What Would Cause Inaccurate Blood Sugar Test Results?
Tests to diagnose blood sugar levels generally are reliable. In most cases, though, because blood sugar varies normally during the day and night, identifying inaccurate results can be a problem. A few causes of inaccuracy do exist.
Glucose Monitors and Test Strips
Getting results on a home glucose monitor that report the blood sugar tests inaccurate may be caused by outdated test strips, strips not stored at room temperature, not washing your hands carefully with soap and water before testing, not enough blood for the test or problems such as calibration of the monitor itself. 
On August 13, 2009, an FDA advisory warned healthcare professionals and patients that one kind of testing strip known as GDH-PQQ (glucose dehydrogenase pyrroloquinoline quinone) could produce “falsely elevated glucose results" on patients consuming non-glucose sugars. 
In this condition, abnormal and inaccurate low blood sugar may occur because of “Raynaud’s phenomenon, peripheral vascular disease, and shock," according to a report by M. El Khoury in Endocrine Practice. The condition stems from a lowered flow of blood in the capillaries and elongated transit time of blood sugar. The report cautions about erroneous diagnoses of low blood sugar in thie individuals. 
A U.S. Army report by E. A. Mann and others published in the Journal of Trauma found blood sugar tests inaccurate— “consistently elevated" in patients with anemia (low hematocrits). The error rates in their tests were between 15.4 percent and 22.3 percent in all brands of meters tested. 
While not strictly speaking an inaccurate result, a situation referred to as the “Dawn Phenomenon" can produce misleading blood sugar test results. In most people, starting in the early morning hours, Cortisol, known as the “stress" hormone, begins to rise, peaking around 7:00 A.M. Tests show that elevated Cortisol can cause increased insulin resistance, leading to a later rise in blood sugar. Some individuals may have an exaggerated Cortisol increase in the early morning. When someone like this gets an early morning fasting blood sugar test, the result may look abnormal, but after fasting all day you may find just a normal diurnal variation of glucose levels. 
Thiazide Diuretics and Other Antihypertensive Drugs
Multiple studies show that two major treatments for hypertension can decrease insulin sensitivity and thereby raise blood sugar levels. Treatment with one these antihypertensive drugs may lead to glucose intolerance and elevated blood levels, which are considered probably reversible when replaced with other types of medication. In one sense these results are inaccurate because they may not indicate a true, long term problem. 
Alternative Glucose Tests
Diabetics certainly need to monitor their glucose level on a schedule recommended by their doctor. However, another blood test is equally important. This is the glycosolated or glycated hemoglobin test. Also called the HbA1c test, this measures the average blood sugar level over the past 2 to 3 months. Normal levels fall at 6 percent or less. Diabetics should try to keep the HbA1c at 7 percent or less. 
An Oral Glucose Tolerance test, which involves drinking a sugar solution with a known level of sugar and then testing after two hours, or sometimes at 30 minute intervals up to two hours can provide more accurate results. 
. MayoClinic.com: Diabetes: Glucose Monitors
. FDA Advisory: GDH-PQQ Glucose Monitoring Technology
. El Khoury, M., et al. Endocrine Practice, “Pseudohypoglycemia: a cause for unreliable finger-stick glucose measurements," 2008 Apr;14(3):337-9. http://www.ncbi.nlm.nih.gov/pubmed/18463040
. Mann, E.A., et al., “Error rates resulting from anemia can be corrected in multiple commonly used point-of-care glucometers," The Journal of Trauma, 2008 Jan;64(1):15-20 http://www.ncbi.nlm.nih.gov/pubmed/18188093
. Plat, L., et al., “Effects of morning cortisol elevation on insulin secretion and glucose regulation in humans," AJP –Endocrinology and Metababolism Online 1996: 270: E36-E42
. Ramsay, LE, W.W. Yeo, and P.R. Jackson, “Influence of diuretics, calcium antagonists, and alpha-blockers on insulin sensitivity and glucose tolerance in hypertensive patients," Journal of Cardiovascular Pharmacololgy,"1992;20 Suppl 11:S49-53; discussion S53-4.
. Mayo Clinic: Diabetes: Tests and Diagnosis
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