A1C Glucose Relationship

Submitted by Thiruvelan on Fri, 08/17/2012
A1C Blood Glucose Relationship

Why my A1C & glucose are unrelated? If you are one among them, then read on for clarification.

A1C and glucose level is closely related; however, often, it appears to be unrelated.

The Relation between A1C and blood sugar

The A1C test is a blood test that reflects the average blood glucose level over the last two to three months. The glucose test is a measure of BS level in that instance. 

Now, imagine your vehicle speedometer, it shows speed in kph (kilometers covered per hour). Imagine odometer shows total kilometers covered from day one to the present date.

Your glucose test shows blood glucose at that instant during a day. At many other instances, you may have a lot of blood glucose highs or lows. That is why you end up with an unrelated hba1c as compared with glucose reading.

Why are my A1C and blood glucose unrelated?

A1C and blood glucose are two different but related things. For unknown reasons, some peoples' A1C may not reflect their average blood glucose.

The A1C test has some limitations. Let us look at an example:

Imagine two people (A & B) have an A1C of 7% and estimated average blood sugar of 154 mg/dl or 8.6 mmol.

Say A’s blood sugar has been on a roller coaster. It soars up to 300 mg/dl or 17 mmol before crashing back down (low blood sugars) to 40 mg/dl or 2 mmol. On average, these severe highs & lows average out to about 150 mg/dl. Say B has 90 days of blood sugars around 150 mg/dl or 8.6 mmol.

Which one has a higher risk of complications, A or B? The problem with the A1C test is that frequent low in blood sugars can lead to a lower A1C. If you want to reduce complications, it is crucial to maintain good BS most of the time. Even a slight high A1C is better if there are fewer or no low blood sugars.

What is the relation between Fasting Glucose and HbA1c?

A1C has a less consistent relationship with fasting glucose levels.

Wong TY, Liew G, Tapp RJ, Schmidt MI, Wang JJ, Mitchell P, Klein R, Klein BEK, Zimmet P, Shaw J . Relation between fasting glucose and retinopathy for diagnosis of diabetes: three population-based cross-sectional studies. Lancet 2008; 371: 736– 743.

What does it mean if you have high A1C and healthy blood sugar?

Normal BS reading indicates good BS at that instant and appears to be in tight control. However, high A1C indicates your BS at other instances in unhealthy and not in strict control.

Alternatively, it means your diabetes control was not well past 1 or 2 months that appeared in your A1C result.  However, presently, you are in reasonable control that is indicating by your glucose meter.

What does it mean if you have normal A1C and high blood sugar? 

High BS reading indicates unhealthy BS at that instant and appears to be not in proper control. However, normal A1C indicates your BS at other instances are healthy, and you are inadequate to control.

Alternatively, an excellent A1C means, you are in proper diabetes management for past 1 or 2 months.  However, high blood sugar indicates you are presently not inadequate diabetes control.

Alternatively, you may have many low BS that gives good A1C and High BS.

What to do if blood sugar and HbA1c results in consistent disagreement?

Consult your doctor to confirm whether you are doing the self-monitoring blood sugar correctly or your health conditions affect A1C result (A1C error).  

When do you consider the HbA1c test as inaccurate?

Your HbA1c is conflicting with home glucose monitoring. For example, the blood glucose levels have been between 300 to 400s, and an HbA1c of 6% the two do not reconcile. There may be a medical condition affecting the test results. Consult your doctor; he may order a CBC test (complete blood count) to know the cause of the false A1C result.

Why sometimes A1C number appears to be wrong?

Two and a half months back, a patient's fasting blood sugar 90+, postprandial BS 120+, and HbA1c 6.4%. At present, his numbers are fasting BS 96, postprandial BS 121, and HbA1c - 5.5%. How his A1C value reduced from 6.4 to 5.5 in just 2.5 months? He is on the same diet and lifestyle for the past 2.5 months. Do last readings seem to be wrong?

This 5.5% is equal to 111, so it agrees with the FBS/PPBS range 96/121. Two months back, A1C of 6.4 is equal to 137, so it does not agree with the FBS/PPBS range 90/120.

Recent 1 month has contributed 50% influence in the A1C and recent 50 days contribute to over 90% of A1C result.

The impact of new RBC over the last 30 days on A1C is around 50%. So, if you switched to a low carb high-fat diet in the start, the significant impact should have come by the latest 50th day, 90%+. In practice, A1C is a reliable indicator of average blood sugars over the last 90 days, especially the last 60 days.

What can influence A1C test accuracy? Can an A1C test give false results?

A1C result depends on the health of hemoglobin. A less common form of hemoglobin (variant) or unhealthy cells can give a false result.

  1. Unhealthy hemoglobin conditions affect A1C accuracy - sickle cell anemia, thalassemia, iron deficiency, blood transfusion, kidney failure, or a similar disease.
  2. Some hemoglobin variant causes A1C error. Most people with a hemoglobin variant does not have any symptoms, and unknowing carry it.
  3. Higher than actual a1c may be due to Uremia and acetylated hemoglobin (massive dose ingestion of salicylates).
  4. Kidney disease, liver disease, hemoglobinopathy, and recovery from blood loss decreases A1C number.
  5. Vitamins C and E supplementation reported lower A1C by inhibiting glycosylation.
  6. Low A1C has an association with Iron-deficiency anemia. Baseline HbA1c level in anemic patients was lower than that in the control group. Additionally, a significant increase in the patients' HbA1c levels after two months of anemia treatment. Reference: "Effect of Iron Deficiency Anemia on Hemoglobin A1c Levels." Ann Lab Med. 2012 Jan; 32(1):17-22.
  7. Low A1C found in diabetic and non-diabetic pregnant women due to short erythrocyte lifespan.
  8. A1C accuracy can also report being influencing by vitamin C and E intake. To avoid errors, avoid taking these supplements for about 24 hours before the A1C test.
  9. Abnormal hemoglobin lifespan causes A1C error. If you have abnormal hemoglobin, then your HbA1C may be low in hemolytic anemia. The life span of RBC is short in hemolytic anemia so HbA1C may be on a lower side. In polycythemia and post-splenectomy patients where the red cell life is longer than usual. Thus HbA1C may be on the higher side. So be cautious when interpreting A1C results in these conditions.