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Hemoglobin A1c Fact Sheet

What is Hemoglobin A1c?

Hemoglobin (Hb) is the compound in the red blood cells that transports oxygen. One of the types of hemoglobin (Hb) is called HbA; HbA1c is a specific subtype of HbA. Glucose binds slowly to Hb and produces glycosylated Hb. There are several types of glycosylated hemoglobin measures (including total glycosylated Hb and HbA1), but HbA1c, which is formed when HbA is glycosylated, is now considered the best and standard measure.

The higher the blood sugar, the faster HbA1c will be formed, resulting in higher HbA1c levels. Red blood cells circulate 60-120 days, and the HbA1c level is in part affected by blood sugar levels over a three-month period. However, it is heavily weighted to levels over the past 45-60 days.

Why is HbA1c important?

What do the test results mean?

Additional information on interpretation of HbA1c values and recommendations can be found in "Clinical Practice Recommendations 2002" of the American Diabetes Association (Diabetes Care Vol 25; Suppl 1; pp S35-S37, 2002).

What are the different HbA1c assays used at the University of Michigan (U-M)?

What are the differences, advantages and disadvantages of the HPLC versus immuno-turbidimetric HbA1c assays?

The assays should achieve very similar values and are both calibrated to the DCCT standard. Upper limits of normal for all HbA1c assays is about 6.2 but can vary slightly between specific machines.

HPLC assay (TOSOH system)

Assay technique = High Performance Liquid Chromotography (HPLC)
Storage limit (at 4°C) = One week


  1. Assay type used in DCCT
  2. Traditional Standard (more "clinically" pure)

Immuno-turbidimetric assay (Cobas Mira by Roche)

Assay technique = Immuno-turbidimetric
Storage limit (at 4°C) = One month


  1. More "analytically" pure approach to measurement
  2. Longer storage time possible

What are total glycosylated hemoglobin and HbA1?

HbA1c is just one type of glycosylated hemoglobin. The HbA1 measure includes all types of glycosylated HbA (including HbA1c) and total glycosylated hemoglobin includes virtually all types of hemoglobin affected by blood sugar level. Based upon analyses of 225 split sample specimens run by the CIC between April and October of 2000, conversion formulas between HbA1c and TGHb were calculated.  The correlation between the two measures was excellent (0.966), however, assays can vary from laboratory to laboratory, so it should not be assumed that the formulas given below can be generalized to other laboratories.

            1) HbA1c = (0.705 * TGHb) + 1.117, and
            2) TGHb = (1.325 * HbA1c) - 0.803.

Recommendation for Studies that were initiated using the TGHb Assays at the Chemistry Laboratory:


U-M Chemistry Laboratory
     Jason Whalen
     Phone: 734-763-1025

U-M Pathology
     Donald Giacherio
     Phone: 734-936-6775

Updates to the Hemoglobin A1c Fact Sheet
     Rod Hayward
     Phone: 734-647-4844

Standardization of Glycohaemoglobin Measurements.

The diversity of assay methods (e.g., HPLC, Immune-based assays, Mass spectrometry, Capillary electrophoresis), instruments, calibration approaches and standards have traditionally posed potential difficulties and inaccuracies in comparing results from measurements of glycated hemoglobins.

The National Glycohaemoglobin Standardization Program, based on the HPLC reference method of the Diabetes Control and Complications Trial for the measurement of HbA1c, has developed a program to provide traceability to the DCCT measurements through a national reference laboratory network. This program also has allowed laboratories and commercial manufacturers to develop traceability of their assays and results to the DCCT standards. The NGSP's efforts have resulted in a much-improved ability to compare Glycohaemoglobin results obtained through different assays and instruments.

The NGSP also has increased the international awareness for similar standardization programs across countries; current efforts are underway to achieve a unified system of standards or conversion formulas that will allow for an accurate and unified interpretation of HbA1c data. Additional information on NGSP policy and certification may be obtained at