Normal Blood Test Values

Written by rene f. najera
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Normal Blood Test Values
Normal blood test values vary from one lab to another (Image by Flickr.com, courtesy of hobvias sudoneighm)

When a health care provider looks at a lab report, he or she may determine a blood test is abnormal based on the normal or reference range given to the provider by the laboratory. These ranges are differ from laboratory to laboratory because the ranges are based on the population the laboratory serves. Determining these normal ranges for blood test values is done with statistics.

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Clinical Laboratories

Clinical laboratories are associated with a health care facility or provider. These types of laboratories provide testing of specimens collected from facility patients. Tests include analysis of body fluids to determine their chemical composition. The cells in the samples are also analysed to determine their size, shape and number. When the laboratory reports the results to the provider, the results are placed in the context of a reference range.

Reference Ranges

A reference range is a range of values of what is being analysed in a sample. For example, the reference range of glucose (sugar) in the blood may be 65 to 110 milligrams per decilitre (mg/dl). Anything outside this range is considered to be abnormal. Abnormal values, however, are not necessarily bad. Health care providers look at these values in the context of the patient and lab test. For example, a blood glucose value of 113 mg/dl may not be cause for concern, while a value of 300 mg/dl may merit medical intervention.

Differences in Reference Ranges

Reference ranges vary slightly from one laboratory to another because of the population the laboratory serves. For example, a laboratory in a retirement community may see different values than a laboratory in an inner city. The same goes for laboratories in different countries or continents. Nevertheless, humans as a whole have a reference range that is very broad. To adjust that range to something to be used within a laboratory, laboratories look at what is known as a normal population.

Normal Population

A normal population is a group of people in a population who are the average of what is seen in the population being served. For each laboratory test performed at the lab, samples are collected and analysed in the members of the normal population. Their results give an average result. For example, their average glucose might be 80 mg/dl. A statistical analysis will yield a standard deviation, or how much the sample values deviated from one another, on average. Ninety-five per cent of the values will fall within two standard deviations of the average value. For example, if the standard deviation is 7.5 mg/dl, 95 per cent of the glucose values will be about 15 mg/dl away from the average of 80 mg/dL.

Normal Range

Given that 95 per cent of the values in a normal population will be within two standard deviations of the average, the laboratory uses this range as its normal, or reference, range. This assures that, for the population the laboratory serves, 95 per cent of people will be within that range. Anyone outside that range will be so because they have a condition. Few laboratories use three standard deviations, which covers 99 per cent of the population. This is why some laboratories will say a glucose of 120 mg/dl is normal, while others will say it is a high concentration.

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