A material which is used to verify the performance of an assay intended to be used for the quantitative measurement of multiple blood gas, haemoximetry and/or electrolyte analytes in a clinical sample. Analytes detected may include partial pressure of oxygen (pO2), partial pressure of carbon dioxide (pCO2), pH, sodium, potassium, ionized calcium, chloride, glucose, lactate, total haemoglobin (totHb), haematocrit, and haemoglobin derivatives such as reduced haemoglobin (RHb), methaemoglobin (MetHb), sulfhaemoglobin (SHb).