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Written by Jordan R. Wagner BSN, RN, CDCES
Laboratory work is very common in managing diabetes, it is used to evaluate trends and make treatment decisions. Yet, many people have little to no understanding of how their physiology works or why certain lab tests are ordered/recommended by their provider. While the list goes on and on of the various labs that can be checked, this blog post will look at some of the most commonly ordered lab tests for someone living with diabetes / diagnosing diabetes.
Glucose:
This test is done through 3 primary methods: capillary, venous, and by continuous glucose monitor (CGM). It allows the person
to see exactly where their glucose level is at that moment.
Capillary blood glucose, simply put, is checking your glucose level with a finger poke and measuring it on a glucometer (glucose meter). It allows a person to track their levels and evaluate trends.
Venous blood glucose is collected via a traditional lab draw and is going to be the most accurate way to measure a glucose level. However, it is not convenient to have a lab draw every time someone wants to see what their glucose level is, thus, this is typically only done in a lab, hospital, or clinic.
Finally a CGM, allows the wearer to see their glucose level every 1 to 5 minutes depending on the brand worn. Of note a CGM measures interstitial glucose and not blood glucose. This means that during rapid changes in blood glucose (i.e. just ate carbohydrates or took a rapid acting insulin), the CGM number could lag behind blood glucose numbers by 15 to 20 minutes. When in doubt, check your glucose with a finger stick.
Hemoglobin A1c:
The A1c, or Hemoglobin (Hb) A1c, or glycated HbA1c is a lab test that looks at your red blood cells (RBCs), specifically at the hemoglobin proteins. These proteins are responsible for carrying oxygen throughout the body, but are prone to glycation (glucose sticking to them). The test looks at this and will come back with a %, and through a calculation can be converted into an estimated average glucose level (eAG). For those interested here is the calculation for eAG in Mg/dL: 28.7 x A1c - 46.7 = eAG
Because a RBC “survives” for 2-3 months, this test gives you an average blood glucose level over the last 2-3 months. Think of the A1c like a bird flying over a forest. You can see what the forest generally looks like, but not the close up details. A glucose check with a finger stick is like a hiker on the ground in the forest seeing the closer up details. Another way to think of A1c is like a “diabetes report card.” A few bad days won’t ruin your A1c, it’s about consistency.
For health care providers, when evaluating A1c levels, ask the patient/client about their blood sugar ranges/trends. Is the person having high sugars but a close to normal A1c? This could be a sign of frequent hypoglycemia. Trends help to learn the bigger picture in relation to the A1c.
This is a valuable lab test, but it is not perfect and may be inaccurate in certain situations such as anemia, sickle cell disease, kidney disease, pregnancy, etc... If any of these conditions are present a finger poke will be a more accurate way to measure glucose control.
C-Peptide:
Insulin and C-peptide are often mentioned together and that is for a good reason, without C-peptide there is no insulin. To fully understand this, we have to go back to the basics of some biochemistry… I promise this won’t be a detailed academic conversation.
Before insulin is even in the picture there are precursors. Preproinsulin which was discovered in 1974 by Dr. Hans E. Weber, is the biologically inactive precursor to insulin. Preproinsulin is converted to proinsulin by signal peptidases (enzymes that remove parts of proteins). Proinsulin contains an A-chain and B-chain connected together by C-peptide. Proinsulin is converted to insulin through the cleaving of the C-peptide. The end result is insulin and C-peptide being secreted by the secretory vesicles (portion of a cell that secretes chemicals into the blood) of the pancreatic beta cells in the islets of Langerhans.
Because the C-peptide remains in the blood much longer than insulin (longer half-life) it is much easier to detect on a blood test. If there is low C-peptide it is likely safe to assume there is low insulin production from the pancreas. High C-peptide may indicate over production of insulin possibly contributing to low blood sugar.
Fructosamine:
The Fructosamine test a.k.a. glycated albumin, or glycated protein test is similar to the A1c test, however, it measures glucose control over 2-3 weeks. This shorter period of time is related to the shorter half life of primarily albumin and other various serum proteins compared to hemoglobin. Because this is a relatively short time period it is not used diagnostically to diagnose diabetes like the A1c, however, it is valuable to see if changes in eating habits or medications are making a difference without having to wait a full 2-3 months for the A1c. It is also commonly used in situations where red blood cells may be impacted such as sickle cell disease, anemias, kidney diseases, or pregnancy. For those of you who are interested, the eAG can be calculated with this equation, eAG = 0.59 x fructosamine(μmol/L)-9.6.
Autoantibodies:
Type 1 diabetes is an autoimmune condition (the immune system attacks its own body) marked by the destruction of the insulin producing beta cells of the pancreas. Various types of autoantibodies are responsible for this attack. There are 4 commonly looked for autoantibodies: islet cell autoantibodies (ICA), insulin autoantibodies (IAA), glutamic acid decarboxylase (GAD65), and protein tyrosine phosphatase (IA2). Generally speaking, the presence of autoantibodies in high numbers is clinically more significant than identifying any particular autoantibody. There is a good chance that if autoantibodies are present, the person has type 1 diabetes.
Creatinine:
Creatinine is the byproduct of muscles using creatine for energy. Like all systems, there is a waste product and this is creatinine and because it is almost exclusively excreted from the body by the kidneys it can serve as an important lab test to assess kidney health. Typically, high serum (blood) creatinine levels indicate poor kidney health. Why measure kidney health with diabetes? Well, one of the leading causes of kidney disease is uncontrolled diabetes.
Blood Urea Nitrogen (BUN):
Similar to creatinine, BUN is a waste product that comes from the liver after proteins are broken down for energy. Urea nitrogen is transported to the kidneys after leaving the liver through the blood. Healthy kidneys will filter this out and blood levels should be low. A high level of BUN can also indicate kidney problems.
Estimated Glomerular Filtration Rate (eGFR):
Imagine that you work on a farm and your job is to clean the animal stalls. If you slowly clean the stalls, the animals would be making them dirty again before you finish the job, thus, the end result is a dirty if not dirtier stall than when starting. This is like the glomerular filtration rate. The eGFR looks at how fast the kidneys are filtering waste products. Everyday our bodies create waste products that need to be excreted. If they are not filtered out at an adequate rate, they build up in the blood. Like the name implies, this is an estimate and should not be solely used in the diagnosis of kidney disease. For those who are interested, the commonly used formula to calculate the eGFR came out of a study called the Modification of Diet in Renal Disease. The MDRD formula is as follows: 186 x (creatinine/88.4) x (Age) x (0.742 if female) x (1.210 if black).
The Takeaway:
There are many lab tests that could be ordered to evaluate diabetes control. In this blog post, we’ve reviewed a few of the more commonly ordered tests (there are many more that could be looked at). If you are curious about any of these lab tests or feel you haven’t gotten the answers you are looking for, talk with your trusted provider about ordering some of these tests. Take control of your health, learn about why the test was ordered and learn how to maintain it or improve it. Remember, your healthcare provider works for you, "you are more than just a number... You are a priority." You’ve got this!
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