Frequently Asked Questions12
12 – What are Cut Off levels?
All drug tests and healthcare screens have a cut off level. So what does this mean?
The easiest way to think about cut off levels is to imagine a speed camera set to 30 mph in a built up area. Below 30 mph the camera won’t respond, but above 30 mph the camera will flash. The ‘cut off level’ is 30 miles per hour. The cut off level is the value at which a negative result becomes positive.
Just as speed has units of miles per hour, medical test results also have units.
In this section of the website we will refer mainly to drug tests, and a drug concentration is measured in nanogrammes per millilitre, or ng/ml.
A nanogram is a thousand millionth of a gramme, or 0.000000001 gram.
Why Do Tests Have a Cut Off Level?
MHS tests are very sensitive. We could detect even the tiniest of traces of a drug in urine, saliva or hair, but the test would be too sensitive, because it would detect traces of environmental contaminants.
It is estimated that 95% of banknotes, and most public toilets are contaminated with cocaine, so if you’ve handled either, the chances are you could have a trace of cocaine on your hands, and possibly in your body.
If you visit a night club, go to a party, or even if you travel on public transport, it is possible you will have inhaled some cannabis smoke too. But this trace of drug in your body is due to environmental contaminant, not from deliberately abusing a drug.
This could leave you with a few nanogrammes of drug in your body, but if you really were a drug user, you would probably have tens of thousands of nanogrammes present.
So to avoid wrongly accusing someone, drug tests are set to only record a positive test result above a set cut off level, just like the 30 mph setting on the speed camera. Drug cut off levels are set internationally by SAMHSA in America, although recently the European Union have set their own values too.
These values are chosen to make sure only the true positives are recorded by the test, so anyone who is exposed to environmental drug exposure need not worry about being wrongly accused of drug taking. Just like speed cameras which should not hold any fear for a responsible motorist.
How do our tests control the Cut Off Level?
Our drug test format is known as ‘lateral flow’, because when you dip a test strip in a sample, or put a few drops of test liquid in one of our cassettes, a complex binding reaction occurs as the liquid flows laterally along the membrane.
We control the cut off level by carefully adjusting the number of binding sites on the membrane. The antibody is sprayed on so the quantity can be determined very accurately.
ACCURACY AND COST
In any analytical method or measuring process, the more accurate it is the more it costs. For example the manufacturer of a speed camera has to consider:
- Choice of the technology in the camera
- Quality of the laser detector
- Accuracy of the markings on the road
- Thickness of the markings on the road
- Parallax issues with a passing car not in line with the camera
- Curvature and secondary reflections off the car (we’re told the new Ford GT40 is so streamlined that it is invisible to the camera, just like a ‘stealth’ vehicle)
The camera manufacturer also has to bear in mind that your indicated speed depends on the amount of air you have in your tyres, and also the car speedometer won’t be very accurate. In fact in the 1970’s many speedometers were deliberately calibrated to read high, to fool the driver into thinking the car was especially sporty.
Manufacturing is a balancing act between accuracy, reliability, ease of use and cost. But we won’t compromise quality, that’s why in trials our products usually compare very favourably with laboratory results.
How important are Cut Off Levels?
The cut off level is like a speed limit. The speed camera is really there to stop people driving irresponsibly, for example travelling at 60 mph in a 30 mph zone. Drug cut off levels are there to avoid wrongly accusing someone of drug abuse.
And both of these limits can change too, just like cut off levels. Pretty annoying if yesterday you were caught doing 38 mph in a 30 mph limit only to find that today the road has been reclassified to a 40 mph limit.
In fact, recently SAMHSA changed the opiates cut off from 300 ng/ml to 2000 ng/ml because some tests were being triggered by the traces of opiates in poppy seeds used to decorate some bread products.
We manufacture both cut off levels, as well as different cut off levels for cocaine, methamphetamines, and some special cut off levels for matching the unusual cut off levels some laboratories have chosen. In fact we can specially manufacture whatever cut off level you want, provided the order is large enough.
The cut off level is an arbitrary value that has been set to avoid falsely accusing someone of drug use. But laboratories tend to focus on it because their business is to provide accurate quantitative results. To them, a result of 1450 ng/ml is just as important as a result of 1460 ng/ml, but this level of accuracy is not needed in screening processes, where we just need to know whether someone is positive or negative.
So, when laboratories test the accuracy of rapid tests, they challenge them with control standards that have concentrations around the cut off level. Unfortunately, some laboratories use a totally different set of cut off values in their machines, which could be why rapid test results occasionally don’t match laboratory results. But this will only happen when samples are very close to the cut off level. In these cases it is usually more ethical to give people the benefit of the doubt.
Screening versus laboratory analysis
Screening needs to be quick, inexpensive and accurate in screening out all the negatives, and identifying the positives. These positives are then analysed by a laboratory to confirm the result. Each technology has its own place in screening, in fact we sell to a number of laboratories who use our products to make their screening easy and cost effective whilst maintaining their accuracies by using GC/MS on just the positive samples.
It is important to remember that our immunoassay tests will not be fooled by any other substances – they have excellent cross reactivity tolerance.
The antibodies used in our tests are so accurate and reliable there is virtually no risk of a false result unless of course the urine contains a drug with the same structure. For example our morphine test will detect a significant dose of codeine because codeine metabolises to morphine in the body.
FALSE POSITIVES OR FALSE NEGATIVES?
A speed camera is a compromise of all the factors described above, so it is impossible to make a camera that has pinpoint accuracy every time. The camera has to be made to a performance tolerance.
Assume a speed camera is set to trigger at 32 mph. If a camera detects someone who is travelling at 31 mph and records them falsely at 33 mph, that is a FALSE POSITIVE. The motorist is recorded as positive when they were negative.
If the camera fails to record someone when they were travelling at 34 mph, that is a FALSE NEGATIVE. The motorist was over the limit but the camera failed to detect them.
False negatives and false positives are rarely an issue with our tests unless the urine or a control sample has a drug level very close to the cut off value, when the line will be faint. But the chances of a urine sample being very close to cut off at the time of test are remote.
SUMMARY
To summarise on cut off levels, rapid tests work just like a speed camera:
The vast majority of the population will have zero drugs in their system, and they have no cause for any concern when performing a drug test.
A small proportion of people will be exposed to environmental drug exposure perhaps because of their job or because of the friends they keep. The cut off level will prevent them from producing positive results.
People who used drugs in the last few days (or cannabis in the last few weeks) but have abstained since will have a reducing drug level. At some point they will have a cut off level around the cut off level. Statistically the number of people in this group will be very small indeed.
People who use drugs will be detected accurately and reliably
Download a PDF showing Cut Off Levels and Retention Times
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