Measured GFR in Routine Clinical Practice-The Promise of Dried Blood Spots

Abstract

Accurate determination of glomerular filtration rate (GFR) is crucial for the diagnosis of kidney disease. Estimated GFR (eGFR) calculated by serum creatinine and/or cystatin C is a mainstay in clinical practice and epidemiologic research but lacks precision and accuracy until GFR <60 mL/min/1.73 m². Furthermore, eGFR may not precisely and accurately represent changes in GFR longitudinally. The lack of precision and accuracy is of concern in populations at high risk for kidney disease, as the dissociation between changes in eGFR and GFR may lead to missed diagnoses of early kidney disease. Therefore, improved methods to quantify GFR are needed. Whereas direct measures of GFR have been too cumbersome for screening and ambulatory care, a practical method of measuring GFR by iohexol clearance using dried capillary blood spots exists. In this review, we examine the current literature and data addressing GFR measurements by dried capillary blood spots and its potential application in high-risk groups.

Keywords: Dried blood spots; Glomerular Filtration Rate; Iohexol; Iothalamate.

Figure 1. Comparison between GFR-DBS and GFR by iohexol clearance in plasma

Bland–Altman plots of the mean difference (dashed line) and 95% CI (dotted line) between each DBS by 2-time points and 5-time points minus the gold standard (GFR by iohexol clearance measured in plasma) plotted against the mean of the two measures. Figure adapted and modified from Maahs DM, Bushman L, Kerr B, Ellis SL, Pyle L, McFann K, et al. A practical method to measure GFR in people with type 1 diabetes. J Diabetes Complications. 2014;28(5):667-73.

Figure 2. Example of GFR-DBS Sample Collection Algorithm

Figure 3. Examples of Unacceptable, Acceptable and Ideal DBS

The DBS on the left represents an unacceptable spot, due to the small amount of blood, layering and abrasion of the sample which degrades sample quality. The DBS in the center is an acceptable spot for analysis – the blood does not quite reach the edges of the spot but it is adequate in size to provide reasonably accurate results. The DBS on the right is an example of an ideal blood spot which is uniform in color and touches the edges of the entire circle.