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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"/>
== Creatinine Biosensors ==
Creatinine is a key indicator of renal function and is measured using various methods. The Jaffe reaction involves creatinine reacting with alkaline picrate to form a measurable orange-red complex, but its drawback lies in low specificity, due to interference from substances like glucose and bilirubin. <ref name = "Ref38">Creatinine - SensUS Wiki. (n.d.). https://wiki.sensus.org/index.php?title=Creatinine
</ref> Also enzymatic techniques are used for creatinine detection, e.g. creatininase amidohydrolase or creatinine deaminase in conjunction with other enzymes to convert creatinine to creatine and subsequently produce measurable hydrogen peroxide. <ref name = "Ref38"/> While enzymatic sensors are specific and sensitive, they have their drawbacks in terms of lack of stability and sensitivity to changes in pH, temperature and humidity. <ref name = "Ref38"/>
Commercially available analytical systems, such as Abbott's i-STAT system and Nova Biomedical's StatSensor CREAT, leverage enzymes and electrochemistry to provide creatinine measurements, offering a linear correlation between current and creatinine concentration. <ref name = "Ref38"/>
Potentiometric creatinine biosensors have been developed using different immobilization techniques and enzyme combinations. Potentiometric biosensors for creatinine detection rely on creatinine iminohydrolase (CIH) and subsequent ammonia detection. The sensors exhibit a linear range of 0.02 – 20.0 mM and a minimum detection limit of 10 µM, with 30 – 60 s response time. <ref name = "Ref37">Pundir, C., Kumar, P., & Jaiwal, R. (2019b). Biosensing methods for determination of creatinine: A review. Biosensors and Bioelectronics, 126, 707–724. https://doi.org/10.1016/j.bios.2018.11.031
</ref>
== References ==
