An enzymatic method for the determination of plasma formate concentration is described. Formate dehydrogenase is used to reduce NAD+ to NADH in the presence of formate. The resulting NADH then reduces the dye resazurin to resorufin, a reaction catalyzed by the endogenous diaphorase of the plasma. The generated resorufin is then measured fluorimetrically by exciting it at 565 nm and quantitating the emitted light at 590 nm. The method uses the patient's plasma as the blank and as the matrix for the construction of a patient-specific formate calibration curve. The blank contains all components of the assay system except the formate dehydrogenase. Formate concentration is determined from the calibration curve, constructed by adding known quantities of sodium formate to the plasma base, and plotting the fluorescence intensity against formate concentration. The assay which is sensitive to a formate level of 7 mg/L should find application in cases where formate is a metabolite.