Zika and dengue viruses remain significant public health threats. These viruses share the same Aedes (Stegomyia) mosquito vectors and geographic distributions but infections cannot be readily distinguished clinically and need to be differentiated from each other, and from other circulating arboviral and non-arboviral pathogens, using laboratory tests. This document provides guidance on current testing strategies for Zika and dengue virus infections with updates to the previous interim guidance for laboratory testing for ZIKV, addressing pregnant and non-pregnant patients respectively, and incorporates current guidance for dengue virus diagnostic testing. The choice of laboratory assays and interpretation of test results require careful consideration of epidemiology, patient history, and limitations of existing diagnostic tests. This interim guidance is for use by staff of laboratories testing for Zika and dengue virus infections and for clinical practitioners and public health professionals providing clinical management or surveillance.