i have a not so pretty idea for dilution leme know what u all think

[Burnt Toast]

I assumed GC/MS was the only was to test for creatinine.

Thats incorrect. The GC/MS is not used for this purpose. The GC/MS is used only to confirm drug positives and two types of adulterants: Pyridine and Glutaraldehyde.
Specific gravity and creatinine is checked using colorimetry (the typical validity dipstick/cup/cassette) for the initial validity analysis. Confirmation methodologies for creatinine doesnt exist in the drug testing field. Therefore, labs check the S/G using Refractometry to "confirm" any diluted/non-human samples, in which the refractometer provides a quantitative value, measured in four decimal places, as prescribed by the regulations set by the DHHS.

Do they always do validity testing for creatinine and SG?

Creatinine and S/G are part of every validity check. And as stated previously, validity checks have become SOP in recent years.