An in-line tester (100) comprising: an inlet (16), connectable, in use, to an aspirator tube (18)； an outlet (20) connectable, in use, to a vacuum source (22)； and a chamber (14) interposed between the inlet (16) and the outlet (20), the chamber (14) comprising: a first tester (108) arranged such that fluids drawn into the chamber (14) by the vacuum source (22) come into contact therewith； a second tester (122) arranged such that fluids drawn into the chamber (14) by the vacuum source (22) pass through it； and a reservoir (114) interposed between the first tester (108) and the second tester (122)； wherein the first tester (108) comprises a first colorimetric substance adapted to exhibit a colour change in the presence of a target substance, and wherein the second tester (122) comprises colorimetric capnometer, which exhibits a colour change in the presence of carbon dioxide, the in-line tester (100) further comprising a liquid-stop device (52) which, when dry, permits the passage of gas from the inlet (16) to the outlet (20), but when wetted by a liquid, inhibits or prevents the passage of fluids from the inlet (16) to the outlet (20).
