

Scheduled Time:
Date: Fri, September 12, 2008
Time: 12:00 PM EDT