External proficiency testing programmes in laboratory diagnoses of inherited metabolic disorders

Introduction: This paper shows the importance and value of external proficiency testing programmes in monitoring and improving a laboratory's diagnostic skills. It reviews and documents the wide variety of inherited metabolic disorders (IMDs) encountered in the programmes organised by the Human Genetics Society of Australasia and the College of American Pathologists. Materials and Methods: The programmes used actual patient specimens to assess a laboratory's ability to provide diagnoses based on laboratory tests results and brief clinical information. Participating laboratory was also required to suggest additional test(s) to confirm diagnoses. Results: The results of diagnoses on 116 samples were reviewed. Altogether 49 IMDs were encountered, including 26 organic acidurias, 16 aminoacidurias, 3 urea cycle defects, 5 mucopolysaccharidoses, and 1 each of mucolipidosis and purine disorder. Our report for 21 of the 116 samples (18.1%) deviated from the actual diagnoses. Deviations from the final diagnoses were recorded along with the reasons for them. The main reasons for the deviations were: the lack of standards for recognising metabolites of pathognomonic significance, absence of characteristic metabolites in samples collected during treatment, the presence of misleading unusual metabolites, inadequate clinical information, and inability to perform additional tests due to insufficient specimens. Conclusions: The programmes provided a wide variety of IMDs, some of which we have yet to encounter in our patients. They also enabled us to learn about the varied biochemical manifestations at different stages of disease and the identity of previously unidentified metabolites. They enhanced our knowledge and experience and improved our diagnostic skills.