Urinary N-glycoproteins from the PRM-SSS discrepancy: a novel diagnostic tool for tubular injury

Tubular injury is a key pathological feature driving the progression of both acute and chronic kidney disease. This study aimed to elucidate the mechanism underlying the discrepancy between two common urinary total protein assays (the pyrogallol red-molybdate [PRM] and sulfosalicylic acid‑sodium sulfate [SSS] methods) and to evaluate its potential as a novel biomarker for tubular injury.

We analyzed 489 clinical urine samples using both assays. Proteins associated with the discrepancy were profiled by mass spectrometry and SDS-PAGE. To validate the role of glycosylation, detection of the native α1-microglobulin and uromodulin was assessed before and after PNGase F-mediated deglycosylation. Diagnostic performance was evaluated in patients categorized by the urinary albumin-to-protein ratio (uAPR).

Despite a strong correlation between methods (r = 0.986), a systematic bias existed. Mass spectrometry demonstrated that the SSS method underestimates N-linked glycoproteins, which was reversible upon deglycosylation. In patients with proteinuria (150-2000 mg/L), the PRM-SSS discrepancy was significantly greater in those with tubular injury (uAPR≤0.4) versus glomerular injury (uAPR>0.4). The PRM-SSS discrepancy distinguished these groups with an area under the curve of 0.93, corresponding to a sensitivity of 84.8% and a specificity of 93.2%.

The PRM-SSS discrepancy originates directly from differential detection of N-linked glycoproteins. Leveraging this inherent difference provides a cost-effective, readily available biomarker for tubular injury, suitable for large-scale screening and potentially enhancing early detection.