Drug testing using hair shows what drugs have been used over an extended time-frame of months, rather than days – which is the timeframe associated with urine and saliva (oral fluid) testing. It is much more sensitive to occasional and intermittent use: where urine testing might miss a ‘party habit’ every two weeks, hair testing will not. It is that much-improved confidence of detection that established the value of hair testing in children-at-risk cases.
The retrospective window of detection is particularly valuable in those family cases where drugs are a risk factor. A hair test can help to prove abstinence over months much more reliably than a urine test. A single hair test may be more expensive than a single urine test, but it requires at least eight urine tests to be confident of surveying a month’s use, or 25 tests for three months. Also, hair testing encourages compliance with court instructions, as the only way that can guarantee beating the test is the total removal of all hair.
When the hair sample is sufficiently long, multiple sections can be tested. Each section then represents blocks of several weeks and any changing pattern of drug use may be revealed by detecting increasing or decreasing levels over longer time periods. Hair testing cannot reveal on which day drugs were used. It can give an indication, but the evidence will inevitably be corroborative rather than definitive. Similarly, the test will not give a precise answer to the questions “How many times were drugs used?” or “What was the dosage used?”.
The testing laboratory should be able to give an indication, based on the cumulative results of its testing experience, of the level of drug use. However, all drug tests should be interpreted in the light of a clinical interview by a skilled drugs counsellor.
In cases where no head hair is available body hair may be used, but the timescales are much less clear. In effect any drug found in body hair is ‘smudged’ over a much greater time window, because the rate of growth is in effect much more variable, making any decision as to when drugs may have been used impossible. However, there is potentially greater value in a negative drug result from body hair, like pubic hair, as the result represents a longer time period in comparison to head hair.