Time to complete mixing for the measurement of glomerular filtration rate from single bolus plasma 51Cr-EDTA clearance.
METHODS
In the measurement of glomerular filtration rate from the plasma clearance of 51Cr ethylenediamine tetraacetic acid by using the slope-intercept method, the first sample is conventionally taken at 2 h, the time by which it is generally assumed that the clearance curve has reached a single exponential. We examined this assumption by comparing the slopes, alpha 12 and alpha 23, based, respectively, on samples at 2 and 3 h, and at 3 and 4 h.


RESULTS
In 421 patient studies in whom the first sample was taken between 110 and 130 min after injection, the mean ratio, alpha 12/alpha 23, was 1.101 (SEM 0.011) which is significantly higher than unity (P<<0.001). The relationship between alpha 12/alpha 23 and the slope, alpha 13 (which is a measure of filtration function already indexed for body size) based on all three samples was negative. By modelling the relationship between mixing time and alpha 13 it was shown that this relationship suggests delayed mixing of the indicator throughout its distribution volume and is inconsistent with irreversible binding of indicator to plasma protein. A significant positive association was observed between alpha 12/alpha 23 and age, but this is largely explained by a generally poorer filtration function in the older age group since low levels of alpha 13 theoretically predict a longer mixing time. In 188 patient studies in whom the first sample was taken more than 130 min after injection, the mean ratio, alpha 12/alpha 23, was 1.055 (SEM 0.017) which is still significantly higher than unity (P<<0.005) but significantly less than the ratio based on studies in which the first sample was taken at 110-130 min (P<0.02). The ratio alpha 12/alpha 23 still showed a significant relation (positive) with age but not with filtration function.


CONCLUSIONS
This error in the estimation of glomerular filtration rate to which delayed mixing leads will make its greatest impact when using the simplified slope-only technique, but can be minimized by delaying the first blood sample.
Bird
Nicholas J
NJ
Department of Nuclear Medicine, Addenbrookes Hospital, Cambridge, UK.
Henderson
Bird L
BL
Lui
Dominic
D
Peters
Michael
M
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D002860:Chromium Radioisotopes / Q000493:pharmacokinetics*; D004492:Edetic Acid / Q000737:chemistry / Q000493:pharmacokinetics*; D005260:Female; D005919:Glomerular Filtration Rate / Q000187:drug effects*; D006801:Humans; D007223:Infant; D008297:Male; D008855:Microscopy, Electron, Scanning; D008875:Middle Aged; D013997:Time Factors
D002860:Chromium Radioisotopes; D004492:Edetic Acid