How about interfering with the CYA reading?

In the spring 1996 issue of the JSPSI Journal, Wojtowicz discusses the interferences in CYA testing. He suggests that if stabilized chlorine is used then "ideally the available chlorine should be reduced with thiosulfate prior to precipitation since some of the CYA is in the form of chloroisocyanurate" as the method has been calibrated for unchlorinated CYA solutions.

Prior to the chlorine reduction he suggests a 10 ppm chlorination (or using Oxone) to oxidize any oxidable materials that may interfere with the test - compounds such as uric acid may precipitate with melamine, for example.

But all this may be splitting hairs given the intrinsic precision (or lack thereof) of the CYA test.