Activation of an enzyme in a bodily fluid is detected based on the amount
of cleavage of a substrate for the enzyme. The substrate is tagged with
two fluorescent dyes--a donor and an acceptor. The tagged substrate is
presented to the bodily fluid. A device emits energy at a first
wavelength into the bodily fluid, and detects energy at second and third
wavelengths emitted by the dyes in response to the energy at the first
wavelength. Prior to enzymatic cleavage of the substrate, the acceptor
emits energy at the third wavelength in response to energy at the second
wavelength received through fluorescent resonant energy transfer (FRET)
from the donor. After enzymatic cleavage of the substrate, the donor
emits energy at the second wavelength. The device can determine the
concentration of activated enzyme within the bodily fluid based on the
relative intensities of energy at the second and third wavelengths.