Educators > Cut offs, screens and confirmation

Cut offs, screens and confirmation

A modern testing laboratory wants to provide a good service at a good price.

However, the lab can be constrained by the following:

Throughput – robot sample handling and chemical test methods need to suit the lab and the client. No client would wait 6 weeks to get the best analytical result.  Similarly, no client would pay £5000 per test, especially in the workplace sector where the vast majority of tests are of course negative.

The client enters into a Service Level Agreement with the laboratory. Almost always this means the lab tells the client which drugs at which levels can be screened and confirmed. Very few clients know their Mexedrone from their Mephedrone.

This drug list or “panel” is usually based on immunoassay tests which allow negative (drug free) samples to be quickly screened and reported.

Logically, the client expects the vast majority of his/her employers to be drug free, so receiving a quick report of “no drug use detected” makes perfectly reasonable business sense.

Bear in mind, this “non detected” applies ONLY to the drug panels that are observed in the screen. The donor could be overdosing on Cocaine and be deemed “negative for our panel” if Cocaine was not part of the panel.  This detail is key.

If this lab screens a sample as presumptive positive using their screening method, then a second portion of the sample will go forward for a confirmatory test.

This confirmation test is almost always mass spectrometry which offers both sensitivity (low reporting concentrations) and specificity (no ambiguity) for a report to the client.

Dependent on the sensitivity of the screening technique, ELISA, HEIA, EMIT a screening cut off will be determined.

Mass spectrometry will offer a lower confirmation cut off, and will be reported as the final result.

A sample which is lower than the screening concentration for a particular drug, will NOT go forward to mass spectrometry analysis.

In practice, that means a urine sample containing 280 ng/ml of diazepam, will be screened as negative, if the screen cut off is 300 ng/ml.

Even though the mass spectrometer used in the lab could accurately measure (and report) 20 ng/ml, the donor sample would never see the mass spectrometer, as it is screened negative at the initial test stage.


This is the way the larger test labs work.

Download PDF