A body-mountable pyruvate sensing device includes an electrochemical sensor embedded in a polymeric material configured for mounting to a surface of an eye. The electrochemical sensor includes a working electrode, a reference electrode, and a reagent localized near the working electrode that selectively reacts with pyruvate. Application of a voltage between the working electrode and the reference electrode causes a current related to a concentration of pyruvate in a fluid to which the electrochemical sensor is exposed; the current is measured by the body-mountable device and wirelessly communicated.