Optimum sample times for single-injection, multisample renal clearance methods.

The best choice of sample times for measuring renal function in adults by single-injection multisample plasma clearance methods was determined by Monte Carlo simulation, using a two-compartment model with parameters chosen to fit average values (from published clinical studies) for each of the three radiopharmaceuticals 99mTc-MAG3, 99mTc-DTPA and 131I-ortho-iodohippurate. Random errors were added and the simulated data were then fit to a two-exponential model using a weighted nonlinear curve fitting method. The calculated clearance values were compared with the original values to determine random and systematic errors for different selections of sample time for each radiopharmaceutical at various levels of renal function. The results show that for research-level accuracy with a GFR agent such as 99mTc-DTPA, plasma sampling must begin by 10 min after injection and continue at least 3 hr (in adults). With an ERPF agent such as 99mTc-MAG3 or 131I-OIH, sampling must begin by 5 min and continue for at least 90 min. Six logarithmically distributed samples are sufficient.