There are factors that may produce inaccurate urine creatinine results:
Exposing the sample to excessive light and elevated temperature, which can change the composition of some particles.
Bacterial growth
Alkaline urine pH favors less creatinine. At a urine pH of 3.7, the conversion of creatine to creatinine is favored, and at about a pH of 5.0, the conversion of creatinine to creatine is favored. At low pHs, the equilibrium position is displaced towards creatinine and the opposite is true for higher urine pH. Above pH of 6.0, the equilibrium is associated with approximately equimolar quantities of creatine and creatinine.
Glucose, which can affect pH and urine creatinine (Low urine pH is significantly associated with abnormal glucose tolerance and useful for identifying patients at considerable risk for diabetes. The amounts of urea and creatinine decrease as disease evolves as glucose increases in urine.)
Contrast agents used in scans and other procedures
Recent exercise
Some foods, such as beetroot, rhubarb, blackberries, and food colorings
Some medications, including ibuprofen, chloroquine, and metronidazole