The use of a monophosphate ester of a phthalein compound as a substrate to obtain a specific measure of activity and/or amount of Tartrate-Resistant Acid Phospatase 5b (TRAP 5b) in a sample, wherein the phthalein compound is a compound having the formula: wherein: X is CO or SO 2; R 1 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 2 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 3 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 4 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 8 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 9 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 10 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; and each R 11 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; wherein: n is 0, 1 or 2; R 5 and R 6 are each independently hydrogen or R 7; and R 7 is selected from hydrocarbyl, -(CH 2) k -heterocyclyl, -(CH 2) k -C -heterocyclyl, any of which is optionally substituted with 1, 2, 3, 4 or 5 substituents independently selected from halogen, cyano, amino, hydroxy, -COOH and esters thereof (e.g. -C -C 1 - 6 alkyl), phosphoramidite, C 1-6 alkyl and C 1-6 alkoxy; and k is 0, 1, 2, 3, 4, 5 or 6. X is CO or SO 2; R 1 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 2 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 3 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; R 4 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 8 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 9 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; each R 10 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; and each R 11 is selected from hydrogen, halogen, trifluoromethyl, cyano, nitro, -OR 5, -C R 5,-C OR 5, -OC R 5, -S n R 5, -N(R 5)R 6, -C N(R 5)R 6 and R 7; n is 0, 1 or 2; R 5 and R 6 are each independently hydrogen or R 7; and R 7 is selected from hydrocarbyl, -(CH 2) k -heterocyclyl, -(CH 2) k -C -heterocyclyl, any of which is optionally substituted with 1, 2, 3, 4 or 5 substituents independently selected from halogen, cyano, amino, hydroxy, -COOH and esters thereof (e.g. -C -C 1 - 6 alkyl), phosphoramidite, C 1-6 alkyl and C 1-6 alkoxy; and k is 0, 1, 2, 3, 4, 5 or 6.