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thank you for replying.
the sensitivity and specificity of test i was refering to is as follows.
the sensitivity of RAST is not as high. meaning it can miss some cases of pateints with allergy. ( meaning a negative test does not always rule out the disease )
specificity is quite high, meaning if test comes out to be positive, it is more likely to be true positive ( meaning patient is allergic, meaning a positive test rules in the disease most of the times )
to answer why, this is just the inherent property of a test used to diagnose a condition. no test ( almost none ) has 100 % sensitivity or 100 % specificity. ( because some other unknonwn particles in blood can react with the reagants and give false positive or false negative test results )
I am sorry if i confused you. please let me know if i should explain more