In kidney diseases, tubule-interstitium has become much more relevant, as formerly only the glomerulus was considered to have the main importance. Kidney's tubular atrophy and interstitital fibrosis is now recognized as long term prognostic value. We aim to evaluate the function of the kidney's tubule-interstitium through furosemide excretion after intravenous administration of this drug, and correlate the rate of excretion of furosemide with interstitial fibrosis findings in scheduled kidney biopsy for patients with chronic kidney disease.

All patients scheduled for kidney biopsy will be invited to participate in a standard isovolemic furosemide stress test before the biopsy procedure. The latter will consist in 1.5 miligrams per kilogram of weight of intravenous furosemide administration, with urinary output follow up and measurement. The urinary output will be replaced intravenously with normal saline to avoid dehydration and/or hypotension. A standard 5ml sample of blood will be taken before the test and aliquots of urine before and after furosemide administration will be frozen.

After completion of the formely mentioned test, patients will proceed to their scheduled kidney biopsy along with interventions considered appropiate by their attending physician.

Kidney biopsy sample will be examined by histopathology service and the degree of interstitial fibrosis will be described. The degree of fibrosis will be correlated with the degree furosemide excretion and urinary output.

Inclusion Criteria:

• Accept to participate (informed consent)
• Legal age or older
• An estimated glomerular filtration rate greater than 15ml/min/1.73m2, calculated by CKD-EPI.

Exclusion Criteria:

• Known Alergic reaction to furosemide
• Contraindication to kidney biopsy
• Patient already in renal replacement therapy