Elements in serum serve as important biomarkers and reflect the well-being of an individual. The Key Comparison CCQM-K139 (Elements in human serum) aimed to enable participating National Metrology Institutes (NMIs) and Designated Institutes (DIs) to demonstrate their competence in the determination of elements (electrolytes and essential elements) in human serum. This is in line with the IAWG’s five year plan that a comparison under the Measurement Category 10 (Biological Fluids and Materials) is organised in order to best cover current and future CMCs. Table A summarises the list of measurands covered in this comparison, as well as the list of NMIs/DIs that registered and submitted their results for each measurand. For the analysis of sodium, copper, selenium and phosphorus, most of the participating institutes employed microwave-assisted digestion and acid digestion (with or without heating) sample dissolution. For the analysis of chloride, in addition to the microwave-assisted digestion and acid digestion, a wider variety of techniques were employed. These included matrix separation, alkaline extraction and coulometric titration. For sodium and phosphorus, inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES) were the two most commonly used instrumental techniques. Other techniques used included ion chromatography (IC) and flame atomic absorption spectrometry (FAAS). For chloride, copper and selenium, the most widely used instrumental technique was ICP-MS. Other techniques used included IC, titration, microcoulometry and ICP-OES. In this key comparison, the participating institutes have demonstrated the inorganic core capabilities in human serum matrix as listed in Table B.