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Diabetes tests
(Chart) Antibodies testing comparison between type 1 diabetes, type 2 diabetes, and latent autoimmune diabetes in adults (LADA)
... that C-peptide is released in equal amounts to the level of natural endogenous
(from the pancreas) insulin?
... that bottled insulin (exogenous insulin) contains no C-peptide?
... that certain tests for specific antibodies can help predict who might be at risk for developing type 1 diabetes, type 2 diabetes, and latent autoimmune diabetes in adults?
Assay - In relation to medical testing, an assay is an analysis done to determine the presence of a substance and the amount of that substance.
Hypokalemia - abnormally low level of potassium in the blood
Insulinoma - a rare tumor in the pancreas that
produces too much insulin. Also called insuloma or islet cells adenoma.
Insulinomas usually occur as single, small tumors in
adults and are very rare in children. But those children with hyperinsulinism have
multiple pockets of overactive insulin-secreting cells in the pancreas, rather
than a single tumor.
About 5-10% of insulinomas are cancer- ous. People with the genetic syndrome called multiple endocrine neoplasia
Type I (MENI) are at risk for develop- ing insulinomas.
C-peptide testing is not widely used and may not be available in every laboratory. There are two main methods of doing the C-peptide test: RIA (radioimmunoassay) and ICMA (immunochemi- luminometric assay). These two methods have different normal
ranges, as well as sensitivities and specificities, and are not interchangeable. If you are going to have a series of C-peptide tests performed, they should be done at the same laboratory using the same method.
Even though they are produced at the same rate, C-peptide and insulin leave
the body by different routes. Insulin is processed and eliminated by the liver,
while C-peptide degrades and is removed by the kidneys. Since the half-life of
C-peptide is about 30 minutes to insulin’s 5 minutes, normally there will be
about 5 times as much C-peptide in the bloodstream as insulin. Add to this the
fact that a person’s kidneys and/or liver may not be clearing insulin and
C-peptide out efficiently and you end up with an inherent imprecision in the
C-peptide test. It can give your doctor important information about your beta
cells and insulin production, but it is not perfect.
What is a C-peptide test and what does it measure?
C-peptide is an inactive amino acid subunit of insulin present in direct proportion to the amount of natural endogenous insulin present. (Insulin itself can also be measured in an insulin test). By measuring the levels of C-peptides, the level of insulin can also be calculated.
The C-peptide test, also known as Connecting peptide, insulin C-peptide, proinsulin C-peptide test, is a blood test conducted to help determine how much insulin a person's pancreas is producing. This test can also help determine if the insulin produced is being used effectively by the body (i.e., if insulin resistance is a problem).
Sometimes, a 24-hour urine test is also requested. This simply means that you collect all your urine over a 24-hour period.
IOH Health Tip:The letter "C" in C-peptide, stands for "connecting." The C-peptide test is not performed by all labs and doctors may rely on other tests. why
Why is this test helpful?
IOH Health Tip:You will need to fast
for a C-peptide blood test if the results will be used to evaluate hypoglycemia.
Your doctor may order a C-peptide test to make or confirm a diagnosis of diabetes, if you are newly diagnosed with type 1 or type 2 diabetes, or suspects insulin resistance. Some doctors also order this test for women with polycystic ovarian syndrome (PCOS) to determine if they are insulin resistant. It is also used to help diagnose the cause of recurrent hypoglycemia.
This test might also be conducted before starting certain medications, like Glucophage or Byetta, to determine if the medication is warranted. Sometimes, the C-peptide test will be conducted about 3 months after starting a medication to see if there has been any changes in insulin level and function. C-peptide blood and urine levels also may be monitored to check your
renal function and C-peptide clearance rate.
Type 1 diabetes- When newly diagnosed with type 1 diabetes, a C-peptide test can measure residual beta cell function. This may help determine how much insulin should be given. Subsequent C-peptide tests may be used to determine when absolute insulin deficiency occurs, or, in some cases, when type 1 patients cross over into type 2 and develop insulin resistance. However, bottled insulin (exogenous insulin) does not contain C-peptide.
Type 2 diabetes - Previously called adult-onset diabetes, type 2 diabetes is often initially marked by an over production of insulin due to insulin resistance. This occurs when the body's ability to use insulin becomes impaired -- it "resists" the normal action of insulin so the body produces more insulin to compensate. Patients newly diagnosed with type 2 diabetes are usually treated with oral antidiabetes medications and changes in lifestyle (some doctors may also prescribe Byetta, an injectable drug that is not insulin but helps regulate normal insulin production in the body).
Eventually, type 2 diabetes can lead to an underproduction of insulin or the body becomes so resistant to insulin that normal blood sugar ranges cannot be maintained. When type 2 diabetes cannot be controlled with lifestyle and medications alone, a person will require daily insulin injections. The C-peptide test may be used to monitor beta cell activity (how much insulin is being produced) and capability (how effectively your body uses insulin) over time. This can help your doctor determine if and when to begin injected insulin therapy.
Insulin resistance, metabolic syndrome, polycystic ovarian syndrome - C-peptide tests may sometimes be ordered in other metabolic disorders that affect insulin levels, insulin sensitivity and carbohydrate metabolism. This test might be used as a tool to determine if a person is insulin resistant.
Hypoglycemia - C-peptide tests can be used in
conjunction with testing insulin and glucose levels
to help diagnose the cause of recurrent, documented, hypoglycemia and to monitor its treatment. The
C-peptide test may be used to help differentiate between excessive insulin production
and excessive insulin being injected. It can also aid in diagnosis of insulinomas (tumors
of the islet cells in the pancreas that can produce uncontrolled amounts of
insulin and C-peptide).
Hypoglycemia can be caused by administering too much insulin, certain antidiabetes medications, alcohol consumption, inherited liver enzyme deficiencies, liver or kidney disease, or insulinomas.
Surgery or pancreas damage - If your pancreas has been surgically removed or suffered from physical trauma, your
C-peptide levels may be monitored to verify the effectiveness of treatment and
continued success of the procedure.
Transplants - If you have had a pancreas transplant or an islet cell transplant (still considered experimental as of May 2006) your doctor will order C-peptide tests to monitor your progress. Transplants are performed with the hope that the patient's ability to make insulin will be restored and the C-peptide test will reflect any changes since your transplant.
What can C-peptide test results indicate?
Because
reference values are affected by many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. Therefore, there is no standard reference range for the C-peptide test because "normal" values vary from lab to lab. Your lab report should include the specific reference range
for your test.
The level of C-peptide in the blood must
be also interpreted along with the results of a blood glucose test. Both these tests will
be done at the same time and comparing the two results can provide important information for your doctor.
High levels of C-peptide generally mean high levels of insulin. During a glucose tolerance test (GTT), there
will often be a temporary 5 to 6 fold increase in C-peptide levels. However, high C-peptide levels may be caused by:
Low C-peptide levels are seen when insufficient insulin is being produced by the beta cells. This might mean type 1 diabetes or latent autoimmune diabetes that has progressed towards insulin deficiency.
Low C-peptide levels may also be seen when production is suppressed by exogenous insulin (insulin that is injected) or with suppression tests that involve substances, such as epinephrine. Diuretics and alcohol intake also may cause low levels in some cases.
The following chart gives a general overview of what your test results might indicate.
Insulinomas[2] (pancreas tumor), certain medications such as sulfonylureas or meglitinides[3]
High
Low
Addison's disease, liver disease, a severe infection, or exogenous insulin therapy
Low
Low
Chart notes [1] Over time, persons with type 2 diabetes may have low levels of C-peptide. [2] Increasing levels of C-peptide after the surgical removal of an insulinoma
may indicate that the tumor has returned or that the tumor has spread to other
locations in the body (metastasized). [3] Both C-peptide and insulin are removed from the body
by the kidneys. C-peptide levels may be increased in a person with kidney
failure.
What is a C-peptide stimulation test?
When using a C-peptide test to diagnose diabetes, your doctor will also take into consideration your age, weight and how long you have had symptoms. But sometimes additional testing might be needed to distinguish type 1 diabetes from type 2, especially in persons over age 25. When there is need for further clarification, a C-peptide stimulation test may be done to help distinguish between type 1 and type 2 diabetes.
How to the test is done:
A blood sample is drawn to measure C-peptide before the stimulation test begins.
Glucagon (another hormone created by the pancreas) is injected to cause the liver to release glycogen which in turn will raise blood glucose levels. However, unlike when a person with diabetes is unconscious and might receive an injection of glucagon under the skin, for this test glucagon is injected into the vein.
Shortly after the injected glucagon is given, another blood sample is drawn.
What the test might show:
If a person has type 1 diabetes they are unable to produce insulin in response to the elevation in blood glucose and blood glucose will be high and C-peptide will be low.
If a persons has type 2 diabetes, C-peptide levels will be increased because the pancreas is
still producing insulin.
What can affect the accuracy of C-peptide tests?
Many things can affect the results of both C-peptide and blood glucose tests. Some of the things that can affect results of the C-peptide test include:
Kidney failure (C-peptide levels may be increased in a person with kidney
failure)
Obesity, which can cause increased C-peptide levels because of the increased
production of insulin associated with obesity (insulin resistance)
Recent radioactive scans within 1 week of a C-peptide test
How might using Byetta affect a C-peptide test?
A few patients using Byetta may have an increase in their value to over 10 after 3-4 months of administering Byetta, This rise over 10 indicates that
values that have returned to normal. However, about most patients will experience a decrease of approximately 20% in their values.
What causes this decrease in values?
A decrease in values may occur when there has been an increase in all of the islets cells (alpha, beta, delta, D1, and PP cells). This is not an indication that Byetta is not working, but because the process of cell growth simply will take more time.
If I am taking Byetta, how often will my doctor order a C-peptide test?
Many physicians will have a C-peptide test done about every three months until you reach normal values, and continue to run the test every three months for the first year. Beyond that point, testing will be based on various factors unique to your own medical situation as determined by your doctor.
Richard R. Rubin, Ph.D., C.D.E. Psyching Out Diabetes: This book examines the psychological obstacles of diagnosis such as panic,
fear, anger, shame, and guilt and how to put them into perspective. Through
proper maintenance, diet, and education, this book is tailor-made for the
diabetic or anyone with a diabetic in their life who wants to overcome the
negative emotions associated with this disease and learn the coping skills
necessary to integrate diabetes into their daily life. Dr. Rubin is a faculty member of the John's Hopkins Medical School.
