Stabilization of labile analytes in reference materials

A multianalyte control kit comprising:

• a first container holding at least one unstable control analyte, wherein the at least one unstable control analyte is lyophilized in the form of beads;
• and
• a second container holding at least one stable control analyte in a base matrix solution from a urine sample from which the unstable control analyte has been removed, wherein the base matrix solution comprises an antibiotic, and wherein the base matrix solution is the urine sample from which creatinine has been removed or processed urine, wherein the processed urine is generated by a method comprising contacting the urine sample with creatinase to remove creatinine from the urine sample.