A collection fitting, system, and method for sampling fluid from a tissue site is described. The collection fitting includes a housing having a central passage, a first union, and a second union. The first union and the second union are in fluid communication with the central passage. The collection fitting also includes a plunger disposed in the housing and movable between a sampling position and a bypass position. The plunger includes a bypass passage configured to fluidly couple the first union and the second union through the central passage in the bypass position. The collection fitting also includes a cap having a boss configured to be inserted into the central passage. The boss includes a first inlet and a second inlet and inserts into the central passage to move the plunger. The first and second inlet fluidly couple the first union and the second union in the sampling position.
