Measurement of the amount of tumor necrosis factor (Tnf), a multifunctional proinflammatory cytokine that belongs to the protein superfamily of the same name, in a specified volume of fluid which has escaped from blood vessels and been deposited within tissues or on tissue surfaces as a result of inflammation, a usually localized, protective reaction of tissue to irritation, injury or infection, characterized by pain, redness, swelling and sometimes loss of function.