Acquired factor XI deficiency in a child with membranoproliferative glomerulonephritis.

We describe a 7-year-old male with membranoproliferative glomerulonephritis who presented with nephrotic syndrome and subsequently developed factor XI (fXI) deficiency. An association between these conditions has not been described previously. In this case, fXI deficiency was caused by an antibody to fXI that enhanced clearance of the protein from plasma. Loss of fXI in the urine did not appear to be involved. Antibody-mediated clearance of prothrombin or factor X can cause acquired deficiencies of these proteins. This is the first report, to our knowledge, of an antibody that causes fXI deficiency by this mechanism.