Measurement of glomerular filtration rate

Technetium-99m-MAG3 and technetium-99m-DTPA: Renal clearance measured by the constant infusion technique - Old news?

BACKGROUND

Accurate, precise and straightforward methods for measuring glomerular filtration rate (GFR) and/or renal plasma flow (RPF) are still in demand today. The time-consuming constant infusion technique (CIT) is the gold standard and preferred for research, whereas the simple, but less precise, single injection technique (SIT) is used in clinical settings. This study investigated the use of ^99m Tc-DTPA and ^99m Tc-MAG₃ by CIT as a measure of renal function. We developed and evaluated a model to balance the primer dose and infusion rate in an attempt to obtain plasma steady state as quickly as possible.

METHODS

14 healthy subjects received ^99m Tc-DTPA and 6 hypertensive patients received ^99m Tc-MAG₃ in a standardized protocol. All participants had an eGFR above 60 ml/min and none had fluid retention. An intravenous primer injection of the relevant tracer was followed by a sustained infusion over 4.5 h with the same radiopharmaceutical. Blood and urine samples were collected at fixed intervals.

RESULTS

^99m Tc-DTPA clearance reached steady state after 210 min (plasma clearance 78 ± 18 ml/min, urine clearance 110 ± 28 ml/min), whereas ^99m Tc-MAG₃  clearance achieved steady state after 150 min (plasma clearance 212 ± 56 ml/min, urine clearance 233 ± 59 ml/min).

CONCLUSION

Constant infusion technique with fixed primer and infusion rate using ^99m Tc-MAG₃ is feasible for research purposes. The longer time for reaching plasma steady state using ^99m Tc-DTPA makes CIT with this tracer less optimal. If the primer/sustained balance can be optimized, for example using a priori SIT information, ^99m Tc-DTPA as tracer for CIT may also be feasible.

Renal and non-renal clearances of iothalamate

An evaluation of the literature indicated that certain aspects of the disposition kinetics of iothalamate, important to the accurate determination of glomerular filtration rate in dogs and humans, remain to be resolved. The simultaneous clearances of iothalamate and inulin in 5 dogs were determined at three steady-state iothalamate plasma levels (2, 10, and 40 micrograms ml-1) following various rates of intravenous infusion. The iothalamate clearances, both renal and non-renal, were concentration-independent. The overall mean non-renal clearance was 18 per cent (ranging from 9 to 25 per cent) of its plasma clearance. The mean iothalamate/inulin renal clearance ratio was about 0.84 with individual values ranging from 0.72 to 0.95. The significant (4-26 per cent) plasma protein binding of iothalamate in these dogs was the main reason for the lower-than-unity clearance ratios obtained. The literature indicates the existence of up to 25 per cent of non-renal elimination in humans with normal renal function; this is comparable to the present results obtained with dogs but contrary to the assumption, sometimes reported in the literature that non-renal elimination is essentially absent in humans. Binding of iothalamate to plasma proteins from humans was not found in the present study. The above results suggest that for accurate glomerular filtration rate determination in humans and dogs, especially for those with renal impairment, renal clearance rather than plasma clearance should be used, and in the case of dogs it should also be corrected for plasma protein binding. Iothalamate in plasma and urine was analysed by a simple, micro high-performance liquid chromatographic method with UV detection.

Relationship between fat-free weight and urinary 3-methythistidine excretion in man

The relationship between body composition determined by densitometry and urinary 3-methylhistidine (3-Mehis) output was studied in 16 healthy men (range 23--52 yr) who consumed a self-selected meat-free diet for 3 days and collected a 24-hr urine sample on the third day. Urinary 3-Mehis and creatinine outputs were 225 +/- 8 mumoles/day (mean +/- SEM) and 1.6 +/- 0.1 g/day, respectively. Mean fat-free weight (FFW) was 61.5 kg (range 46.9--76.4 kg). FFW correlated more closely with urinary 3-Mehis excretion (r = 0.89, p < 0.001) than with urinary creatinine excretion (r = 0.67, p < 0.1). These results suggest that endogenous urinary 3-Mehis may be related to muscle weight, which is a major component of FFW.

Assessment of individual renal function in children using 99mTc-DTPA

Differential renal function assessment using 99mTc-DTPA was carried out in 12 children aged between 3 weeks and 11 years who had undergone surgical procedures which allowed separate access to the urine output from each kidney. The results correlated well with those obtained by measuring individual kidney creatinine clearances. The best result was obtained using the technique of deconvolution analysis (r = 0.98; P less than 0.001). We conclude that the method is accurate and recommend its use in the management and follow-up of patients with asymmetric renal disease.

The assessment of glomerular function: creatinine clearance or plasma creatinine?

Plasma creatinine concentration is superior to creatinine clearance for the detection of abnormal glomerular function, of changes of glomerular function in patients with chronic renal disease. Creatinine clearance should be abandoned as the routine assessment of glomerular function.

The molecular configuration of inulin: implications for ultrafiltration theory and glomerular permeability

A space-filling model of the inulin molecule, based solely on physico-chemical data in the literature, indicates that the gross shape of the solvated molecule is a cylinder having semi-length 25 A and radius 10 A. The axial dimensions of this model very nearly equal the equivalent estimates (25 and 10.5 A) previously determined by studying restricted passage of inulin through artificial membranes of known pore size (E. Middleton, J. Membrane Biol. 20:347, 1975). The good agreement between these independent in-vitro studies--neither of which depends on ultrafiltration theory--strongly suggests that glomerular pore size is considerably greater than predicted by the theory.

Application of pharmacokinetic methods to the drug residue profile

A pharmacokinetic approach to the delineation of a drug residue profile in food-producing animals has been presented. It is recognized that the determination of a drug withdrawal period is one of the costly developmental procedures in drug development for food animals; thus it is believed that in the early developmental phases, a thorough pharmacokinetic characterization of a drug in the target species would greatly facilitate the design and quality of studies conducted in the later phases of new drug development. It is suggested that drugs that are intended for use in food animals can be characterized kinetically in less costly studies, the results of which might be used to determine the feasibility of developing a drug that might cause serious residue problems. It is also suggested that the pharmacokinetic modeling of a drug in the target animal may provide an essential data base for calculating dosage rates and intervals that directly relate to the efficacy aspects of drug development.

Pharmacokinetics of 125I-iothalamate and 131I-o-iodohippurate in man

The pharmacokinetics of 125I-iothalamate (IOT125I) and 131I-o-iodohippurate (OIH131I) have been studied in patients with varying degrees of renal insufficiency. Both compounds have been shown to obey two-compartment model kinetics after intravenous administration. The clearances are closely related to renal function, but poor correlations were obtained between renal function and compartmental distribution characteristics. This study provides further evidence that IOT125I and OIH131I may be used as indicators of renal function in patients after a single intravenous injection.