The terms “False Positive” and “False Negative” can be confusing, however we have developed a very simple scheme to explain these terms.
For any process that is based on two tests (in this case we are using an “Initial Test” and a “Confirmation Test”) – there are four possible outcomes (see below under “Confirmation Test”).
We rarely hear about the terms “True Positive” or “True Negative” because these results are correct. In these cases the “true” part is normally ignored and the results are simply referred to as being “Positive” or “Negative”.
On the other hand, the terms; “False Positive” and “False Negative” are used frequently – especially the term: “False Positive” because it quickly describes the outcome that the initial “positive” finding was wrong (or at least does not agree with the first result).
In drug testing, where incorrect screening/ initial results can sometimes be encountered, these can be further described as follows:
False Positive Drug Test
This describes an initial (or screening test) that produced a “positive” result; which after further analysis by a secondary process results in a negative test result. However due to the implications of referring to the screening test as “positive” (because it is not confirmed yet) the very use of the term “positive” is often frowned upon. In fact the Australian & New Zealand Standards specifically preclude any inference that an initial test was “positive”. As a result – terms such as “non-negative” and “laboratory referral” are often used instead.
There are a myriad of factors which can cause “False Positive” test results, however we will address that in another post. In the meantime, an interesting thought to ponder is the possibility that the secondary “Negative” test result could actually be wrong (by performing a third test & assuming that this third test is correct) , which then leaves us with the first test actually being a “True Positive” and the second test a “False Negative”. If you’re lost – the major point to remember is the fact that any classification of “False” or “True” results can always be subject to further analysis – which can turn a “True” result into “False” and vice versa. Of course it is quite unlikely to encounter an incorrect confirmation test in a properly accredited laboratory – but the availability of an additional “referee” sample affords additional confidence in the event of a disputed confirmation test result. There is the possibility of performing additional testing as further verification, so that the above scenario, although unlikely; could occur.
False Negative Drug Test
This classification of drug test results are in fact rarely encountered because it is not common to run confirmation tests on negative test results (with the exception that in some Standards such as the Australian Saliva Testing Standard – a percentage of negative samples have to be sent to the laboratory as a quality check – to determine any “False Negative” results – or to participate in an external Proficiency Program – but this is another topic for another post).
The terms False Positive and False Negative are used in a myriad of different situations. Some common applications include:
- pregnancy tests
- security alerts such as alarms
- antivirus programs
- quality control