A drill stem test (DST) is a procedure for isolating and testing the surrounding geological formation through the drill stem. The test is a measurement of pressure behavior at the drill stem and is a valuable way to obtain important sampling information on the formation fluid and to establish the probability of commercial production.

In oil and natural gas extraction, the drill stem includes the drill pipe, drill collars, bottomhole assembly, and drill bit. During normal drilling, fluid is pumped through the drill stem and out the drill bit. In a drill stem test, the drill bit is removed, a drill stem test tool is added, and 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 or cased well, to isolate them for testing.) The drill stem test 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, the tester valve is opened, and testing begins.