The bone-derived hormone fibroblast growth factor 23 (FGF23) controls phosphorus and vitamin D homeostasis and is used as a marker in chronic kidney disease/dialysis research. It is rarely requested diagnostically, but important in case of unexplained hyperphosphatemia. For determination of FGF23, we currently rely on four immunoassay manufacturers. Immutopics, Kainos, Millipore and DiaSorin have all marketed assays for determination of the intact 251 amino-acid long protein. Additionally, Immutopics has developed an assay which determines both the intact protein and the 72 amino-acid C-terminal fragment of FGF23, formed after cleavage by furin. Unfortunately, these five assays are neither standardized nor harmonized and differ substantially in their reported values. This, in turn, has led to scepticism regarding their suitability in research and particularly clinical diagnostics. One factor that attributes to their lack of comparability and forthcoming questionable legitimacy is the supposed instability of intact FGF23 post-venipuncture. This protein instability may emerge in whole blood, before centrifugation and removal of blood cells and clotting factors, and/or after centrifugation, in serum or plasma. Various studies have reported their finding regarding this matter, but results have not always agreed. We have therefore performed additional experiments to elucidate some of the inconsistencies and account for the still remaining hiatus we observed in the available literature. We concluded FGF23 instability does not seem to be an issue any longer after introduction of the 2^nd generation intact and C-terminal FGF23 assays by Immutopics and the automated intact FGF23 assay from Diasorin. Addition of protease inhibitors in the collection tubes to prevent degradation is not required. Centrifugation and subsequent removal of blood cells and clothing factor should nonetheless not be delayed, but completed within 30 min to one hour to prevent regression of the antibodies’ effectiveness to recognize FGF23.