The C-Peptide test measures how much
insulin is released from the pancreas, when stimulated by glucose to do
so. According to Mosby’s Manual of
Diagnostic and Laboratory Tests – Second Edition, “The exogenously administered
insulin suppresses endogenous insulin production.” More accurately, exogenous insulin controls blood glucose
concentrations, thus preventing stimulation of endogenous insulin release,
which may in effect suppress endogenous insulin production.
The C-Peptide test does not
indicate whether the beta islet cells of the pancreas are working or not, nor
whether there are in fact beta cells present.
C-Peptide is not a test for insulin production, but a test for insulin
released from the pancreas.
To understand how the pancreatic
beta cells are working one needs to look at the C-Peptide test and at the blood
glucose level at the time of test, and consider all the factors that I have
previously listed that contribute to hyperglycemia, to arrive at an objective
conclusion.
As a blood glucose load increases,
the C-Peptide levels increase.
Of course insulin users (type 1)
with good blood glucose control are C-Peptide negative or of low value, let me
explain:
A child has a cold-like infection
and his blood glucose levels increase to astronomical heights (lets say 500
mg/dl). This is considered an emergency
situation (because of ketoacidosis). So
after a quick finger test in the emergency room the patient is put on insulin
(intravenously) according to standard procedures.
The high blood glucose levels,
caused by infection, are lowered to normal levels again with exogenous
insulin. The child is told not to
eat. 6-10 hours pass and finally an
endocrinologist comes around to examine the patient, then he orders a C-Peptide
test. And of course it comes back
negative or have low value: confirming
type 1 diabetes.
This is a false conclusion because
the exogenous insulin is controlling the blood glucose and the child has not
eaten in 6-10 hours so there is no new glucose entering the blood stream
through the intestines. The pancreas
has no reason to release insulin (measured by the C-Peptide test). Hence, the result is C-Peptide negative or
of low value.
Insulin pump users with good blood
glucose control will also have these same misinterpreted results.
The only way for insulin using
diabetics to prove in spite of poor or of good blood glucose control and also
of the Glycohemoglobin or Hemoglobin A1c test (HbA1c - a test for average long
term blood glucose, about three months) that they have in fact Type 2 Diabetes
which is a curable condition, and not Type 1 Diabetes, is to stimulate the
pancreas to release insulin before taking the C-Peptide test.
Increase the blood glucose above
normal for at least one hour before taking the blood test.
This should help exculpate the
pancreas and prove that maybe the beta cells are there and functioning
normally, proving type 2.
I have done the C-Peptide test
recently; it was 6.2 nanograms per milliliter (ng/ml). -- This is above normal,
and proves type 2. My blood glucose at the
time of the C-Peptide test was about 230 mg/dl (not intentionally). -- I credit
this fact for giving me the result that convinced my doctor that I was really a
type 2 diabetic.
I was using 70 units of Lantus® 24
hour basal insulin a day and my pancreas was also releasing insulin above the
normal range. This was evidence that there was a very heavy glucose load
present. Since I was on a no
carbohydrate, low calorie diet and my basal insulin was dosed correctly, the
heavy glucose load was an unexplainable factor. The sudden need to drop the amount of insulin that I was using
right after starting on antibiotics proved that this very heavy glucose load
was due in fact to an infection.
The C-Peptide test was done first;
the antibiotics were started a week later after the nasal and sinus symptoms
came out.
According to Mosby's Manual of
Diagnostic and Laboratory Tests - Second Edition
2002 by Kathleen Deska Pagana,
Ph.D., RN & Timothy J. Pagana, MD,
FACS - Pages 186-188
Mosby’s Inc. – St. Louis, MO USA
ISBN 0-323-01609-X
“Fasting range is: 0.78 - 1.89
ng/ml (0.26 - 0.62 nmol/L SI unit)
Range one hour after a glucose
load is: 5.00 -12.00 ng/ml” (During a glucose tolerance test)
According to my lab report, the
normal reference range is
0.6 - 3.2 ng/ml. This reference range may be misleading for
doctors because it is incomplete.
I conclude that the complete human
C-Peptide range as opposed to the "normal" range is in fact
0.6 - 12.0 ng/ml, keep in mind
that the normal values are for a non-diabetic person.
The values will be lower in an
insulin user and higher in a type 2 oral agent user (agents that promote the
release of insulin from the pancreas) when blood glucose levels are controlled.
The C-Peptide test is mostly used
to diagnose and evaluate patients who are hypoglycemic (produce too much
insulin) as well as those who have insulinomas (insulin producing tumors). The test is also used to see if a normal
person is secretly using insulin.
They may ask patients to fast for
this test (to reduce glucose levels in the blood) to establish a baseline in
hypoglycemia cases. This is currently a
standard procedure for this test.
In diabetics however the baseline
has to be the opposite, to increase glucose levels in the blood, in order to
prove type 2. Fasting may not yield the
intended results. Do not fast.
What I understand from these two
sources about my own condition is that if I were normal, non-diabetic, these
results would mean that my pancreas releases insulin above the normal reference
range. However, I have a low glucose
tolerance threshold. My 6.2 ng/ml C-Peptide falls at the bottom of the 5-12
ng/ml glucose test range.
I can only presume that people who
can tolerate a high carbohydrate diet would have scored a 12 ng/ml on the C-Peptide
test for glucose tolerance. Perhaps
because they have a higher genetic glucose tolerance (more beta islet cells),
and the blood sugars are better controlled under extreme conditions such as
infection and poor diet, they may in effect have better immunity against
disease.
The values of the C-Peptide test
and blood glucose separately do not give an absolute picture of what is
actually happening, they must be considered together. Even when they are considered together they still may not present
a complete picture of the diabetic condition.
Let me add one more factor to the picture, which may be relevant: The
metabolic rate as controlled by the thyroid gland, which is different in each
person.
As I use less exogenous insulin,
and more of my own natural insulin I feel a positive change in my mood that has
been absent for a long time. I
attribute this to the fact that more C-peptide is now being released along with
insulin from my pancreas. Perhaps
C-peptide has a physiological effect on mood.
Bottled insulin does not contain C-Peptide and using it suppresses
natural insulin release and probably C-Peptide that is released with it. This may explain why I have noticed that my
mood was great, even thought I had high blood glucose at times and a dull mood
when having normal glucose levels controlled by bottled insulin. The fact that an insulin dependent diabetic
feels good after eating a meal may be proof that there is endogenous insulin
release, because C-Peptide is also being released.
On the subject of diagnostic tests, I found that Mosby's
manual contains more complete information about the C-Peptide test than other
sources that I looked up.