Correction of the slope-intercept method for the measurement of glomerular filtration rate.

OBJECTIVE: Glomerular filtration rate (GFR) is frequently assessed using the slope-intercept method by fitting a single exponential to plasma samples obtained 2-5 h after injection. The body surface area (BSA)-corrected one-pool clearance (CO,BSA) overestimates true GFR (CT,BSA) because it fails to sample the full plasma curve, and values of CT,BSA are usually estimated from CO,BSA using the Brøchner-Mortensen (BM) equation. An improved equation, CT,BSA=CO,BSA/(1+fBSA×CO,BSA), with fBSA a fixed constant, was proposed by Fleming, but subsequently Jødal and Brøchner-Mortensen (JBM) reported that fBSA varies with BSA. We report data for a large group of individuals who underwent GFR investigations with sampling of the full plasma curve. The aims were to validate the JBM equation with independent data and assess whether replacing the BM equation with a BSA-dependent correction based on Fleming's equation can increase the accuracy of the slope-intercept method. METHODS: Plasma data were analysed for 142 children and adults aged 0.6-56 years who underwent technetium-99m-diethylenetriaminepentaacetic acid GFR investigations with blood samples taken between 5 min and 8 h after injection. Values of CO,BSA were calculated using the 2, 3 and 4 h data. Values of CT,BSA were calculated by integrating the plasma curve between 5 min and 4 h and extrapolating the terminal exponential. Individual values of fBSA were calculated using the relationship fBSA=1/CT,BSA-1/CO,BSA. Nonlinear regression was used to fit the function fBSA=f1×BSA and find the best-fit values for f1 and n. Scatter and Bland-Altman plots were drawn comparing the various formulae for correcting slope-intercept GFR. RESULTS: The trend for fBSA to decrease with increasing BSA was highly significant (Spearman's test: RS=-0.31; P=0.0002). When the data were fitted by nonlinear regression, the best-fit values (95% confidence interval) of the model parameters were n=-0.13 (from -0.21 to -0.04) and f1=0.00191 (from 0.00183 to 0.00200). CONCLUSION: The results confirm that fBSA varies with BSA and provide independent values of the parameters f1 and n. Differences from GFRs calculated using the original JBM equation were small and not clinically significant. The BM equation also performed well for CT,BSA less than 125 ml/min/1.73 m. However, there was a small number of children with CT,BSA greater than 150 ml/min/1.73 m for whom the JBM formula provided more accurate estimates of true GFR than did the BM equation.

UK audit of glomerular filtration rate measurement in 2001.

OBJECTIVE: To investigate the consistency of glomerular filtration rate (GFR) calculation from plasma sampling in the UK. METHODS: Ten patients' data sets from plasma sampling measurements of GFR were distributed throughout the UK. The data included count rates from four samples taken between 2 and 4 h after injection, a diluted sample of injected dose for standardisation, the patient's height, weight, age and sex. Participants were asked to use the routine method to calculate GFR and express the results in absolute terms (i.e. in millilitres/minute) and normalized for body surface area (ml/min/1.73 m2). Supplementary data were also requested relating to workload, method used and normal range. Intercentre variability was assessed by calculating the root median square (RMedS) deviation of each GFR from the median for that data set. Centres using a particular analysis method were grouped together and the RMedS deviation of each result from the median for that group and that data set was calculated. The influence of using normalized data and number of samples was also studied. RESULTS: Seventy-nine returns were received. For the normalized data, the overall RMedS variability was 5.8 ml/min/1.73 m2. This decreased significantly to 0.6 ml/min/1.73 m2 when results were grouped by analysis method. Results were similar for non-normalized data. A small but significant decrease in error with the number of samples was observed. CONCLUSION: Considerable variability in GFR values obtained at different centres in the UK for a given set of data was observed. Nearly all this variability was due to different methods of analysis. If methodology were standardized then intercentre variability in GFR analysis could be reduced dramatically. Radionuclide techniques are confirmed as being the method of choice if an accurate value of GFR is required.

UK audit of left ventricular ejection fraction estimation from equilibrium ECG gated blood pool images.

PURPOSE: To examine the variability of results obtained from computer analysis of left ventricular gated blood pool (LVGBP) images by nuclear medicine centres in the UK. METHODS: Twelve data sets of LVGBP images were distributed via commercial software suppliers to nuclear medicine centres in the UK. Two of the data sets were duplicates and three were acquired from the same patient with different total counts in the images. The quality of the images was also variable and two images had poorly defined left ventricular walls. A questionnaire was used to identify the parameters used during the analysis and to give an indication of the number of LVGBP scans per year routinely carried out by each centre as well as report the results obtained from the analysis. RESULTS: Results were received from 63 nuclear medicine centres using 77 computer systems. The vast majority of participants (57) carried out fewer than 10 scans per month. Only two centres performed more than 30 scans per month. Sixteen centres did not quote a minimum normal value for left ventricular ejection fraction (LVEF) and 36 did not record a maximum value. The remainder recorded between 0.40 and 0.60 for the minimum of normal range and 0.60-0.90 for the maximum of normal range. Analysis of returns showed that LVEF estimates for the data sets were highly variable between centres and computer systems. The overall standard deviation of results compared to the mean for each study was 0.076. Approximately half this variation was due to systematic variation between centres. The overall precision taking into consideration this systematic variation, was 0.055. Lower variability was found between studies with higher overall counts and this was highly significant.

Guidelines for the measurement of glomerular filtration rate using plasma sampling.

These guidelines have been adopted by the British Nuclear Medicine Society.