FGF23 and neoplasm: This may be related to the relatively long half-life of intact FGF-23 (58 ± 34 minutes).(18) When a tissue secretes a factor with a relatively long half-life, the sampling must take place relatively close to the tumor, and it is likely in the cases categorized here as true negatives that we were not able to get the catheter tip close enough to the tumor.