We use total creatinine as a measure for our 24 hr urine collections to see if the specimen is a full specimen. I doubt your specimens are timed specimen. For instance, you could have your patients void in the morning when they wake and tell them they will be giving a urine specimen 2 hrs later. The total creatinine in that sample should be greater than 100 if the patient has normal kidney function and they are not emaciated. You must also collect the entire amount eliminated.
Some urine tests have a patient void into a cup and discard the rest in the toilet.
This would not give you the information you need. Again, with a spot urine test, there is no reference range. I would not accuse your patients of tampering with there urine specimens but rather monitor its collection. I played college athletics and during drug testing , the collection of the urine specimen was monitored. Please consult your physician for diagnostic and treatment options pertaining to your specific medical condition.