Testing Explained

Modern screening methods are designed to rapidly identify a sample as either negative or presumptive positive if the substance is above a set cut-off concentration. Negative samples are deemed to not require further investigation, whilst samples that are presumed to be positive will require testing by a different analytical technique, known as a confirmation test.

Our screening methods are based on established immunoassay technology and can be conducted in a laboratory or with a rapid diagnostic test at the point of care. Immunoassays use antibodies as reagents to measure the amount of a particular drug or drug group in the sample.

Antibodies bind to the drug by recognising a distinct three dimensional shape. In simplest terms this can be thought of as a key in a lock. The key (drug) has a distinct shape which will only fit into a suitably matched lock (antibody). This concept is shown in figure 1, where each coloured block is a different drug (with a different shape) reacting to a matching antibody.

Figure 1: A stylised representation of an antibody recognising one of the shapes. Notice that each shape will only bind to one antibody.

One of the key concepts within drug testing is the application of a cut-off level. This is the point which segregates a test result as being either positive or negative.

For drug screening tests, a cut-off is chosen that will optimise drug detection but minimise the number of false positive results. It is important to note that a negative sample doesn’t mean that it is drug free; it might contain a drug at an amount that is lower than the defined cut-off.

If a drug test is reported as screen positive or presumptive positive, this merely shows a response, which is usually because a drug is present. It cannot show how much drug was taken or be correlated to any degree of impairment. As this is a screening tool, all presumptive positives require a confirmation test.

Figure 2: A visual representation of a cut-off level and window of detection.

The actual time a drug will remain detectable in a sample will depend on some or all of the following:

• The amount of drug taken
• How frequently the drug is taken
• The nature of the drug itself
• An individual’s metabolism and general health
• The amount of fluids taken since taking the drug
• The amount of exercise taken since taking the drug
• Genetic variations that affect a response to drugs

For example if a person smoked a single spliff the cannabis could remain detectable in urine for no more than 2–3 days and may even be as short as one day depending on the strength of cannabis. However, if their cannabis use is habitual and heavy it is stored in the fatty tissues, resulting in a much wider detection window (sometimes up to 30 days).

The data supplied below acts as a guideline and should be interpreted very carefully due to the large number of factors that can influence the amount of drug over time. The quoted windows of detection indicate how long a drug can be detected. This doesn’t indicate that the drug will be detected for this long in all cases.

Figure 3: A visual representation of a cut-off level and window of detection.

A screening test uses established immunoassay technology to measure the amount of a particular drug or drug group in the sample. Antibodies bind to the drug by recognising a distinct three dimensional shape. In simplest terms this can be thought of as a key in a lock. The key (drug) has a distinct shape which will only fit into a suitably matched lock (antibody). Once the antibody is bound to the drug a colour change occurs, the greater the intensity of this colour change the larger the volume of the drug.

Some drug families share a common shape and so an antibody raised to recognise a drug within that group will detect or cross react with all of the drugs within the family.

There are many hundreds of thousands of drugs available worldwide, many of which are broken down by the body into drug metabolites. With so many over the counter (OTC) and prescription only medicines (POM) available, there is a possibility that some of these drugs and their metabolites might have a similar 3-dimensional shape to other drugs and will be detected by the immunoassay. For this reason it is standard practice to perform a confirmation test and request details of a donor’s declared medication after a presumptive positive screening result.

Figure 4: A drug family can share a common shape to which an antibody will bind. Here four different drugs share a triangular section and therefore bind to the top portion of the antibody.