Blood Sugar Glucose Measurement

By Dr Ananya Mandal, MD

Imbalances of blood sugar are common among patients of diabetes mellitus. Diabetes indicates persistently high blood sugar that may cause damage to various organs like the kidney, heart, small arteries and the eyes (retina).

Diabetics are also prone to sudden drops in blood sugar called hypoglycaemia. To monitory these fluctuations blood sugar measurement is vital in diabetic individuals.

Some of the considerations in blood glucose measurement include:

Sample

Glucose can be measured in whole blood or serum (ie, plasma). Earlier blood glucose was measured in whole blood. Nowadays serum is extracted from blood and glucose is measured in the serum. Whole blood and serum blood glucose is often different. Red blood cells have higher concentration of protein than serum and serum has higher water content and more dissolved glucose than whole blood. To obtain blood glucose in serum from figures in whole blood, it is multiplied by 1.15.

Blood is collected from a vein (usually in the crook of the arm). The blood sample is collected into vacuum tubes. Blood sample needs to be collected from a different arm other than the one where there is the intravenous line to prevent confusion of the results with the intravenous fluids. After meals the levels in the veins are somewhat lower than capillary or arterial blood. The estimate is by about 10%.

The surrounding temperature before processing affects blood glucose level estimation. At refrigerator temperatures, glucose remains relatively stable for several hours in a blood sample. At room temperature (25 °C), a loss of 1 to 2% of total glucose per hour is seen in whole blood samples.

If the blood is allowed to clot the glucose in the sample gets metabolized by the blood cells unless the cells are separated. If there are higher numbers of red or white blood cells there is excessive glycolysis in the sample with substantial reduction of glucose level. This occurs if the sample is not processed immediately and leads to a faulty result.

To prevent such losses blood samples are collected in Fluoride tubes (ie, gray-top) since fluoride inhibits glycolysis. Red-top serum separator tubes can also be used for samples after being centrifuged isolating the serum from cells.

Timing of the test

Blood sugar is measured at various points of time to give an idea about the body’s blood glucose regulation system. The primary test is the fasting blood glucose. (FBG). This is measured after overnight fasting. Blood glucose normally is lowest early in the morning after 6 to 8 hours of fasting overnight.

Two hours post prandial blood glucose or PPG is the next common test. After a carbohydrate rich, full meal, two hours are allowed to elapse before blood is taken again for estimation of glucose. This test gives an estimation of glucose handling by the body.

Other tests include oral glucose tolerance test (OGTT) and intravenous glucose tolerance test (IVGTT) wherein a fixed amount of glucose is administered orally or intravenously respectively and repeated blood sugar tests are performed to check on the body’s glucose handling.

Another important test is the glycosylated haemoglobin (HbA1C). This test gives an idea about fluctuations of glucose in blood over a period of last three months.

Blood glucose can also be self-monitored by the patient using meters or hand help portable monitors. Blood glucose is co-ordinated with urine glucose test as well.

Methods of blood sugar measurement

The earlier method used to measure blood sugar is a chemical method that exploits the ''nonspecific reducing'' property of glucose in a reaction with an indicator substance that changes color when reduced. There are other blood compounds like urea that also have reducing properties, this technique may yield faulty results. The method currently in use, utilizes enzymes specific to glucose and is less likely to yield errors of this kind.

The two most common employed enzymes are glucose oxidase and hexokinase. The enzyme is embedded on a strip or a test strip. When the blood is applied onto the strip the strip changes in composition and color. This strip is then inserted into a meter for a reading. Test strip shapes and their exact chemical composition vary between meter systems and cannot be interchanged.

Abnormal glucose readings

Abnormalities of fasting blood glucose is indicative of problems in the multiple control mechanism of glucose regulation. There are several influences on blood glucose level apart from diabetes and food intake. These include infections, physical or psychological stress, prolonged exercise etc.

Causes of Abnormal Glucose Levels
Persistent Hyperglycemia Transient Hyperglycemia Persistent Hypoglycemia Transient Hypoglycemia
Reference Range, FBG: 70-110 mg/dl
Diabetes Mellitus Pheochromocytoma Insulinoma Acute Alcohol Ingestion
Adrenal cortical hyperactivity Cushing's Syndrome Severe Liver Disease Adrenal cortical insufficiency Addison's Disease Drugs: salicylates, antituberculosis agents
Hyperthyroidism Acute stress reaction Hypopituitarism Severe Liver disease
Acromegaly Shock Galactosemia Several Glycogen storage diseases
Obesity Convulsions Ectopic Insulin production from tumors Hereditary fructose intolerance

 

Reviewed by , BA Hons (Cantab)

Further Reading

Last Updated: Dec 3, 2012

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