Abstract: An apparatus for automatically drawing a blood sample from a sealed container having an internal pressure below that of atmospheric pressure comprises an aspirating needle which serves to pierce the sealed container to aspirate liquid from the container, and to vent the interior of the container to the surrounding atmosphere through a suitable vent line connected to the needle. Such apparatus further comprises a vacuum pump connected to the needle through an aspiration line for aspirating blood from the sealed container. To warn the user of the possibility that fluid (e.g. cleansing diluent) in the aspirating needle has been drawn into the sealed container in the event of a vent line failure, apparatus is provided for detecting the pressure in the container immediately after the aspirating needle has pierced the container seal, and for comparing this pressure with a predetermined nominal value (typically, the negative pressure rating of the vacuum pump).