Correction of slope-intercept glomerular filtration rate measurement without scaling for body size

The aim of this study was to evaluate a slope-intercept glomerular filtration rate (GFR) one-compartment correction method based exclusively on the rate constant (α2) of the exponential between 2 and 4 h post-injection that requires no scaling for BSA.

The correction factor is 1/([C.α2]+1). C depends on the difference between one-compartment-corrected and uncorrected GFR, so varies with different correction procedures. Patients were in four groups: group 1 (Cr-EDTA; n = 141) and group 2 (Tc-DTPA; n = 47) had sampling at 2, 3 and 4 h. Groups 3A (Tc-DTPA; n = 168) and 3B (Tc-DTPA; n = 361) gave nine samples up to 480 min. C was calculated from GFR corrected using Brochner-Mortensen (BM) without prior BSA-scaling (CBM; GFRBM), after BSA-scaling then reverse-scaling as per British Nuclear Medicine Society (BNMS) guidelines (CBNMS; GFRBNMS), and after correction using the equations containing 'f' described by Fleming (CFlem; GFRFlem) and Jodal and Brochner-Mortensen (CJBM; GFRJBM). In group 3A, C (C9) was determined from GFR measured from all nine samples (GFR9) and from seven samples (C7) up to 240 min. In 3B, GFRC, corrected using 1/([C9.α2]+1), was compared with GFRBM, GFRBNMS, GFRFlem and GFRJBM against GFR9 (gold-standard).

C derived from these one-compartment correction formulae ranged from 25 to 32 min. In group 3, C7 and C9 were 28 ± 11 and 38 ± 14 min (P < 0.0001). Biases of GFRBM, GFRBNMS, GFRJBM, GFRFlem and GFRC against GFR9 were 2.7, 1.5, 4.2, 3.4 and 0.4 ml/min. Corresponding precisions were 9.3, 7.3, 7.0, 6.7 and 7.6 ml/min.

Correction using α2 avoids BSA scaling, has a low bias against gold-standard GFR and does not over-correct at high GFR.