It was also known as a belt monitor, this belt was similar to a medical Feinberger in monitoring its wearer's life signs. To read data from the monitor, medics touch a small control on the buckle and then examine a readout panel under a small flap on the device. The belt monitor is also equipped with a short-range data transmitter, which allows ship-based medical computers to also monitor the wearer. Because the ship's computers pick up the signals through induction into its intercom wiring system, they cannot also determine the wearer's location. Each buckle's data signal is coded, and so a Medical Officer can check the basic life signs of a specific crew member without calling him in for a checkup.
This device is no longer standard-issue for Starfleet personnel, but Intelligence agents can acquire them if they desire. (FASA RPG module: Star Fleet Intelligence Manual: Agent's Orientation Sourcebook)
In the year 2273, Leonard McCoy considered perscan units to be Orwellian. He thought they would prevent the crew from seeing a doctor when they felt ill, having assumed sickbay was already monitoring them. He also felt it was a technical barrier to the intuitive insights a doctor would have during a physical exam. McCoy intended to have them removed in the next uniform design. (TOS novel: Ex Machina)
McCoy’s perscan unit saved his life later in the year 2273. McCoy had been kidnapped by Anton Zauber, a criminal who had been surgically altered to look like McCoy. Zauber took over McCoy’s role aboard the USS Enterprise, keeping McCoy drugged and hidden aboard the ship to answer questions. Suspicious of his change in attitude and behavior, Christine Chapel reviewed McCoy’s perscan data from the previous two weeks and discovered abrupt changes to his vital signs during his visit to Tarsus II, suggesting that he was a completely different person. Yet transporter logs showed that McCoy had beamed back to the ship from Tarsus II. That knowledge led Spock to deduce where Leonard McCoy was hidden before Zauber could kill him. (TOS comic: "The Real McCoy")