The present invention provides an improved method of preparing a 4,7-di-O-alkyl chromogenic ketoside of N-acetylneuraminic acid (Neu5Ac) for use in detecting influenza virus types A and B. The ketosides are substrates that are selectively cleaved by a neuraminidase on influenza virus, but not be neuraminidases found on other viruses or on bacteria. The synthesis is efficient and provides large quantities of the ketoside for commercial development. The synthesis includes a step of alkylating the 4- and 7-hydroxyl groups of a protected alkyl ester alkyl ketoside derivative of Neu5Ac by a process that comprises contacting the derivative with a composition comprising an alkyl halide to form a 4,7-di-O-alkyl protected alkyl ester alkyl ketoside derivative of Neu5Ac. The synthesis alternatively includes protecting the 8- and 9-hydroxyl groups of an alkyl ester alkyl ketoside derivative of Neu5Ac by forming an 8,9-epoxide protected alkyl ester alkyl ketoside derivative of Neu5Ac.