**Preface to "Biomarkers of Renal Diseases"**

The National Institutes of Health (NIH) Biomarkers Definitions Group has defined a biomarker as "A characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention." For acute or chronic kidney diseases, the ideal biomarker should, among others, show rapid and reliable changes with the progression of the disease and be highly sensitive and specific, be able to detect injury to the different segments of the nephron, and be rapidly and easily measurable. Creatinine, for instance, is not a good renal marker since acute injuries would not show changes in filtration rate until the progression of the disease allows its accumulation. Similarly, in chronic renal disease, the elevation in serum creatinine is a late indicator of the reduction in glomerular filtration. Other conventional biomarkers such as proteinuria, cell cylinders, and fractional excretion of sodium have shown a lack of sensitivity and specificity for the early recognition of acute kidney injury, leading to the need for and the enormous interest surrounding the possibility of using other biomarkers with the ability to perform early detection, differential diagnosis, prognostic assessment, response to treatment, and functional recovery. In this Special Issue, we have published reviews and experimental papers showing significant advances in the field of renal biomarkers.

> **Joaqu´ın Garc´ıa-Esta ˜n, Felix Vargas** *Editors*
