A method cannot be considered valid unless it has been tested against a positive sample.

He was stopped and gave a positive drink-drive sample.

"We never expected to find positive samples" of pathogens, the team's biological specialist said.

Processors are now required to hold the entire day's output from the production line where a positive sample was found.

Previously the plant was required to hold only the production from two hours on either side of the positive sample.

The low number of positive samples speaks to the positive effect that testing has had.

The only positive sample found was taken from the Princeton mailbox.

These included the only sample positive on culture for M tuberculosis.

Most of the positive samples were presumably taken from homosexual men, the survey takers said.

The test line will show as a coloured band in positive samples.