Determination of 51Cr-EDTA clearance between 15 and 40 ml/min/1.73 m2 by a single plasma sample

A single-sample method for calculation of 51Cr-EDTA clearance (Cl) between 15 and 40 ml/min/1.73 m2 (ClB) is introduced. It was tested in 109 consecutive patients and compared with a previously published single-sample method for calculation of Cl for Cl greater than or equal to 30 ml/min/1.73 m2 (ClA). When Cl less than 40 ml/min/1.73 m2 the results were more precise when calculated as ClB than when calculated as ClA (p greater than or equal to 0.05). The ClA values were more precise than the ClB values when Cl was greater than or equal to 40 ml/min/1.73 m2 (p less than 0.01). Used in combination (ClA/B) there was a close correlation with Cl (r = 0.98) for the whole range of tested clearance values. It is concluded that Cl can be precisely determined by combined use of the two methods, provided Cl greater than 15 ml/min/1.73 m2 and the single blood-sample is drawn between 240 and 300 min after the injection. If ClA/B less than or equal to 15 ml/min/1.73 m2 it is recommended that a second blood-sample be drawn after 24 h and that Cl is calculated by a conventional slope method.