Among the instruments of the 23rd century biobed was the K3 indicator. This instrument, not normally active, measured the level of pain experienced by a patient. While treating Spock for neural parasite infiltration, Doctor McCoy activated this indicator to illustrate to Captain Kirk how much pain the parasite was inflicting. The indicator rose variably between 75% to 85% of its maximum; McCoy commented that it was "no wonder the poor devils go mad"; implying that this was a considerable degree of pain. When Spock continued to speak the indicator increased to nearly 100%. (TOS: "Operation -- Annihilate!")