49 CFR § 40.91 - What validity tests must laboratories conduct on primary specimens?
(a) You must determine the creatinine concentration on each primary specimen. You must also determine its specific gravity if you find the creatinine concentration to be less than 20 mg/dL.
(b) You must determine the pH of each primary specimen.
(c) You must perform one or more validity tests for oxidizing adulterants on each primary specimen.
(d) You must perform additional validity tests on the primary specimen when the following conditions are observed:
(1) Abnormal physical characteristics;
(2) Reactions or responses characteristic of an adulterant obtained during initial or confirmatory drug tests (e.g., non-recovery of internal standards, unusual response); or
(3) Possible unidentified interfering substance or adulterant.
(e) If you determine that the specimen is invalid and HHS guidelines direct you to contact the MRO, you must contact the MRO and together decide if testing the primary specimen by another HHS certified laboratory would be useful in being able to report a positive or adulterated test result.