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

Aim 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 (alpha(2)) of the exponential between 2 and 4 h post-injection that requires no scaling for BSA. Methods The correction factor is 1/([C.alpha(2)]+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-51-EDTA; n = 141) and group 2 (Tc-99m-DTPA; n = 47) had sampling at 2, 3 and 4 h. Groups 3A (Tc-99m-DTPA; n = 168) and 3B (Tc-99m-DTPA; n = 361) gave nine samples up to 480 min. C was calculated from GFR corrected using Brochner-Mortensen (BM) without prior BSA-scaling (C-BM; GFR(BM)), after BSA-scaling then reverse-scaling as per British Nuclear Medicine Society (BNMS) guidelines (C-BNMS; GFR(BNMS)), and after correction using the equations containing 'f' described by Fleming (C-Flem; GFR(Flem)) and Jodal and Brochner-Mortensen (C-JBM; GFR(JBM)). In group 3A, C (C-9) was determined from GFR measured from all nine samples (GFR(9)) and from seven samples (C-7) up to 240 min. In 3B, GFR(C), corrected using 1/([C-9.alpha(2)]+1), was compared with GFR(BM), GFR(BNMS), GFR(Flem) and GFR(JBM) against GFR(9) (gold-standard). Results C derived from these one-compartment correction formulae ranged from 25 to 32 min. In group 3, C-7 and C-9 were 28 +/- 11 and 38 +/- 14 min (P < 0.0001). Biases of GFR(BM), GFR(BNMS), GFR(JBM), GFR(Flem) and GFR(C) against GFR(9) 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. Conclusion Correction using alpha(2) avoids BSA scaling, has a low bias against gold-standard GFR and does not over-correct at high GFR.