In order to ensure the delivery of a service of the highest possible quality, it is an essential requirement that laboratories undertake strict internal quality control (QC) measures as well as participate in external quality assessment (EQA) schemes. For any given test, a critical part of the internal QC process involves the establishment of reference intervals using samples taken from normal individuals, and then calculating limits representing the 95% range. This forms the basis for assessment of abnormal test results, which will in turn impact on laboratory performance in proficiency testing exercises in EQA programmes. Whereas for plasma-based assay systems, variability in performance in EQA exercises is usually determined by measurement of a coefficient of variation (CV), results of genetic testing is usually measured in absolute terms. Despite this, results of genetic EQA programmes confirm that errors in testing do occur, as much because of inadvertent sample switching and transcription errors as to analytical mistakes. EQA programmes involving identification of mutations by DNA sequencing, such as haemophilia, is made difficult by the high information content of sequence data. Nevertheless, results show that errors are usually made in the naming of the mutations, indicating that this is an evolving and poorly standardized area. Developing countries face particular challenges in the encouragement of laboratories to participate in local EQA programmes, as well as in relation to the logistical issues of sample provision, distribution and result collation in an effective and affordable manner.