Krumova, Stefka, et al. “Dried Blood Spots As a Clinical Samples for Laboratory Diagnosis and Surveillance of Vaccine-Preventable Diseases in Bulgaria”. American Scientific Research Journal for Engineering, Technology, and Sciences, vol. 51, no. 1, Jan. 2019, pp. 183-91, https://asrjetsjournal.org/American_Scientific_Journal/article/view/4611.