Effect of serum volume and time of exposure to gel barrier tubes on results for progesterone by Roche Diagnostics Elecsys 2010.

To the Editor: Inert thixotropic gel is commonly used in blood collection tubes for separation of serum or plasma from the cellular components of blood. Eichhorn et al. (1 ) in 1997 suggested that gel tubes (Sarstedt) used in their laboratory were generally reliable, but they found incomplete separation in 3.1% of tubes. Plain tubes are preferred for therapeutic drug monitoring because of the potential for binding of drugs to the gel (2 ). Smith (3 ) in 1985 and Hilborn and Krahn (4 ) in 1987 found that progesterone decreased with time as serum remained in contact with the separator gel. In contrast to these findings, we recently encountered an apparent increase in progesterone concentrations when blood was collected in glass serum separator tubes (SST with Clot Activator; Becton Dickinson) and progesterone was measured by the Roche Diagnostics Elecsys 2010. To further investigate these findings, we evaluated the effect of SST on several assays performed on the Elecsys 2010. All test samples (treated in compliance with the Human Investigation Committee) were assayed for progesterone, estradiol, luteinizing hormone (LH), folliclestimulating hormone, creatine kinase MB isoenzyme, and troponin T. Blood from three volunteers was collected in both SST (16 3 100 mm) and plain glass (13 3 100 mm) Vacutainer tubes and was allowed to clot for 30 min before centrifugation. Serum drawn in SST remained on the separator gel, whereas serum drawn in plain tubes was transferred into 13 3 100 mm glass tubes. Samples were kept capped at room temperature and tested at 0, 2, and 4 h after collection; they were then refrigerated and tested at 24 h. In an additional progesterone study, serum volumes of 2, 5, and 10 mL were allowed to sit in SST, and progesterone was assayed at 0, 2, and 4 h. Measured progesterone was markedly higher in the sera in SST tubes than in sera in plain tubes (Fig. 1, top panel). The effect of gel barrier on progesterone was greater when tubes contained smaller serum volumes (2 or 5 mL), and the effects increased with time (Fig. 1, bottom panel). There were no significant differences between SST and plain tube results for estradiol, LH, follicle-stimulating hormone, prolactin, creatine kinase MB isoenzyme, and troponin T. We also had access to blood samples collected simultaneously in plain and SST tubes from five patients on digoxin. The time from blood draw to testing for this group ranged from 6 to 8 h. These samples were assayed for digoxin, estradiol, progesterone, and prolactin. We found no clinically significant difference between plain and SST patient sample results for digoxin, estradiol, or prolactin, but the trend for progesterone results was similar to those in the top panel of Fig. 1, with progesterone results higher by 0.6–2.2 mg/L for the SST samples. We conclude that Elecsys progesterone results are falsely increased when blood is collected in these SST containers and that a component of the inert gel is probably released into the serum to produce the false increase. It is difficult to conclude that this represents an interference with the electrochemiluminescent measurement because there was no similar interference for the other assays tested, and the mechanism is unknown. Although the absolute increases in serum progesterone values are small, without the support of estradiol and/or LH values, a pa-