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Mr. McMinn
What metal ion levels are considered high and
to be of concern for a patient to seek out
treatment?
People report being told by their surgeons that
they have "elevated levels of cobalt" ranging
from single digit numbers (one person reported
9.2 for example) up to numbers as large as
2400+. What is the point at which one should
have serious concern? If one person is living a
normal life with 2400+, is it really significant
if you have a level of 10 or 20? Or does the
cobalt affect people in different ways?
What is considered to be toxic level of Cobalt
in blood, some say 5.0 - others 7.0?
There is a lot of confusion in this area because
of the different measurement techniques and
specimens used, and the units in which they are
expressed. Different cut-off levels have been
advocated by different authors varying from 2 to
25 parts per billion, ppb (34 to 424
nanomoles-per-litre, nmol/L) of cobalt, none of
which is based on any valid test of systemic
toxicity.
Provided the correct specimen is used and
analyzed by the correct method (refer to Q1),
metal ion levels provide a reasonable measure of
bearing wear. In most cases it can be said that
a patient with high metal ion levels is more
likely to have an adverse reaction than someone
with regular levels.
The MHRA (medicines and healthcare products
regulatory agency) the statutory body that
regulates resurfacing devices in the UK
advocates the figure of 119 nmol/L cobalt (or 7
parts per billion, ppb) for resurfacings. This
was not arrived at, by performing the kind of
pharmacokinetic tests that are carried out for
drug toxicity. It was based on the distribution
and variations of cobalt levels in patients with
well-functioning metal-on-metal (MoM)
resurfacing. Levels above this do not indicate
systemic toxicity. They have used it more as a
rough guide to identify potential cases for
local debris related reactions only.
It is suggested that it is reasonable to
reassure patients with less than 7ppb that they
had a well functioning MOM hip and monitor those
with greater than 7ppb. It does not mean that
this level causes systemic toxicity. It only
implies that among patients who develop a local
reaction the blood levels are often greater than
7ppb.
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