In oil and natural gas extraction, the drill stem includes the drill pipe, drill collars, bottomhole assembly, and drill bit. A drill stem test (DST) is a procedure for testing the surrounding geological formation through the drill pipe.

Working in El Dorado, Arkansas, in the late 1920s, E.C. Johnston and his brother M.O. Johnston developed the first drillstem tester in 1927 and subsequently refined it in the early 1930s. The test is a measurement of pressure behavior at the drill stem and is a valuable way for an engineer to obtain important sampling information on the formation fluid and to establish the probability of commercial production.

In the 1950s, Schlumberger introduced a method for testing formations using wireline. The Schlumberger formation-testing tool, placed in operation in 1953, fired a shaped charge through a rubber pad that had been expanded in the hole until it was securely fixed in the hole at the depth required. Formation fluids flowed through the perforation and connecting tubing into a container housed inside the tool. When filled, the container was closed, sealing the fluid sample at the formation pressure. The tool was then brought to the surface, where the sample could be examined. In 1956, Schlumberger acquired Johnston Testers and continues to perform drillstem tests around the world.

During normal drilling, fluid is pumped through the drill stem and out the drill bit. Instead, in a drill stem test, fluid from the formation is recovered through the drill stem, while several measurements of pressure are being made.

The basic drill stem test tool consists of a packer or packers, valves or ports that may be opened and closed from the surface, and two or more pressure-recording devices. (A packer is an expanding plug which can be used to seal off sections of the open well, here to isolate them for testing.) The tool is lowered on the drill pipe to the zone to be tested. The packer or packers are set to isolate the zone from the drilling fluid column, and testing measurement begins.