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Educators > Uncertainty
Few people outside of the laboratory sector understand the mathematical uncertainty in the results that laboratories report.
These "uncertainties" is the term used to account for all the variables in the process of the analytical sampling and testing. All the small errors added up to determine how wide a span of possible concentration ranges correct the result could be.
In reality if the lab reports a number, they typically are reporting the low concentraion in a range of numbers. For example, 5 ng/ml may actually mean 7 +/- 2.
The laboratory is reporting a 5 or a "greater than 5" result.
These expanded uncertainties of measurement can be quite large. 30 - 40% is quite common. This means that the labs report a “greater than x” number where x is at the lower concentration range of the result. Each drug will have its own uncertainty range within the method used.
For example, an instrument may report a result of 5.0, if the uncertainty measurement is 40% then the actual concentration range of the possible result lies in the range 3.0 to 7.0 That is 5.0 +/- 40%. The laboratory will then report a result of “greater than 3”. The smaller the uncertainty then the more precise and repeatable the lab processes are.
Again, this is a good indicator of lab performance and is something the client should ask at the SLA stage.
If the lower end of the uncertainty range fell below the confirmation cut off then the lab would correctly report the sample as negative.
This is important to remember. Laboratories never report zero.
Below limit of quantitation
Are typical terms used.