β2 microglobulin is a part of the Major Histocompatibility Complex Class 1 (!) molecules that our cells use as I.D. badges. As a cell dies or gets damaged there is a certain chance that this molecule will float off in the blood stream. Since the MHC 1 molecule (and thus β2 microglobulin) is part of almost every cell, there is always some β2 microglobulin detectable in the blood stream. And even though the kidney normally hates to filter proteins, it can't help but filter small ones like β2 microglobulin (and creatinine for that matter). However, it is normally totally reabsorbed and catabolised by the renal tubular cells, so that you don't lose precious protein into your urine.
This is where it gets interesting. If there were a tubular problem in the kidneys (e.g. acute tubular necrosis), β2 microglobulin would still be filtered by the glomeruli, but its reabsorption and catabolism would be impaired. Thus, finding it in the urine is indicative of tubular dysfunction.
On the other hand, finding an elevated level of β2 microglobulin in the blood tells us roughly the same thing as finding an elevated creatinine level in the blood - namely that you are likely to be dealing with renal dysfunction (e.g. chronic kidney disease) that involves (at least) the glomeruli, since you aren't filtering enough of it.
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