A method for preprocessing transaction records indicative of subscriber activity on a telecommunication network for subsequent downstream processing is provided. The method includes receiving a file including a plurality of transaction records each having a plurality of fields, wherein each field contains at least one character. The characters of selected fields, or all fields, are analyzed to determine whether each of the characters is of a desired format. When any one of the characters is of an improper format, a first flag is set to indicate the existence of an erroneous character within the selected field of the transaction record. Each of the plurality of transaction records are also analyzed to determine whether each transaction record is within a desired category of transaction records. When any one of the plurality of transaction records within a file is not within the desired category of transaction records, a second flag is set to indicate an erroneous transaction record within the file. If either flag is set, the method creates a modified version of the file and overwrites any bad data with data in a format proper for further processing by downstream transaction record processing systems such as call detail record billing systems.