An apparatus disclosed herein includes a downhole tool having a body defining an interior bore and an exterior surface. The example apparatus also includes first and second fluid ports to provide respective fluid paths between the exterior surface and the interior bore. The first fluid port is to receive wellbore fluid from an annulus of a wellbore in which the downhole tool is to be disposed and the second fluid port is to receive formation fluid extracted from a subterranean formation. The formation fluid is to mix with the wellbore fluid to form an output mixture in the interior bore. The example apparatus also includes a controller to control a concentration of the formation fluid in the output mixture.