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What are all those lines and spaces? Understanding bar code technology.

A BAR CODE is typically composed of pairs of rectangular bars and intervening spaces of varying widths. Each pair is known as a character. A single character can stand for a space, number, letter, or other graphic symbol.

The light from a scanning device such as a light pen, wand, or laser is swept across the bars to read them. Black bars absorb the light; white spaces reflect the light back to the scanner. The instrument then transforms the analog patterns of light and dark into electrical impulses that are measured by a decoder and translated into binary digits for transmission to a computer.

The manner or arrangement in which data are encoded into the bars and spaces of an individual code is called its language or symbology. Each code block comprises a unique combination of bars and spaces that represent a number of characters as well as start and stop patterns. Numeric symbologies encode only numbers, whereas alphanumeric symbologies encode both numbers and letters of the alphabet.

* Glossary. A symbology is called discrete if its characters can stand alone and can be decoded independently from those around them. They are separated by intercharacter gaps containing no information.

A continuous code contains no intercharacter gaps. It begins with a bar and ends with a space. The end of one character is indicated by the beginning of another. The absence of intercharacter gaps in continuous symbologies gives them greater density, so that less space is required to encode information. This conservation of "real estate" makes continuous symbologies a better choice for the clinical laboratory than discrete ones.

A symbology is termed self-checking if a printing defect will not cause one character to be scanned as another. A check character is a predetermined symbol used by a scanner to verify that the appropriate data have been decoded. Accurate scanning is assured through the use of patterns of bars and spaces called start and stop codes, which indicate where each bar code begins and ends. The term bidirectional is used to describe a symbol that can be scanned from either the left or the right side without affecting the decoded information. Almost all of the symbologies that are used today are bidirectional.

* Common symbologies. Of the 30 existing bar code symbologies, the ones used most widely in the laboratory--approved by the Health Industry Business Communications Council (HIBCC)--are Codabar, Interleaved 2 of 5 (I 2 of 5), Code 39, and Code 128.

Codabar is a numeric, self-checking, discrete symbology. It has been widely used in blood banks for years and was the first to be used in the laboratory, but is currently being phased out. Each Codabar symbol contains 16 characters that use four different start-and-stop codes. Codabar is fairly complex in that each character consists of four bars and three intervening spaces. Each character is the same width when printed at its highest density. A variation called rationalized Codabar is fully compatible with Codabar but has a higher density and uses only wide and narrow element widths.

Interleaved 2 of 5 is a high-density, self checking, continuous numeric symbology consisting of pairs of digits. The first digit of each pair is encoded in the widths of the bars; the second is in the widths of the spaces. Each character contains five bars and five spaces. The name of the symbology is derived from its physical setup: Both bars and spaces are arranged two wide and three narrow. Each digit contains a unique arrangement of I 2 of 5.

Every I 2 of 5 symbology has a start code (two narrow bars and spaces) and a stop code (one wide bar, one narrow space, one narrow bar). The interleaving effect is achieved through successive pairs of digits.

Even though this symbology is self-checking, partial scans can result in decoding as a valid, shorter I 2 of 5 symbol. For this reason, only an even number of digits can be encoded; odd numbers must be padded with zeros. The use of a check character is also recommended.

Code 39, also called 3 of 9, was the first alphanumeric symbology to be developed. Partly because it can be produced on a variety of printers, Code 39 is now widely used. It is discrete, self-checking, and variable in length, with a total of 44 characters. Code 39 received its name from the qualities of each character: three wide elements and six narrow elements, totaling nine. An asterisk is used for a start-and-stop code. Characters are separated by gaps between characters, which may vary in size.

Code 39 was the first code adopted by the HIBCC. The appeal of that code is derived from its complete alphanumeric character set. A disadvantage is that nine bars and spaces are required to define each character; this poses a problem when space is limited, as it is on a specimen tube or slide label.

Code 128 is a high-density, alphanumeric, continuous symbology with variable lengths and multiple element widths. Each character consists of three bars and spaces, for a total length of 11 black or white modules. Each of its 106 printed characters can carry one of three different meanings, depending on the character set used. Three different start characters tell the scanner which character set will be used first. Three shift codes allow character sets to be changed within a symbol.

Code 128 is popular in the clinical laboratory because it is capable of placing a maximum number of numeric characters in a limited number of bars and spaces. This condensed-space symbology is ideal for applications (such as microscope slides) in which space conservation is important.

* Analyzer capabilities. Figure 5 lists several laboratory analyzers, chosen at random, and the bar code symbologies their manufacturers' literature states the instruments will read. For each analyzer, one symbology will provide the highest read rate. Printer vendors who specialize in medical laboratory applications know which bar code symbologies work best with which automated analyzers.

* System performance. Having a successful scan rate demands optimal performance from every component of the bar code system, whether human, electronic, or mechanical: the printer, the phlebotomist who draws the specimen and places the label on the tube, the medical technologist who puts the specimen on the analyzer, the scanner within the analyzer, and the LIS.

If every component performs within design tolerances, the system should work well. Every component should be designed to exceed minimum specifications. The printer should be of the highest possible quality. Using a top-notch printer will assure good scans even if the label is slightly off center or the tube is slightly out of place on the sampler. Having a well-trained staff is a prerequisite.

The first step in assuring the best chance of a successful read (that is, greater than 99%) is to print the highest-quality bar code available. Printers that are capable of printing 200 dots per inch (dpi) work best in clinical laboratory applications and may help compensate for any limitations of the scanner.

* Catching on. Although the retail industry caught on to the advantages of bar coding technology in the 1960s, the health care profession has lagged behind. Fear of change and the temporary inconveniences that go with it are among the greatest deterrents to any innovation.

Many manufacturers of analyzers used in the laboratory have taken the first step toward automating specimen processing by integrating bar code readers into their products. An increasingly large number of clinical laboratories has developed successful applications. Will your lab be next?
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Kasten, Bernard L.
Publication:Medical Laboratory Observer
Date:Dec 1, 1992
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