How about deuterated water (D20)? http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0032137

 Dear Laura,

Techniques for measuring renal function employing  a number of exogenous [inulin, iothalamate, ethylenediaminetetra-acetic acid (EDTA) and iohexol] and endogenous filtration markers (creatinine and cystatin C) [Radioactive & Non-radioactive] have been studied and are discussed in detail in the following review article entitled "Measurement of renal function in patients with chronic kidney disease ":

The compounds in bold (iothalamate & iohexol) are nonradioactive.

Br J Clin Pharmacol. 2013 Oct; 76(4): 504–515.

Published online 2013 Jun 26. doi:  10.1111/bcp.12198

PMCID: PMC3791974

Measurement of renal function in patients with chronic kidney disease

Euan A Sandilands,1 Neeraj Dhaun,2,3 James W Dear,1,2 and David J Webb2

Author information ► Article notes ► Copyright and License information ►

Abstract

Chronic kidney disease affects millions of people worldwide and is associated with an increased morbidity and mortality as a result of kidney failure and cardiovascular disease. Accurate assessment of kidney function is important in the clinical setting as a screening tool and for monitoring disease progression and guiding prognosis. In clinical research, the development of new methods to measure kidney function accurately is important in the search for new therapeutic targets and the discovery of novel biomarkers to aid early identification of kidney injury. This review considers different methods for measuring kidney function and their contribution to the improvement of detection, monitoring and treatment of chronic kidney disease.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3791974/

Hoping this will be helpful,

Rafik

NGAL levels clearly correlate with severity of renal impairment, probably expressing the degree of active damage underlying the chronic condition. For all these reasons, NGAL may become one of the most promising next-generation biomarkers in clinical nephrology and beyond.

NGAL:

Neutrophil gelatinase–associated lipocalin

iohexol clearance is to date considered the best way,

regards,

Maurizio Salvadori