Indican interference in bilirubin assays: a classical solution still applies.

There was no difference in mean decay time in samples of controls and patients. We found a significant change in ACTH plasma concentrations with time, as in other studies (1, 5), but this change was much smaller than expected. Unlike another investigation (5 ), we studied samples not only from healthy volunteers but also from patients with high ACTH concentrations. Our study did not show a difference in the mean rate of hormone concentration change in highACTH samples vs normal samples. In our study, for up to 24 h the decline in the measured ACTH concentration was 10% even in whole blood stored at room temperature. Given the analytical imprecision of 15%, commonly accepted for immunoassays, a 10% change in the hormone concentration attributable to preanalytical factors seems not to be a major problem in a clinical setting. We therefore confirm stability of ACTH in EDTA plasma for 24 h as previously reported (5 ) for a manual radioactive version from the same assay from the same manufacturer. Sample temperature during the preanalytical phase appears to have less influence on measured ACTH concentrations than does time to centrifugation. We speculate that enzymes involved in EDTA degradation are not inhibited sufficiently at 4 °C. Although the mean decay in measured ACTH concentration after storage for 24 h at room temperature without centrifugation was only 10% [mean (SD), 9% (11%)], the decrease was 20% in samples from 3 healthy volunteers and was not prevented by storage at 4 °C. No relevant change occurred in any of the samples during the first 4 h, however. For clinical practice we therefore recommend that centrifugation and separation of plasma supernatant be performed within 4 h of sample collection. Cooling of samples seems to be much less effective. Thus, the preanalytical procedure can be simplified without risking clinically relevant changes in measured hormone concentrations. We acknowledge Brigitte Mauracher, Rita Schwaiger, Sarina Meurer, and Juliane Ramisch for excellent technical assistance.