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The GFR in mL/min can be calculated with the following formula: '''GFR''' = ( U<sub>X</sub> · V̇ ) / P<sub>X</sub>. Here, U<sub>X</sub> and P<sub>X</sub> are the concentrations of substance X in urine and plasma in mg/mL respectively, with V̇ being the urine flow in mL/min. Ideally X is a substance that is freely filtered but not secreted or reabsorbed by the kidneys, subsequently having the same concentration in the plasma and glomerular filtrate. <ref name = "Ref21">Pocock, G., Richards, C.D. and Richards, D.A. (2013) Human physiology. Oxford: Oxford University Press.</ref>These criteria are largely met by creatinine, and the creatinine clearance (C<sub>Cr</sub>) obtained from this formula is generally used to measure GFR in clinical practice. <ref name = "Ref22">Delgado, C. et al. (2022) ‘A unifying approach for GFR estimation: Recommendations of the NKF-ASN task force on reassessing the inclusion of race in diagnosing kidney disease’, American Journal of Kidney Diseases, 79(2). doi:10.1053/j.ajkd.2021.08.003.</ref> Other diagnostic tools also include serum creatinine levels (SCr) as in the case of renal dysfunction, the creatinine clearance by the kidneys is reduced and therefore the creatinine concentration in the blood rises . <ref name = "Ref21"/>
== State of the Art ==
=== Matrix ===
The fluid matrix for biosensor measurements in SensUs 2024 is Interstitial skin fluid (ISF). ISF is the most prevalent fluid in the body, making up 75% of extracellular fluid and 15-25% of body weight. <ref name = "Ref24">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I., Felner, E. I., Jones, D. P., Miller, G. W., & Prausnitz, M. R. (2020). Sampling interstitial fluid from human skin using a microneedle patch. Science Translational Medicine, 12(571). https://doi.org/10.1126/scitranslmed.aaw0285
</ref> ISF surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum. <ref name = "Ref25">Samant, Pradnya P, and Mark R Prausnitz. “Mechanisms of Sampling Interstitial Fluid from Skin Using a Microneedle Patch.” Proceedings of the National Academy of Sciences of the United States of America, 2018, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5939066/
</ref> Due to its accessibility and similarity in composition to serum, ISF is a suitable candidate for continuous monitoring <ref name = "Ref23">Friedel, M., Thompson, I. a. P., Kasting, G. B., Polsky, R., Cunningham, D., Soh, H. T., & Heikenfeld, J. (2023b). Opportunities and challenges in the diagnostic utility of dermal interstitial fluid. Nature Biomedical Engineering. https://doi.org/10.1038/s41551-022-00998-9</ref> and is currently used in clinical settings for continuous glucose monitoring (CGM).
== Creatinine Biosensors ==