β-Trace Protein in Serum: A New Marker of Glomerular Filtration Rate in the Creatinine-Blind Range

Recent studies have shown that low-molecular weight proteins in serum seem to be better markers for detecting reduced glomerular filtration rate (GFR) than the conventional measurement of serum creatinine (1)(2). Several proteins, such as ribonuclease, α1-microglobulin, β2-microglobulin (B2M), and cystatin C have been compared with serum creatinine (2)(3). Whereas serum creatinine is increased only after a reduction of ∼50% in GFR, the above proteins, especially cystatin C, are already increased within that so-called creatinine-blind range (1)(4)(5)(6). Recently, another low-molecular weight protein, the β-trace protein (BTP), isolated primarily from cerebrospinal fluid (7), was shown to be increased in patients with renal diseases (8)(9). However, there is no information on the relationship of BTP and a standard measure of GFR such as inulin clearance. Therefore, to investigate the potential clinical usefulness of BTP for early detection of reduced GFR, we have performed corresponding measurements of BTP and inulin clearance. The study included 115 diabetic patients (44 women; mean age, 53.4 years; 71 men, mean age, 52.9 years); 57 had type I diabetes and 58 had type II. GFR was determined by measuring the inulin clearance. A priming dose of 2.5 g of inulin (Inutest; Laevosan) was administered intravenously within 30 s, followed by a constant infusion (15 mL/h) of inulin at a rate dependent on serum creatinine (infusion rate, 1.5 g/h for serum creatinine concentrations <130 μmol/L; 0.75 g/h for serum creatinine concentrations between 130 and 265 μmol/L; and 0.38 g/h for serum creatinine concentrations between 265 and 450 μmol/L).