Factors that interfere with GHB (HbA1c) Test Results

Posted on January 6, 2010

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Factors that interfere with GHB (HbA1c) Test Results
UPDATED 8/09

Information for physicians and patients regarding HbS, HbC, HbE and HbD traits can be found on the NGSP site under HbA1c Methods and Hemoglobin Variants (HbS, HbC, HbE and HbD traits).
More about hemoglobin variants and HbA1c can also be found at the NIDDK web site:
Sickle Cell Trait and Other Hemoglobinopathies and Diabetes: Important Information for Physicians
For People of African, Mediterranean, or Southeast Asian Heritage: Important Information about Diabetes Blood Tests

Hemoglobin Variants and Derivatives: Genetic variants (e.g. HbS trait, HbC trait) and chemically modified derivatives of hemoglobin (e.g. carbamylated Hb in patients with renal failure, acetylated Hb in patients taking large amounts of aspirin) can affect the accuracy of HbA1c measurements.  The effects vary depending on the specific Hb variant or derivative and the specific HbA1c method.  Table 1 contains information for most of the commonly used HbA1c methods for some of the more common Hb variants and derivatives. Interferences from less common Hb variants and derivatives are discussed in Bry, et al (1).  All entries in Table 1 are based on published information.  In addition, if a product insert indicates clearly that there is inference from a particular factor, then the interference is entered as “yes” and the product insert is cited.   When selecting an assay method, laboratories should take into consideration characteristics of the patient population served, (e.g. high prevalence of hemoglobinopathies or renal failure).

Shortened Erythrocyte Survival: Any condition that shortens erythrocyte survival or decreases mean erythrocyte age (e.g., recovery from acute blood loss, hemolytic anemia) will falsely lower HbA1c test results regardless of the assay method used (23).  HbA1c results from patients with HbSS, HbCC, and HbSC must be interpreted with caution given the pathological processes, including anemia, increased red cell turnover, transfusion requirements, that adversely impact HbA1c as a marker of long-term glycemic control.  Alternative forms of testing such as glycated serum protein (fructosamine) should be considered for these patients.

Other factors: Vitamins C and E are reported to falsely lower test results, possibly by inhibiting glycation of hemoglobin (24, 25); vitamin C may increase values with some assays (25).  Iron-deficiency anemia is reported to increase test results (26).  Hypertriglyceridemia, hyperbilirubinemia, uremia (see carbamylated Hb in Table 1), chronic alcoholism, chronic ingestion of salicylates, and opiate addiction are reported to interfere with some assay methods, falsely increasing results (3, 5, 9, 11, 12, 14, 16-18, 21-22, 27-30).

http://www.ngsp.org/

What conditions may provide falsely high or low results?
According to Dr. Martin J. Abrahamson of the Joslin Diabetes Center, HbA1C is essential in measuring how effective one manages diabetes.Several conditions affect the HbA1C test results.Among the medical conditions that affect HbA1C results include anemia, sickle cell disease and chronic kidney disease.

Red blood cells (RBC) have a short life span. They only last up to 90 days.Those suffering from hemolytic anemia (premature destruction of red blood cells) are affected by falsely low HbA1C results because the RBC has less time to interact or bind with glucose molecules. High levels of vitamin C and E in the blood also decrease the HbA1C results.

Medical conditions such as iron-deficiency anemia and other forms of genetic abnormalities that occur in the hemoglobin may increase HbA1C results. Other factors that may provide high HbA1C results are alcoholism, taking large doses of aspirin, chronic use of opiate-containing drugs, having high levels of blood triglycerides, uremia* and high levels of bilirubin in the blood.

It is also important that the HbA1C be performed by a laboratory that uses a reliable assay. HbA1C testing in the Philippines unfortunately has not yet been standardized with many assays yielding inaccurate results. The International Federation of Clinical Chemistry and Laboratory Medicine is working on global standardization of HbA1C. Have your test done using an assay that has been certified.

Don’t be complacent about your condition. The HbA1C test won’t be enough to ensure control of diabetes. It is only one of the tests you need to do in order to manage the disease. Bear in mind to continually check your blood sugar at home or with your doctor. Aside from blood glucose monitoring, proper diet, regular exercise, medication and doctor visits will do wonders in bringing more than your blood sugar to normal level and manage diabetes.

http://www.allaboutdiabetes.net

Every 1 % increase in your Hgb A1c means a rise of your average blood glucose levels between 30-33 mg %.

HOW IMPORTANT ARE THE LAST 3-4 WEEKS BEFORE PERFORMING THE HgbA1c TEST ?

It is important to know that the previous month before taking the Hgb A1c test is responsible for 50% of the obtained result. The reason is because in the continous birth and removal of our cells the younger red blood cell are predominant and that explain why previous month influence the final result of the Hgb A1c test more than the rather previous 2 months. This phenomenon may explain some unexpeted results in your Hgb A1c test. A positive consequence is that when you are struggling to improve your diabetes control you can see positive changes in you Hgb A1c test as soon as 4 weeks later.

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