When an abnormal hemoglobin is present in the blood
(such as HbS, HbC, HbE, or HbD), it is glycated either
in addition to or instead of HbA1c. Modern glycohemoglobin
analyzers can identify the nonglycated portion
of these abnormal hemoglobins (but not the glycated
portion). Nevertheless (even with such knowledge), it
is very difficult to estimate what the HbA1c would
be if the hemoglobinopathy was not present. This is
even more difficult in patients who have homozygous
hemoglobinopathies where there is no HbA present at
all.