How valid is urine testing?

There are several markers that are used in testing the validity of the specimen. Creatine is always present in urine and measured at expected values. Specific Gravity also has expected values. These two markers are used to indicate diluted or substituted specimens. It is possible to denature substances by raising or lowering the pH of the sample. However, pH also has expected values. If any of these markers are out of the expected range, the sample is considered invalid.