Post-test quality control for the single blood sample technique in glomerular filtration rate measurement in children

While the value of the single blood sample (SBS) method for estimating 51Cr-EDTA plasma clearance has been repeatedly demonstrated, some nuclear medicine physicians are still reluctant to use it because of the lack of quality control parameters.

To present a post-test quality control procedure for the SBS technique in children.

In addition to the SBS clearance calculated using the specific paediatric SBS method, three artificial slope intercept (ASI) method clearances were calculated by assuming the distribution volume as, respectively, 20%, 25% and 30% of body weight. By dividing the injected activity by the distributional volume, the initial plasma concentrations (A0,30%, A0,25% and A0,20%) were calculated. Using these A0 values and the available single sample, ASI clearances were calculated by using the classical slope-intercept method. The working hypothesis of this approach was as follows. In the absence of significant errors, the three ASI clearance values should be close to that of the SBS method. This hypothesis has been tested using both simulated and patients' data.

The results of the simulated study showed that an error in the injected dose produced variable differences between SBS and ASI clearances depending on the clearance values. The effect of an error on the plasma sample also varied as a function of the clearance values. The analysis of patient data revealed that the ASI approach allowed the identification of patients in whom the classical slope-intercept method suggested the presence of a possible error.

A post-test quality control procedure for the SBS GFR measurement is presented. When the SBS clearance shows a difference with the ASI method (> 10 ml . min(-1) per 1.73 m2), the presence of an error is highly probable. A smaller difference, however, does not exclude erroneous data.