I'm not trying to dilute my urine sample, but I get a consistent reporting of low creatinine which they consider "dirty".

I don't know what you mean by diluting your urine sample, but if you mean drinking vast quantities of water, it doesn't make any difference. If you have abnormal kidney function no amount of water will change that. Low creatinine means too much blood in the urine (meaning in lab results, not something you can readily see) and signifies a kidney problem.

Tick

Urine samples are routinely checked for temperature (90-100F), pH concentration (4.5-8.5) creatinine level (>20 mg/dL) and specific gravity (1.003).

Creatinine level <20 mg/dl may indicate dilution. Low creatinine levels are also associated with other physiological conditions, ex. Diabetes.

If the creatinine level is suspect then it should be compared with the other benchmarks.