Urine- and serum beta 2-microglobulin in patients with rheumatoid arthritis: a study of 101 patients without signs of kidney disease.

Serum levels and 24-hour urinary excretion of beta 2-Microglobulin (beta 2-M) was investigated in a group of 101 patients with rheumatoid arthritis (RA), without any other signs of renal disease in past or present, and in a comparable control group. Elevated 24-hour urinary beta 2-M excretion, due to renal proximal tubules dysfunction, was observed in 19% of the patients and not in the control group. There was a significant correlation with clinical signs of extra-articular RA. It is postulated that the observed increase may be an early symptom of renal involvement in RA. Elevated serum beta 2 levels, corrected for glomerular filtration rate, were observed in 44% of the RA patients and only in 3% of the control group and correlated with clinical signs of a more severe RA, as well as with increased 24-hour urinary beta 2-M excretion.