Hope this one would help....
Purified protein derivative standard; TB skin test; Tuberculin skin test
In the United States health care providers use a simple skin test called the PPD (purified protein derivative) to test for exposure to tuberculosis .
Why the test is performed
The test is performed to evaluate infection with tuberculosis.
Normal values
A negative reaction (no induration) or a level of induration below the cutoff for each risk group (e.g., different cutoffs for children, people with HIV, etc.) may indicate lack of infection with the bacteria which causes TB. Unfortunately, this is not a perfect test and up to 20% of people infected with tuberculosis may not have a reaction on the PPD skin test. In addition, individuals with certain conditions which affect the immune system may also have a false negative test (e.g., cancer, recent chemotherapy, late-stage AIDS).
What abnormal results mean
The significance of the reaction depends on the size of the reaction on the skin and the individual. The reaction is measured in millimeters of induration (hard swelling) at the site. For example, a small reaction (5 mm) is considered a positive reaction in individuals with HIV, in individuals on chronic steroids or in close contacts of a person with active tuberculosis. Larger reactions (greater than or equal to 10 mm) are considered positive in individuals with diabetes, renal failure and health care workers, among others. In individuals with no known risks for tuberculosis, a positive reaction requires a 15 mm induration.
Special considerations
This test is widely used -- however its limitations include the fact that there are false negatives and false positives. In addition, a positive skin test does not necessarily mean that an individual has active tuberculosis. Additional studies are needed to verify whether active disease is present.
get more details@
http://www.fwcc.org/TB_BCG.htm
