Urea Excretion at Low Urine Volumes. the Calculation of "minimal" Urea Clearances.

In a previous paper (1), it was shown that the calculation of standard urea clearances from urine volumes of less than about 0.35 ml. per minute gives results which are considerably too low. It was found that the lower the urine volume is, the greater the error becomes, as the calculated standard clearance falls lower and lower. This was demonstrated in 173 adults, including patients with toxemia of pregnancy, Bright's disease of almost all grades of severity, and in normal controls. Relevant published data of other writers fitted in with the trend of points on the graph summarizing these calculated clearances. Most of the data reported in the previous paper were derived from routine renal function tests and represented only two, three, or four successive hourly clearances. It was seen that the source of error in calculating the standard clearances lay in the behavior of the urea concentration ratio (the ratio of urine urea to blood urea, U/B). This ratio stopped increasing when the urine volume fell below the critical limit of about 0.35 ml. per minute. It was suggested that possibly the concentration ratio even reversed its trend and fell with further volume decrease. This was observed in some cases. To answer this question as to the behavior of the urea concentration ratio at low urine volumes, a series of clearances in successive periods is required. These data will be presented here. On the basis of the findings, a new calculation, the " minimal urea clearance " is suggested. This calculation applies for urine volumes up to 0.35 ml. per minute (21 ml. per hour, and 504 ml. per 24 hours).