Inulin single injection clearance. Microsample technique useful in children for determination of glomerular filtration rate

Maturation of Glomerular Filtration Rate in Term-Born Neonates: An Individual Participant Data Meta-Analysis

Background: The evidence from individual studies to support the maturational pattern of glomerular filtration rate (GFR) in healthy term-born neonates is inconclusive. We performed an individual participant data (IPD) meta-analysis of reported measured GFR (mGFR) data aimed to establish neonatal GFR reference values. Furthermore, we aimed to optimise neonatal creatinine-based GFR estimations Methods: We identified studies reporting mGFR measured by exogenous markers or creatinine clearance (CrCL) in healthy term-born neonates. The relationship between postnatal age and clearance was investigated using cubic splines with generalized additive linear mixed models. From our reference values, we estimated an updated coefficient for the Schwartz equation (eGFR(ml/min/1.73m²)=(k*height (cm))/serum creatinine(mg/dl)). Results: Forty-eight out of 1521 screened articles reported mGFR in healthy term-born neonates, and 978 mGFR values from 881 neonates were analysed. IPD were available for 367 neonates and the other 514 neonates were represented by 41 aggregated data points as means/medians per group. GFR doubled in the first five days after birth from 19.6 (95%CI 14.7;24.6) ml/min/1.73m² to 40.6 (95%CI 36.7;44.5) ml/min/1.73m², then more gradually increased to 59.4 (95%CI 45.9;72.9) ml/min/1.73m² by four weeks of age. A coefficient of 0.31 to estimate GFR best fitted the data. Conclusions: These reference values for healthy term-born neonates show a biphasic increase in GFR with the largest increase between days 1 and 5. Together with the re-examined Schwartz equation, this can help identify altered GFR in term-born neonates. To enable widespread implementation of our proposed eGFR equation, validation in a large cohort of neonates is required.

Measured Glomerular Filtration Rate: The Query for a Workable Golden Standard Technique

Inulin clearance has, for a long time, been considered as the reference method to determine measured glomerular filtration rates (mGFRs). However, given the known limitations of the standard marker, serum creatinine, and of inulin itself, and the frequent need for accurate GFR estimations, several other non-radioactive (iohexol and iothalamate) and radioactive (51Cr-EDTA, 99mTc-DTPA, 125I iothalamate) exogenous mGFR filtration markers are nowadays considered the most accurate options to evaluate GFR. The availability of 51Cr-EDTA is limited, and all methods using radioactive tracers necessitate specific safety precautions. Serum- or plasma-based certified reference materials for iohexol and iothalamate and evidence-based protocols to accurately and robustly measure GFR (plasma vs. urinary clearance, single-sample vs. multiple-sample strategy, effect of sampling time delay) are lacking. This leads to substantial variation in reported mGFR results across studies and questions the scientific reliability of the alternative mGFR methods as the gold standard to evaluate kidney function. On top of the scientific discussion, regulatory issues are further narrowing the clinical use of mGFR methods. Therefore, this review is a call for standardization of mGFR in terms of three aspects: the marker, the analytical method to assess concentrations of that marker, and the procedure to determine GFR in practice. Moreover, there is also a need for an endogenous filtration marker or a panel of filtration markers from a single blood draw that would allow estimation of GFR as accurately as mGFR, and without the need for application of anthropometric, clinical, and demographic characteristics.

Measuring GFR: a systematic review

BACKGROUND

No comprehensive systematic review of the accuracy of glomerular filtration rate (GFR) measurement methods using renal inulin clearance as reference has been published.

STUDY DESIGN

Systematic review with meta-analysis of cross-sectional diagnostic studies.

SETTING & POPULATION

Published original studies and systematic reviews in any population.

SELECTION CRITERIA FOR STUDIES

Index and reference measurements conducted within 48 hours; at least 15 participants studied; GFR markers measured in plasma or urine; plasma clearance calculation algorithm verified in another study; tubular secretion of creatinine had not been blocked by medicines.

INDEX TESTS

Endogenous creatinine clearance; renal or plasma clearance of chromium 51-labeled ethylenediaminetetraacetic acid (51Cr-EDTA), diethylenetriaminepentaacetic acid (DTPA), iohexol, and iothalamate; and plasma clearance of inulin.

REFERENCE TEST

Renal inulin clearance measured under continuous inulin infusion and urine collection.

RESULTS

Mean bias <10%, median bias <5%, the proportion of errors in the index measurements that did not exceed 30% (P30) ≥80%, and P10 ≥50% were set as requirements for sufficient accuracy. Based on the GRADE (Grading of Recommendations Assessment, Development and Evaluation) approach, the quality of evidence across studies was rated for each index method. Renal clearance of iothalamate measured GFR with sufficient accuracy (strong evidence). Renal and plasma clearance of 51Cr-EDTA and plasma clearance of iohexol were sufficiently accurate to measure GFR (moderately strong evidence). Renal clearance of DTPA, renal clearance of iohexol, and plasma clearance of inulin had sufficient accuracy (limited evidence). Endogenous creatinine clearance was an inaccurate method (strong evidence), as was plasma clearance of DTPA (limited evidence). The evidence to determine the accuracy of plasma iothalamate clearance was insufficient. With the exception of plasma clearance of inulin, only renal clearance methods had P30 >90%.

LIMITATIONS

The included studies were few and most were old and small, which may limit generalizability. Requirements for sufficient accuracy may depend on clinical setting.

CONCLUSIONS

At least moderately strong evidence suggests that renal clearance of 51Cr-EDTA or iothalamate and plasma clearance of 51Cr-EDTA or iohexol are sufficiently accurate methods to measure GFR.

Determination of inulin clearance by single injection or infusion in children

The reference method to determine the glomerular filtration rate (GFR) in children is the urinary clearance of inulin during a continuous intravenous infusion. Alternatively, the plasma clearance of inulin can be determined, which does not require urine collection. This study compared the determination of the inulin plasma clearance in 24 pediatric patients by two methods: the single injection and the continuous infusion method. In the single injection method 5000 mg/m(2) inulin was administered as bolus injection, and blood samples were drawn 10, 30, 90, and 240 min after administration. For the continuous infusion method inulin was started overnight and blood samples were collected the next day. The inulin plasma clearance determined by the single injection method was on average 9.7 ml min(-1) 1.73 m(-2) higher than the clearance determined with the continuous infusion method (95% CI: 5.3-14.2). The difference between the two methods was smaller at lower GFRs. The difference in results generated by the two methods in children is small and is considered acceptable in clinical practice. For practical reasons, the single injection method with minimum sampling is preferred.

Variability of glomerular filtration rate estimation using single injection methods in Type 1 diabetic patients

AIM

Single-injection inulin clearance (SIIC) methods without urine collection have often been compared with classical constant-infusion inulin clearance methods. Therefore, we used repeated SIIC measurements to prove the hypothesis of a possible overestimation or variability of inulin clearance in Type 1 diabetic patients.

METHODS

Two SIIC periods, glomerular filtration rate 1 and 2 (GFR 1, GFR 2) were performed consecutively on the same day. In diabetic patients, GFR 3 was measured several days later at the same time of day as the GFR 1 had been done. The calculation used a two-compartment modelling system (TCM) which showed excellent agreement to ratio of dose over area under the curve (D/AUC) calculation.

RESULTS

Twelve normotensive Type 1 diabetic patients (mean and SD), age 39 (10.7) years, diabetes duration 20 (10.7) years, HbA1c 8.5% (0.82%), albuminuria 13.8 (15.7) mg/24 h and six controls, age 26 (2.6) years were examined. Healthy controls showed an excellent repeatability between GFR 1 111.8 (11.44) and GFR 2 110.8 (11.41) ml.min-1.1.73 m2-1. However, in diabetic patients there was a highly significant decrease between GFR 1 129.4 (11.86) and GFR 2 118.1 (13.05) ml.min-1.1.73 m2-1. GFR 1 with 129.4 (11.86) ml.min-1.1.73 m2-1 did not differ significantly from GFR 3 with 129.9 (12.40) ml.min-1.1.73 m2-1 in diabetic patients (P < 0.839).

CONCLUSIONS

In contrast to normal subjects, in Type 1 diabetic patients single-injection inulin clearance technique showed a clinically relevant decrease in GFR on two repeated measurements on the same day. Overestimation of GFR by the first inulin clearance may be caused by incomplete inulin distribution in the slow compartment. Diabet. Med. 18, 464-468 (2001)

Small animal model of weightlessness for pharmacokinetic evaluation

As the United States seeks a greater presence in space, physiologic changes associated with space flight become of greater concern. Exposure to a weightless environment has been shown to have numerous effects on body composition and organ function. Alterations include decreases in muscle and liver mass, changes in bone structure and integrity, and fluid shifts markedly affecting cardiovascular functioning. Furthermore, metabolic activity of the liver has been found to be altered in rats after extended periods of weightlessness. As the length of space travel increases, the probability for the need to administer pharmacologic agents to crew members during space flight for prophylaxis or treatment becomes greater. Thus, because of the observed physiologic and metabolic changes associated with weightlessness, it is reasonable to assume that the pharmacokinetics and pharmacodynamics of xenobiotics administered during space flight may be different that those found in 1g environment. To address these possible changes, the development of a model of weightlessness to investigate possible alterations in drug pharmacokinetics and pharmacodynamics before space flight is of importance. The tail-suspended rat is a well-described model of weightlessness. During the time of the suspension, the pharmacokinetics of marker compounds can be used to evaluate changes in hepatic and renal physiology. Rats suspended for different periods allow for the investigation of the length of weightlessness exposure and drug pharmacology. Results from the use of the suspended rat model provide valuable information regarding possible pharmacokinetic and pharmacodynamic changes associated with weightlessness, and therefore, provide space biomedical researchers with a method of investigating drug administration during space flight missions.

The concentration-dependent disposition and kinetics of inulin

The disposition of inulin was studied in 30 healthy male and 10 healthy female volunteers, and 10 patients with stable chronic renal failure (mean creatinine clearance 45 ml.min-1) following intravenous infusion of 70 mg.kg-1 over 5 min. Plasma concentrations fell rapidly initially but the rate of decline decreased continuously over 8 h and a linear terminal elimination phase could not be identified. Inulin was excreted rapidly by the subjects with normal renal function and 97.3% of the dose was recovered in the urine in 8 h. There was a progressive highly significant fall in the renal clearance of inulin after 2 h as plasma concentrations fell below about 150 mg.l-1. Six to 8 h after administration the clearance was less than 50% of the initial value in the healthy volunteers and the corresponding fall in the renal patients was 33%. The concentration-dependent renal clearance of inulin was confirmed in "step-up" and "step-down" constant infusion studies in which clearances were measured at mean plasma concentrations ranging from 35.2 to 186.7 mg.l-1. These studies virtually excluded time, changes in posture and urine flow rate as important factors. There was no statistically significant fall in clearance during the first 2 h and kinetic analysis was based on data obtained over this time.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased glomerular filtration rate in patients after reconstructive surgery on the abdominal aorta

Fifty-three patients undergoing elective reconstructive surgery on the abdominal aorta were included in a prospective study to obtain information on postoperative glomerular filtration rate (GFR) in relation to increased fluid and sodium intake and decreased plasma colloid osmotic pressure (COP). GFR, extracellular fluid volume (ECV) and COP were measured before operation and on the first and fourth day after surgery. GFR and ECV were measured by means of the single injection residue detection method using the inulin analogue polyfructosan-S as marker. The compiled GFR values increased from 92.3 to 103.7 ml min-1 1.73 m-2 (P less than 0.002) on the first day, and to 105.3 ml min-1 1.73 m-2 (P less than 0.001) on the fourth day after surgery. Concomitantly, ECV increased from 8.9 litres 1.73 m-2 (P less than 0.025) respectively. Positive linear correlation was found on the first postoperative day between GFR per 1.73 m2 and ECV per 1.73 m2 (r = 0.33, P less than 0.05) and between GFR per 1.73 m2 and extracellular sodium content per 1.73 m2 (r = 0.38, P less than 0.01). COP was not correlated to GFR per 1.73 m2. The postoperative increase in GFR found in this study is most likely to be the result of increased ECV and renal plasma flow. Whether other factors are also involved remains unknown.

Extracellular volume, renal clearance and whole body permeability-surface area product in man, measured after single injection of polyfructosan

In 22 patients non-compartmental analysis was applied to plasma disappearance curves obtained after a single injection of polyfructosan to measure volume of distribution (Vd), renal clearance (Cl) and the whole body permeability-surface area product (PdS) of the indicator. It was found that introduction of the theoretical plasma concentration at time zero, calculated as the injected amount divided by the plasma water volume independently determined, to the curves was necessary to determine the initial slope of the curve and subsequently calculate the PdS. This 'initial phase' approximation had negligible effect on the calculations of Vd and Cl. The approximation, however, allowed extension of the non-compartmental analysis to include calculation of the whole body PdS. The whole body PdS for polyfructosan was found to be 0.68 ml/100g X min or 429 ml/min. Assuming a capillary surface area of the whole human body of 50 cm2g-1, the average whole body diffusional permeability coefficient, Pd, for polyfructosan was found to be: 0.23 X 10(-5) cm X sec-1.