This is certainly a difficult one. I go along with Chris' view about us
being asked for a consult on the sample and of doing 'whatever is reasonable
to answer the question put to us'. I certainly have no qualms about adding
on tests which are of relevance to the condition being investigated (adding
thyroid antibodies onto a TFT is an example), but if the request is to
assess glycaemic control, then the question did not ask us to test for the
completely unrelated condition of haemoglobinopathy. It just so happens that
our best test of glycaemic control involves haemoglobin, but I am sure the
patient with diabetes does not realise that!
Interestingly, yesterday our haematologist did not have a problem about
adding on Hb electrophoresis either. Having slept on it, he is now not so
keen!
Eric
----- Original Message -----
From: Chris Florkowski <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, June 25, 2003 10:26 PM
Subject: Haemoglobin variants in HbA1c samples
Our haematologists have never had a problem test-adding Hb
electrophoresis - the question of a "genetic test" has not arisen in
this context. The ultimate arbiter is mass spectrometry, in our domain
and under our jurisdiction. To answer Eric's specific point, we take
the view that any request that crosses our threshold is a consultation
and we will do (test-add) whatever is reasonable to answer the question
put to us. We turned up one especially interesting Hb variant this way
(vide infra for precis), for which we did get permission from the
patient to publish the findings.
Chris Florkowski; Christchurch, NZ
C M Florkowski, T A Walmsley, S O Brennan and P M George
Haemoglobin Marseille-Long Island and interpretation of HbA1c: which
HbA1c result is the "right answer"?
Postgraduate Med J (2003) 79:174-175
An asymptomatic 44 year old woman was screened for diabetes. Initial
HbA1c analysis by HPLC (Bio Rad Variant) gave a result of 45%,
considered biologically implausible. Immunoassay (DCA 2000) was 2.8%,
also considered implausible. Affinity chromatography (Primus
Corporation, Kansas City, MO, USA) gave 4.6%. Mass spectrometry (VG
Platform; Micromass, UK) confirmed the presence of Hb Marseille-Long
Island (methionyl extension of the amino terminus of the ß globin chain
and histidine to proline substitution at position 2 in the ß chain)
This substitution of a positively charged amino acid with a neutral one
results in a net loss of one positive charge. It thus has lower affinity
for the ion exchange resin and coelutes with the HbA1c peak resulting in
an artefactually high reading.
The ß globin chains from Hb Marseille-Long Island have an amino
terminal that is modified in a way that does not permit recognition by
the antibodies of the DCA 2000 assay. Given that the subject is
heterozygous for this variant and that half the ß chains are therefore
normal, then arguably the "correct" answer might be obtained by doubling
the DCA 2000 result. This would give a value of 5.8%, which is
plausible.
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