Urinary carbonic anhydrase 1 as a marker of hematuria in IgA nephropathy

<p dir="ltr">To the Editor, In immunoglobulin A nephropathy (IgAN) hematuria is a risk factor for the development of kidney failure and a decrease in hematuria is associated with improved kidney survival [1]. Hematuria can be quantified by (automated) conventional bright field microscopy of the urinary sediment. However, because erythrocytes lyse rapidly in urine, it is difficult to reliably measure hematuria in urine samples that are collected and centrally stored in large multicenter studies. As a result, treatment effects on hematuria are often not reported in clinical trials. Additionally, it has been reported that automated microscopic analysis underestimates the severity of hematuria in kidney diseases [2–4]. Therefore, there is a need for a quantitative marker of hematuria that can be measured in a standardized manner in stored urine samples.We investigated whether carbonic anhydrase 1 (CA1) is a useful marker of hematuria that can serve as an alternative to microscopic erythrocyte quantification in urine. Carbonic anhydrases (CA) are zinc-metallo enzymes, responsible for catalyzing the reversible hydration of carbon dioxide, of which 12 isoforms exist. CA1 is primarily located in the cytosol of red blood cells and is the most abundant protein in erythrocytes, besides hemoglobin [5, 6]. [Opening paragraph]</p>