APC and hyperinsulinemic hypoglycemia, familial, 4: The significance of protein C arises from published literature on activated protein C. Whilst we did not measure APC in this study, it is well recognised that circulating levels of APC strongly correlate (r = 0.75, p < 0.0001) with levels of protein C antigen in both healthy men and individuals with protein C deficiency suggesting that the circulating protein C concentration is the limiting factor in the rate of activation of protein C [33].