No need for immediate freezing of metal ion blood samples in patients with metal-on-metal hip articulations

INTRODUCTION: A recent British medical device alert suggested monitoring patients with metal-on-metal (MoM) articulations using blood metal ions. These blood samples are usually frozen immediately and shipped frozen for analysis. Simply posting the samples in the mail would lower the costs and simplify logistics of metal ion testing. The aim of this study was to determine whether the metal ion value in the blood is stable when kept at room temperature.

MATERIAL AND METHODS: Eight patients with large-diameter MoM articulations were included. We compared levels of chromium (Cr) and cobalt (Co) in whole-blood samples frozen immediately, after four days and after 30 days.

RESULTS: We found Co ranging from 0.64 to 10.9 µg/l and Cr from 0.76 to 5.16 µg/l. There was no systematic reduction in the mean level of Cr and Co of the eight patients when we compared results from the blood frozen immediately with the blood frozen after four days and after thirty days. There was a tendency towards greater variation (limits of agreement) in the results of the individual blood samples over thirty days, but these increases were non-significant.

CONCLUSION: The variation of Co and Cr ions in blood kept at room temperature for up to thirty days is within clinically acceptable levels for the diagnosis of excess wear.