One of the studies that involves Don Catlin and David Cowan came to this conclusion in 1997 where 6 International Laboratories were used to see if they would come up with the same T/E levels,
"SUMMARY AND CONCLUSIONS The laboratories participating in this experiment esti-
mate T/E with excellent within-laboratory precision; however, in all cases the ANOVA indicates that laboratories significantly differed in their estimate of T/E in urine. Given that many variables remain to be controlled, and that a rigorous method for standardizing the method was deliberately not included, this finding is not surprising. Greater agreement was expected among laboratories on the standard. An important element in this experiment was to gain experience with collaborative research, explicit protocols and logistical factors.
It is evident that to achieve greater agreement between laboratories in the measurement of T/E it will be necessary to control additional experimental variables, to develop a method for correcting the T/E to a standard(s), or both. Later studies will explore some of these possibilities, the feasibility and practicality of independent quantitation of T and E followed by calculation of the mass ratio from these values and the use of a reference standard urine."
https://drive.google.com/drive/folders/1ZBJIt-_Nf0V-j34oIBtxJEtdOT7sPz90