Tricky one. Pretty much every single blood composition parameter is variable on a population level (and over time within a single individual). Probably the least variable in terms of percentage is Na+ concentration. Cl- and Ca2+ are also fairly tightly controlled. But I don't think there's a single compound that will do it. If you're looking for a value to normalise to, your best bet might be a combination of two or three relatively constant parameters

Hello Alexander

You're right. I am looking for a compound to normalize my samples. At first, I was imagining a organic compound (or, like you said, a set of organic compounds) that could absorb to use this parameter as normalization factor. The problem of variability according to time I wouldn't have since the subjects are pretty much the same age. But I will consider another possibilities now using these ions. Thanks for the insights!

The water activity of plasma has very low inter-individual variation, so you could use serum osmolality.

Thank you Jukka! For sure I will consider that too. I checked and the inter-individual variability is supposed to be less then 2%!

Hi Vinicius try some TAG, some of them do not change even under fasting and caloric restriction.

Thanks Du! I will check these compounds too!