People victimized by erroneous drug test results frequently face loss of their jobs and few are in a position to defend themselves adequately, Congress has been told. Mark L. Waple, a Fayetteville, N.C., attorney who has handled more than 500 drug test cases in the past six years, said Wednesday that such a person ``loses the presumption of innocence and is instead burdened with a presumption of guilt.'' Waple said that in at least 75 percent of his cases, ``there was a significant legal or scientific irregularity which substantially affected the reliability of the ultimate conclusion that the individual had in fact used a controlled substance.'' He said his clients have faced threats of incarceration, loss of security clearances and hospital credentials as well as loss of employment, rank and position. Waple told the House Energy and Commerce subcommittee on oversight and investigations that the combination of the ``presumption of guilt'' and complex legal and scientific issues makes it very difficult for a person to defend against the positive test result. ``Very few individuals who test positive for controlled substances will have a proper background either through experience or education to know where to begin to defend themselves,'' he said. ``Drug testing laboratories guard their scientific data, their test results, their mathematical computations and their standard operating procedures very carefully.'' He called for ``complete and total disclosure of all of the data supporting a positive test result.''