How good is the slope on the second exponential for estimating 51Cr-EDTA renal clearance? A Monte Carlo simulation

It has been suggested that the slope of the second exponential of the plasma disappearance curve may be used to monitor changes in renal function instead of plasma clearance calculated using the slope-intercept method. The purpose of this study was to evaluate the magnitude of error in the slope induced by errors in sampling time and in activity measurement, and to compare it with the error observed in clearance. A model of mono-exponential curves based on two blood samples, taken at 120 and 240 min, was created. Normally distributed random errors were introduced into the sampling times and activity measurements. For each setting, the random errors were successively introduced 200 times and the coefficients of variation of the calculated slopes and clearances were determined. Variable errors in slope and clearance were induced by errors in sampling time and activity measurement. In general, the observed errors in the slope were high in the case of low slope values, decreasing progressively for increasing slope values. The errors in clearance followed a different pattern: highest errors were observed in the case of very low clearance, decreasing progressively for higher clearance values and attaining the minimal value at a lambda around 0.006 min-1, which corresponds to clearance of about 90 ml.min-1. The magnitude of the errors then started to increase again for higher clearance. For a large range of clearance values, the errors in the slope were higher than the errors in clearance. The only exceptions were cases with very high clearance rates. In conclusion, clearance calculation using the slope-intercept method should be preferred to that using the slope alone for monitoring changes in renal function.