I have found limited literature listing concentrations of serum AGE levels (ng/ml). Several studies have reported results in AU (absorption units) which cannot be compared between different studies. I would be grateful if anyone could provide me with some reference for measuring AGE concentrations in serum or plasma. Any studies that have used ELISA for the measurement of total AGE would be of most relevance.
You can't really measure total AGE in ng, because it's a heterogenous mixture of a large number of very different structures. All the components probably aren't even fully known yet.
There are even legitimate philosophical questions about what molecules count as "AGE". How long-lived exactly does it need to be in order to qualify? For example this is very relevant to the highly abundant fructoselysine. It's fairly long lived, but given enough time, it does react on. Is it an AGE or not? It depends on who you ask!
The precise mixture, and therefore the extinction coefficient is going to depend on how you make it. In-vitro made AGEs by different methods could well be different from in-vivo AGEs at different donor age & diabetic status. Nobody can know this, without knowing the total molecular composition of their mix. Measuring it is extremely challenging. And it is scientifically impossible to know for sure that you know all of it.
So, that's why people specify the AU. It's what they can measure. The total molecular composition, they cannot measure, let alone agree on. But you are also right -- AU is far from perfect, because some AGEs don't even contribute to the AU (say hello again to fructoselysine!). It is merely the best anyone can do with reasonable effort.
So, there's no answer to your question and you need to deal with that somehow. Good luck :)
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