It is likely that your patient's tolerance to the Good Start formulas relate to their partially hydrolyzed protein. A similar example was published in JACI In Practice in 2015 (1). Importantly, not all cow's milk allergic patients will similarly tolerate this.
1. Lee, T. D., G. Gimenez, G. Grishina, M. Mishoe, H. A. Sampson, and S. Bunyavanich. "Profile of a Milk-Allergic Patient Who Tolerated Partially Hydrolyzed Whey Formula." J Allergy Clin Immunol Pract 3, no. 1 (Jan-Feb 2015): 116-8.
Hope you find this information helpful.
Daniel Jackson, MD FAAAAI