An important factor that must be considered when carrying out drug testing is the application of cut-off levels. All drug screens have a cut off level which is internationally decided by by SAMHSA (Substance Abuse and Mental Health Services Administration).
Drug testing cut-off levels are normally expressed as ng/mL (which means nanograms per millilitre) The cut-off level of a drug test is the ng/mL measurement at which a sample is deemed to be either a positive or a negative test result.
Each drug of abuse has it’s own defined drug concentration level which is the cut-off that has been chosen to minimise the chance of false positive results. An important thing to note here is that a negative drug test result does not necessarily mean that the urine sample is drug free, it just means that the concentration of the drug is lower than the defined cut-off.
When a drug test result proves to be positive, this is only an indicator that drug is present in the sample. What the test cannot tell you is how much of the drug was used or taken and if this had any correlation with the impairment of the individual at the time. It should also be remembered, that this kind of test is a screening tool and another testing method should be used to obtain a definite positive result.