Why can untreated urine be used for urea determination via the diacetyl monoxime method?

The diacetyl monoxime method for urea determination is based on a specific chemical reaction that occurs predominantly with urea rather than other nitrogenous compounds, such as ammonia. In untreated urine, the primary focus is on quantifying urea, which is the main nitrogenous waste product in urine.

Because this method does not measure ammonia, it is ideal for evaluating urea content in urine that may contain other compounds. Ammonia could potentially interfere with or complicate the results if it were being measured as well but since the diacetyl monoxime method is specific to urea, it allows for accurate determinations even in the presence of ammonia.

Therefore, the ability of untreated urine to be effectively analyzed for urea using this method hinges upon the method's exclusive measurement of urea and not ammonia, making it a reliable choice for urea determination in clinical laboratory settings.