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How much insulin is my pancreas actually making?

Ever wonder how clinicians distinguish between type 1 and type 2 diabetes? Though I view the different types of diabetes as more of a sliding spectrum, clinicians are required to diagnose patients as type 1 OR type 2 based on set criteria. Insurance payers then use the established diagnosis to decide what technology and medications they will or will not cover. Yeah...it makes me mad too.

So, what is this criteria?

The easiest way to distinguish between diabetes types is through fasting blood work, specifically the “fasting c-peptide”. This test is a useful marker for insulin production because c-peptide and insulin typically circulate the blood in equal amounts. Because c-peptide is much easier to measure, we can use it to find out how much endogenous insulin is being produced. Normal fasting c-peptide levels fall between 0.8-3.85 ng/mL. Fasting c-peptide levels must be taken with a fasting plasma glucose level to help determine if the beta cells are responding appropriately to the amount of glucose circulating.

When might a c-peptide be low?

Type 1 diabetes. Why? An autoimmune process resulting in beta cell death. These beta cells can no longer produce insulin. Low or no insulin = no or low c-peptide.

Type 2 diabetes. Why? It is progressive. Long standing type 2 diabetes, extreme insulin resistance, and prolonged hyperglycemia will eventually “burn out” the pancreas’ beta cells. When this occurs, diagnosis’ are often changed to type 1 diabetes because these patients need to be treated as type 1’s. Many benefit from insulin pumps, cgms, etc. and unfortunately, these are not well covered under a type 2 diagnosis. In fact, many insurance companies request proof of low endogenous insulin production with a fasting c-peptide of <1.0ng/mL with a fasting plasma glucose of <225mg/dL. Side note: this means that a patient who is maxed out on their oral anti-DM agents and requires multiple daily injections will not qualify for a pump if their c-peptide is >1.0ng/mL or if their BG > 225mg/dL at the time of measure. No, they don’t care if the BG is 400mg/dL and the c-peptide is 1.1ng/mL showing an obvious lack of insulin production. How does that make any sense?

Low blood glucose. This is why a fasting glucose measurement (taken at the same time as the c-peptide is measured) is so important. Someone with low blood glucose SHOULD be producing very little insulin, hence the low c-peptide. In case you’re wondering, no, insurance companies do not have a cut off for low plasma glucose taken with the c-peptide. Technically, a patient without diabetes could have a fasting plasma glucose of 65mg/dL and a c-peptide of 0.9ng/mL and they would qualify for a pump before my type 2 patient mentioned above. It’s normal physiology, people! We need to look at each person individually.

When might a c-peptide be high?

Newly diagnosed type 2 diabetes. Why? The root of type 2 diabetes is insulin resistance. In the beginning stages of type 2, the body is often working to compensate for this insulin resistance by producing extra insulin. If this occurs for too long, it will often lead to pancreatic burnout, as I mentioned above.

Obesity. Excess weight leads to insulin resistance and elevated c-peptide levels.

Kidney disease. Why? C-peptide is removed by the kidneys and when the kidneys are damaged, c-peptide may build up in the blood.

Insulinoma. A very rare insulin producing tumor in the pancreas.

Long term corticosteroid use. This can lead to insulin resistance, elevated c-peptide levels, and an increased risk for diabetes.

When might a c-peptide be normal?

In people without diabetes or insulin resistance. Obviously.

Type 1 diabetes. Why? Honeymooning. People with type 1 diabetes may be diagnosed before complete beta cell failure. During this time, some of the beta cells continue to produce insulin. This can last from months to years depending on the person’s auto-immune processes.

Latent autoimmune diabetes in adults (LADA). Why? These people have one or more autoimmune markers for diabetes (autoimmune diabetes=type 1). Unfortunately, they are often misdiagnosed with type 2 diabetes because they present as adults. Can we all just agree that type 1’s can present in adults and type 2’s can present in kids? Thanks.... According to the ADA, LADA is, in fact, type 1 and should be treated that way. Though the progression to complete beta cell failure is usually slower, complete beta cell failure is inevitable.

Ultimately, the c-peptide test can be very helpful in guiding clinicians and patients in their treatment choices. The problem is in the fact that non-medical/insurance providers are making decisions for us based on a checklist. We are not a list of criteria and we shouldn’t have to “prove” ourselves to qualify for the best technology.

Fewf...I’m done now...as you all can tell I’ve had some frustrating run-ins with insurance companies lately. I have all of the reps on speed dial and the amount of time I spend fighting with insurance and distributors makes me want to quit my job and move here #tahiti ... But then I remember how much I love my patients...so, there’s that.



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