THE DOPE ON DEALERS

The DNA police will soon be on the trail of dope dealers. A new swab
test can reveal if suspects have handled cannabis. It's so sensitive
that there are worries that innocent people could be
incriminated.

The DNA-based test, which will be described fully in a forthcoming
edition of the journal Science and Justice, was developed at the
forensic science unit at the University of Strathclyde. Researchers
identified DNA sequences within the part of the plant cell responsible
for photosynthesis--the chloroplast--that are specific to Cannabis
sativa, but which are not found in any other species.

The discovery makes it possible to test samples, such as a swab taken
from someone's hands, for these sequences. The samples have to be sent
to a lab, where the polymerase chain reaction is used to amplify the
target sequences. If any are found, cannabis must be present.

Because the patented test (WO 98 24929) multiplies the amount of DNA,
it works on minute amounts of a sample, unlike existing techniques.
"This test is as sensitive as you can ever get," says Adrian Linacre,
one of its inventors at Strathclyde. "Theoretically, it could pick up
the presence of just one molecule of cannabis DNA." This means a swab
can be taken from a suspect's hands to see if they have been in
contact with the drug, even if nothing is visible.

However, Linacre says that he has calibrated the test so that only
relatively large quantities of DNA give positive results, in an
attempt to exclude those who have unknowingly been contaminated. But
the threshold level is likely to be controversial.

Chris Evans of Surescreen Diagnostics in Derby, a company that markets
drug testing kits, says it would be easy for innocent people to be
incriminated. "If you have been driving a second-hand car and a
previous owner has been using cannabis, you could get enough on your
hands to test positive," he warns. He also points out that many
banknotes carry trace amounts of drugs (New Scientist supplement, 4
October 1997, p 2).

Linacre is now developing the test to detect the DNA profiles of
different strains of cannabis, such as those grown in South Africa or
Thailand. "We will be able to use genetic evidence to link different
batches of cannabis and trace them back to their original source," he
says.

The development of the cannabis DNA test follows that of a saliva
test, by Oxfordshire-based Cozart, for a number of drugs including
cannabis. Unlike the Strathclyde development, the saliva test is aimed
at users rather than dealers, and is likely to be used by the police,
sports governing bodies and employers (New Scientist, 29 May, p 11).