Potential sources of error in measureing single-nephron glomerular filtration rate.

In recent years single-neplhron filtration rate lhas been measured extensively with micropuncture teclhniques. The values obtained in our own studies, as well as those reported by otlhers, reveal an impressive (legree of scatter. Some of the variation can be attributed to differences in the animals and in their physiologic state. Nevertlheless, values in the same animal in a constant pllysiologic state may differ by as muclh as 100%/. Sucli discrepancies witlhin a single animal could be the consequence of eitlher lheterogeneity of nephrons or experimental error. Althouglh some lheterogeneity of superficial neplhrons may exist, most of the variability in measured SNGFR probably stems from experimental error. Some of the potential soturces of error in the measurement of SNGFR are listed in Table 1. Obvious analytic errors can arise from the measurement of botlh the volume of the collectedI sample of tubular fluid and the concentration of inulin. In our experiments, using radioactive inulin, the total analytic error was 7-8%. This included errors in measurement of volume of fluid samples, loss of fluid d(uring transfer into counting vials, and counting errors. If, lhowever, the total timed collection of tubuilar fluidl was transferred directly into the counting vial, the volumetric errors could be eliminatedI and the total error reduced to the countinig error, whiclh is approximately 4%. MIore important sources of error are obviously related to the micropuncture teclhniques used in the collection of tubular fluid. Timing errors can contribute to some extent, depending upon whetlher the timing of collection begins with the injection of oil into the tubule or the aspiration of the sample. In our experiments the average time between injection of the oil and starting the collection