The disclosure describes a method for detecting conditions indicative of sepsis. In one embodiment of the invention, an increase in the level of 3-chlorotyrosine or 3-bromotyrosine from the normal level in a sample of body fluid or tissue is indicative of early sepsis or infection. In another embodiment of the invention, the level of 3-chlorotyrosine or 3-bromotyrosine is measured or monitored to determine the response to therapeutic treatment of the infective condition in which a reduction in the level that existed prior to the treatment is an early sign or indication that the treatment is working in vivo. In a preferred embodiment, the method of the invention is illustrated in a clinically relevant mouse model of sepsis.