Are urinary levels of free light chains of immunoglobulins useful markers for differentiating between systemic lupus erythematosus and infection?

INTRODUCTION

Active systemic lupus erythematosus (SLE) and infection are often hard to distinguish. We evaluated the urinary levels of free light chains of immunoglobulins (urFLCIg) as a possible laboratory marker to differentiate those conditions.

METHODS

We evaluated 43 patients with lupus nephritis (16-63 years old), with or without concurrent infection (12 with infection), 14 with infectious disease without SLE and 20 with idiopathic Fanconi's syndrome. The Systemic Lupus Erythematosus Disease Activity Index was utilized to establish activity of disease. Levels of urFLCIg kappa and lambda were determined by an immunoenzymometric assay developed in our institution. In order to evaluate proximal tubular dysfunction which could be responsible for increased levels of urFLCIg, we determined the low-molecular-weight protein urRBP.

RESULTS

Levels of urFLCIg in healthy volunteers (median kappa 1.57 mg/l; lambda 0.96 mg/l), inactive SLE (5.36; 4.93) and active SLE (11.82; 23.59) were significantly different; urFLCIg levels or urFLCIg/urRBP ratios could not separate patients with infection from those with SLE.

CONCLUSION

Our data show that convoluted proximal tubular dysfunction was not responsible for the increase in urFLCIg levels. UrFLCIg determination was useful in the detection of SLE activity, but was unable to distinguish activity from infection in this condition.