This document provides detailed guidance on laboratory testing for suspected diphtheria cases during significant outbreaks or in low-resource settings. It aims to supplement and build on other existing WHO guidance documents on surveillance standards, diagnostics and research on Corynebacterium by providing key considerations for laboratories that allow the rationalization and optimization of testing during outbreaks. The recommendations given here have been prepared by WHO in consultation with, and reviewed by, global experts with experience in laboratory analysis of Corynebacterium species and in outbreak settings, or with expertise in developing new technologies for diphtheria research and diagnosis. Unless otherwise stated, the considerations provided apply to diphtheria outbreaks caused by toxin-producing C. diphtheriae with a classical respiratory diphtheria presentation.