Electrochemical assay

The invention provides an electrochemical assay for a phenol analyte in a body fluid sample wherein said sample, or fluid therefrom, is contacted with the working electrode of an electrode assembly comprising a working electrode, a counter-electrode, a voltage supply to said working and counter-electrodes and a current meter for determining the current between said working and counter-electrodes, and wherein a first compound capable of reversible oxidation and reduction is disposed at said working electrode, said first compound being capable in either an oxidized or a reduced form of binding to said phenol analyte, characterized in that said first compound in its oxidized or reduced form comprises a group of structure I R1-NH-C*-(C*-C*)n-C*-QR′ (I) (where n is 0 or 1; Q is 0, S, NH or NR′; C*-(C*-C*)n-C* is a two or four carbon string in a conjugated delocalised electron system optionally substituted by a group comprising R; R is an electron-donating or withdrawing substituent; and R′ is H or a group R, at least one group R′ being a group R).