Urinary fibrin/fibrinogen degradation products measured using an anti‐fibrinogen antibody predict orthostatic proteinuria

Background The aim of this study was to assess the diagnostic value of urinary fibrin/fibrinogen degradation products ( uFDP ) measured using an anti‐fibrinogen antibody in patients with orthostatic proteinuria ( OP ), and their use in differentiating between OP and glomerulonephritis ( GN ). Methods uFDP were measured using first urine in the morning (supine) and non‐first urine during a hospital visit (upright) and then normalized to urine creatinine ( uFDP /Cr, ng/mgCr). We compared (i) OP patients ( n = 16); (ii) those in remission from nephrotic syndrome ( NS , n = 14) and from GN (IgA nephropathy [Ig AN ], n = 14; Henoch–Schönlein purpura nephritis [ HSPN ], n = 12); and (iii) those with active GN (Ig AN , n = 12; HSPN , n = 19). Results The uFDP /Cr ratio increased from supine to upright urine in patients with OP ( P < 0.001), but decreased in one case. uFDP were excreted in supine urine in 94% of OP patients, with no excretion in NS remission patients or in 92% of GN remission patients ( P < 0.001 for both). uFDP /Cr in supine urine was similar between the OP and active GN patients ( P = 0.40), whereas proteinuria in supine urine was in the normal range in all OP patients, but was significantly higher in upright urine in the OP patients ( P < 0.001). In upright urine, urinary protein/creatinine ratio was significantly lower in patients with OP than in those with active GN ( P = 0.005). A uFDP /Cr ratio cut‐off of 1,108 ng/mgCr in upright urine correctly differentiated OP from active GN , with a sensitivity of 87.5% and a specificity of 100%. Conclusion Comparison of uFDP levels in supine/upright urine can be reliable for diagnosing OP and for differentiating it from active GN .