The present disclosure provides methods for determining whether a patient exhibiting cystic fibrosis symptoms, or a patient at risk for cystic fibrosis, will benefit from treatment with one or more anti-cystic fibrosis therapeutic agents. These methods are based on detecting hereditary cystic fibrosis related mutations in small-volume dried biological fluid samples that are collected using a volumetric absorptive microsampling device. Kits for use in practicing the methods are also provided.