Dharmasetiawani, Nani, et al. “Quantitative Analysis of Lactose and Lactulose in Urine by High Performance Liquid Chromatography for Determination of Intestinal Lactase Activity”. Medical Journal of Indonesia, vol. 12, no. 1, Feb. 2003, pp. 8-12, doi:10.13181/mji.v12i1.84.