CST3 and acute kidney injury: Other proteins reflected AKI as also indicated by increased serum creatinine levels in cluster B. The low-molecular-weight proteins complement factor D (CFD) and beta-2 microglobulin (B2M), which normally freely pass the glomerular filtration, were significantly increased in cluster B (Fig. 3b) and correlated with the established marker of renal function, Cystatin C (CST3), as well as with each other (Fig. 3c).