Probable range for whole kidney mean transit time values determined by reexamination of UK audit studies

Inconsistency in the intercentre measurement of whole kidney mean transit time (MTT) has been reported in a previously published UK audit. The main objectives of this study were to identify a probable value of MTT for each kidney in the UK audit data and to find likely reasons for the reported variations.

Datasets of MTT values were obtained by an independent review of the audit data by four experienced practitioners of deconvolution techniques. The deconvolution techniques used included the matrix method, a constrained least squares method as well as a residence time technique. The datasets were compared using t-test, linear regression, and mean difference analysis.

Twelve of a total of 13 datasets showed nonsignificant differences using a paired t-test (P>0.05). For each kidney (x), a collective mean and standard deviation, Mx and SDx, respectively, were calculated from these 12 datasets and a probable range was defined as Mx+/-3SDx. Average SDx/Mx was 3.6% (range 1.5-7.7%). For five kidneys, Mx exceeded the median of the audit results by 3.5-15.3 SDx (P<0.001).

Probable ranges for whole kidney MTT have been estimated with good precision. Underestimation of the area under the plateau of the renal retention function as well as overestimation of the plateau height might have contributed to an underestimation of MTT apparent in some audit results. Visual display of both the renal retention function and the reconvolution curve are suggested as simple quality control measures for analysis software.