Effect of underfilling of tubes with EDTA on PTH assay measured by cobas analyzer

INTRODUCTION: The purpose of this study is to evaluate cobas parathyroid hormone (PTH) measurement effect of plasma samples obtained from underfilled tubes with ethylenediaminotetraacetic acid (EDTA).
METHODS: Two blood collection tubes with K3-EDTA from 67 patients at the same time were taken. One of them was for routine PTH measurement, which was filled to its capacity, while another tube was underfilled. All EDTA tubes were immediately centrifuged at 4°C. Plasma PTH concentrations were measured by electrochemiluminescence immunoassay method on the cobas e 601 (Roche Diagnostics GmbH, Mannheim, Germany) analyzer.
RESULTS: The underfilled sample tubes were grouped according to their being up to 25% (n=9), 25–50% (n=35), and 50–100% (n=23) of the appropriate amount. In all groups of underfilled tubes, the PTH values were found to decrease concerning those fulfilled (p<0.001). The agreement among underfilled and fulfilled for measuring PTH demonstrated a bias of −3.2 pg/mL (−5.9%) for the underfilled tubes.
DISCUSSION AND CONCLUSION: Insufficiently filled blood in tubes with EDTA has a significant effect on PTH levels. However, further studies are needed to show whether this effect is clinically significant.