How is the "CUT-OFF LEVEL" determined in drugs of abuse detection?

Question

Im lookink for a mathmatical equation to understand the CUT-OFF LEVEL.

Answer 1

The 'cut-off' depends on the method of detection used and how sensitive it is.

The cut-off level means the drug concentration at which 50% of samples will test positive. For example, if a test has a cut-off of 50 mg/L and you make a solution of exactly 50mg/L and test it repeatedly, half the time you will get 'positive' and half of the time you will get 'negative'.