Use of ImmobIlizedEnzymesfor GlucoseAnalysisIn a Small Laboratory

and spleen systems cannot be attributed to lack of precision of the assays (p = 0.01). We analyzedpurifiedliverferritin at several concentrations by immunoradiometric assay, using the spleen system, and similarly analyzed purified spleen fernitin, using the liver system. When theresultsforthepurifiedferntins were plotted along with the standards used in the analyses, the two fernitins appeared to be immunologically identical (Figures 2 and 3). The method we used for purifyingthese fernitin preparations involved multiple crystallizations in CdSO4, which probably minimizes differences in isofernitin composition (12). Although some tumor ferritins have demonstrably different isoferritin compositions and different immunoreactivities from that of liver and spleen fernitins (12), the usefulness of serum ferritin as a tumor marker is not now clear,whereas the usefulnessof measuring serum fernitin for evaluating body iron stores has been shown repeatedly. Even though the differencesbetween our two sets of values are statistically significant when the concentrations are very high, these concentrations are so extreme that the differences have no practical clinical significance. The choice of spleen or liver tissue for isolating ferritin might therefore be based on practical considerations.