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 [IgAN], n = 14; Henoch-Schonlein purpura nephritis [HSPN], n = 12); and (iii) those with active GN (IgAN, 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.