Albertine Donker

General Discussion 307 9 and measurement procedure. As a first step, harmonization ensures traceability to a reference system agreed on by convention. The next step is standardization, which leads to the “true” value, by ensuring traceability to the International System of Units. 12,13 Standardization, but at least harmonization, is paramount regarding serum hepcidin measurements procedures. Until now, serum hepcidin measurements procedures had not been standardized nor harmonized. 14,15 Therefore, our studies performed with a standardized hepcidin assay are unique and allow the definition of international reference values and clinical decision limits, and as such have the potential of bringing clinical care and research regarding iron disorders to a higher level. Limitations of this thesis Our studies have several limitations that should be taken into account. These include the retrospective description of patients in our case series. Furthermore, in the diagnostic studies, the majority of patients were of Dutch ancestry and thus of Western European origin. Moreover, in the study on serum reference values of hepcidin in children ( Chapter 7 4 ) the age distribution of the participants was skewed; young children were underrepresented. Another important point is that our patients with IRIDA and XLSA were diagnosed by traditional pathways, following the conventional sequence of history, examination, standard hematological and biochemical assays, possibly a bone marrow aspirate and at last genetic analysis, revealing the relevant gene defect. This approach requires a high index of suspicion from the clinician and might result in selection bias regarding instigating genetic testing for IRIDA, XLSA and other microcytic anemias due to disorders of iron metabolism and heme synthesis. Nowadays, the quick advance of next generation sequence (NGS) techniques (particular whole exome sequencing (WES)) 16-18 is changing the work-up of patients with rare diseases, also in the field of benign hematology. 18,19 Employing WES in the diagnostic process is altering the known phenotypic spectrum of diseases, as illustrated by the case series of a family with multiple females with a macrocytic dyserythropoietic anemia that turned out to be caused by an X-linked ALAS2 defect. This presentation is different from the known classical phenotype of ALAS2 defects as a combination of microcytic anemia and iron loading in male patients . 19 Another example is the description by Khuong-Quang of two siblings suffering from unexplained severe microcytic anemia, hypoferremia and hyperferritinemia, responding to some extent