February 28, 2011
Diagnosis and Symptoms
Question from Mount Pleasant, Texas, USA:
Would a glucose challenged C-Peptide test be definitive in determining type 1 diabetes in my case? If so, could you describe the test procedures and how I prepare for the test (insulin dosing changes, fasting or not, etc.) Here’s my background:
In March 1993, one day before my 42d birthday, I was diagnosed with diabetes. I had lost 50 pounds during the previous eight months prior to diagnosis (20 on purpose, the other 30 “on its own”). At the time of diagnosis, my blood sugar was 560 mg/dl [31.1 mmol/L] and I had all of the classic diabetes symptoms (weight loss, extreme thirst, frequent trips to the bathroom, etc.). Over the next two years, I moved quickly thru oral medications, 70/30 Humulin mix, Humalog (bolus) and Ultralente (basal) multiple daily injections (MDI) with a sliding scale to Humalog and Ultralente MDI using carbohydrate counting. Lantus was substituted for the Ultralente when it came out. Currently, I am using 26 units of Lantus once daily during the evening and +/- 30 units of Humalog (H) daily based on carbohydrate counting. For breakfast, I take one unit of Humalog per six grams of carbohydrates. For lunch, I take one unit of Humalog per nine grams of carbohydrates, And for supper, I take one unit of Humalog per seven grams of carbohydrate. My insulin sensitivity factor is 40. I have had no changes in my insulin dosing for several years. My A1cs have been in the mid- to upper-7% range. Since 1993, I have slowly gained back the weight lost prior to diagnosis (now 245 pounds at 6 feet, 2inches with a stocky, pear-shaped build). Everything else is normal (eyes, kidneys, thyroid, cholesterol, blood pressure) with no evidence of any diabetic complications. I have not had any antibody tests and I have not been officially diagnosed as type 1 (probably LADA). My endocrinologist retired several years ago and I am currently being treated by an internist. We have not seen the need to see a new endocrinologist since I’m doing so well on the current program. Although I understand that a type 1 diagnosis will have no clinical impact, as I age, I want to be able to tell other specialists, surgeons, etc., that I am a type 1 diabetic to avoid any misunderstandings because of my age and size. Is the C-Peptide test the way to go?
Answer:
I agree with you that there may be some benefit in being able to categorize your type of diabetes. For instance, you would not want a person with type 1 diabetes to go without insulin as this may increase the risk of diabetic ketoacidosis in a person with type 1 diabetes. Most of these decisions about type 1 versus type 2 diabetes are made clinically. In your situation, your physician has categorized you as having a form of type 1 diabetes, LADA. The best categorization for a practical determination is with antibodies and C-Peptide response to a meal challenge. The antibody test most people use is the anti-GAD antibody. This antibody, unlike the anti-islet cell antibody and the anti-insulin antibody, stays up at a high concentration for years after the onset of diabetes. A high test is helpful in categorizing you as type 1 diabetes, but a negative test does not totally rule it out. The other test often used is to give a mixed meal challenge, in the form of a nutritional supplement, like Sustacal. This preparation has carbohydrates, protein, and fat and is thought to be more physiologic than just glucose. A C-Peptide test is usually measured after the challenge at roughly 90 minutes. Cut-off values below which represent type 1 diabetes, or C-Peptide negative diabetes, are set by the laboratory. These challenge tests should be done fasting and without any rapid-acting insulin given. These tests are then used to categorize whether you have type 1 or type 2 diabetes.
JTL
