Brady, Clayton

The glomerular filtration rate (GFR) is a key indicator of kidney function, yet no ideal method exists for its direct measurement. This two-part review examines the development of GFR measurement techniques, focusing on their physiologic basis, performance characteristics, and clinical applications. Part 1 reviews endogenous markers used to estimate GFR (eGFR), including creatinine- and cystatin C-based equations, with emphasis on their derivation, limitations, and clinical use. Part 2 addresses exogenous filtration markers and measured GFR (mGFR) methodologies, highlighting comparative performance and specific uses. Emphasis is placed on innovation in marker development, highlighting the need for continued development and standardization to enhance accuracy and clinical utility.

Measured glomerular filtration rate (mGFR) remains the reference standard for precise assessment of kidney function when estimated approaches are insufficient. This second part of a two-part series reviews exogenous filtration markers used to obtain mGFR, including inulin, iothalamate, iohexol, and radionuclide-based tracers, with emphasis on their physiologic handling, analytic methods, accuracy, and clinical applicability. The review further examines emerging fluorescent and near-infrared tracers coupled with transcutaneous monitoring technologies that enable rapid or continuous assessment of GFR. Together, these approaches illustrate the evolution of mGFR measurement toward methods that are more practical, scalable, and responsive to dynamic changes in kidney function, while maintaining analytical accuracy.

Abstract Background Cystinuria is an autosomal recessive disorder resulting in poor proximal tubule reabsorption of cystine in the nephron, increasing the risk of cystine stone formation. A fast, inexpensive assay to screen for urinary cystine is needed because cystine stones are difficult to noninvasively differentiate from more common calcium-containing ones. Tandem mass spectrometry (MS/MS) is sensitive and specific but is labor-intensive and costly. Alternatively, a colorimetric assay is fast and cost-effective; however, creatinine interference is an issue. Methods A published cyanide-nitroprusside colorimetric assay was modified for a high-throughput microplate format. Creatinine interference was reduced using 0.1 mol/L PBS and a standard reaction time of 60 s and was further corrected using a formula derived from the slope of multiple creatinine standard curves. Results The limit of blank was determined to be 2.6 mg/L, the limit of detection 11.9 mg/L, and the limit of quantitation 15.3 mg/L. The analytic measurement range was established as 15.3–100 mg/L cystine. Intraassay and interassay CV was calculated to be 9.6% and 8.0%, respectively, for a high-level cystine concentration (83.6 mg/L). Low-level cystine (36.4 mg/L) intraassay and interassay CV was determined to be 18.1% and 17.6%, respectively. Passing–Bablok regression analysis of colorimetric vs LC-MS/MS results revealed a slope of 1.10 and y intercept of −7.14 mg/L, with an overall bias of 2% by Bland–Altman plot analysis. Conclusions We analytically validated a rapid colorimetric assay suitable to quantify urinary cystine. The effect of thiol drugs on this assay remains to be determined.