A new single-sample, time-flexible, empirically-derived formula for measuring glomerular filtration rate from a...

Measurement of glomerular filtration rate from a single blood sample using filtration markers may be based on empirical or model-based formulae. The former do not require a whole body metric but are sample time-inflexible. The latter include time as a continuous variable but require a whole body metric to estimate extracellular fluid volume. This study describes a new empirical method that is time-flexible and requires no whole body metric.

Data comprise 982 slope-intercept glomerular filtration rate measurements from a single institution in which the correlation coefficient of the three-sample fit was ≥0.999. In 582 measurements, samples were taken at 118-122, 144-148, 178-182, 202-206 or 238-242 minutes. Within each of these five sample time groups, slope, k(t), of the regression of apparent volume of distribution (Vt) against slope-intercept glomerular filtration rate was recorded following exclusion of outliers. The five sampling times correlated closely and exponentially with k(t) (=5.4.e), allowing calculation of k(t) for any sampling time. In a further 341 measurements, glomerular filtration rate from a single blood sample was calculated as [k(t).V(t)] and compared with slope-intercept glomerular filtration rate. Sample time for glomerular filtration rate from a single blood sample is critical, so the most appropriately timed sample was chosen according to slope-intercept glomerular filtration rate/1.73 m.

The current method compared favourably with three previously published model-based methods (Fleming et al., Jacobsson and Gref and Karp), showing better precision, and unlike the other three, no correlation between difference and average on Bland-Altman analysis.

This completely time-flexible, empirically-derived approach to glomerular filtration rate from a single blood sample requires no whole body metric, is simple, equally applicable to children and adults and superior to three model-based methods.