Iohexol as a marker of glomerular filtration rate in patients with diabetes: comparison of multiple and simplified sampling protocols

Iohexol plasma clearance for measuring glomerular filtration rate in clinical practice and research: a review. Part 1: How to measure glomerular filtration rate with iohexol?

While there is general agreement on the necessity to measure glomerular filtration rate (GFR) in many clinical situations, there is less agreement on the best method to achieve this purpose. As the gold standard method for GFR determination, urinary (or renal) clearance of inulin, fades into the background due to inconvenience and high cost, a diversity of filtration markers and protocols compete to replace it. In this review, we suggest that iohexol, a non-ionic contrast agent, is most suited to replace inulin as the marker of choice for GFR determination. Iohexol comes very close to fulfilling all requirements for an ideal GFR marker in terms of low extra-renal excretion, low protein binding and in being neither secreted nor reabsorbed by the kidney. In addition, iohexol is virtually non-toxic and carries a low cost. As iohexol is stable in plasma, administration and sample analysis can be separated in both space and time, allowing access to GFR determination across different settings. An external proficiency programme operated by Equalis AB, Sweden, exists for iohexol, facilitating interlaboratory comparison of results. Plasma clearance measurement is the protocol of choice as it combines a reliable GFR determination with convenience for the patient. Single-sample protocols dominate, but multiple-sample protocols may be more accurate in specific situations. In low GFRs one or more late samples should be included to improve accuracy. In patients with large oedema or ascites, urinary clearance protocols should be employed. In conclusion, plasma clearance of iohexol may well be the best candidate for a common GFR determination method.

Decreased eGFR as a Risk Factor for Heart Failure in 13 781 Individuals With Type 1 Diabetes

BACKGROUND

Impaired renal function is a well-known risk factor of cardiovascular disease, but its relation to heart failure in individuals with type 1 diabetes has been sparsely studied. The aim of our study was to quantify the risk increase for development of heart failure with decreasing kidney function in individuals with type 1 diabetes.

METHODS

Three equations were used to calculate eGFR (estimated glomerular filtration rate) for individuals with T1D and no known heart failure in the Swedish National Diabetes Registry. Proportional hazards regression models were constructed to evaluate the association between eGFR and hospitalization for heart failure (HF).

RESULTS

Among 13 781 individuals (mean age 41.1 [SD 13.3] years at baseline), 330 (2.4%) were hospitalized for HF over median follow-up of 7.0 years. Renal function was normal (eGFR > 90 mL/min/1.73 m(2)) in 67% of individuals according to the Cockcroft-Gault formula, compared to 51% and 41% according to the Chronic Kidney Disease Epidemiology (CKD-EPI) and Modification of Diet in Renal Disease (MDRD) formulas. For eGFR 45-60 ml/min/1.73 m(2), hazard ratios (HRs) for hospitalization (reference >90 mL/min/1.73 m(2)) were 3.18 (95% CI 2.17, 4.65), 2.12 (1.16, 3.08), and 2.44 (1.69, 3.55) using the Cockcroft-Gault, MDRD, and CKD-EPI formulas. With eGFR <30 ml/min/1.73 m(2) there was a HR of 3.78 (2.15, 5.91), 3.44 (2.14, 5.51), and 3.51 (2.21, 5.51) compared to normal kidney function (>90 mL/min/1.73 m(2)).

CONCLUSIONS

In individuals with T1D, risk of hospitalization for heart failure was over 2 times greater at eGFR 45-60 ml/min/1.73 m(2) and more than 3 times greater at eGFR <30 ml/min/1.73 m(2) when compared to normal eGFR.

Intravenous fenoldopam for early acute kidney injury after liver transplantation

PURPOSE

Acute kidney injury remains a serious complication after orthotopic liver transplantation. To date, several 'renal-protective' agents have been explored in this setting but with conflicting and disappointing results. Therefore, our aim is to evaluate the effects of fenoldopam in liver transplant patients with an established renal injury.

METHODS

In this prospective study, intravenous fenoldopam 0.1 µg/kg/min was administered to consecutive liver transplant patients with postoperative (within 7 days from surgery) stage 2 acute kidney injury (AKI) according to the Acute Kidney Injury Network classification. Actual glomerular filtration rate (GFR; calculated by the iohexol plasma clearance), serum creatinine (SCr) and cystatin C (SCyC) were used to assess the effect of the medication on the patients.

RESULTS

During the study, 295 patients underwent liver transplant. Fifty-one patients (17.6%) met the inclusion criteria and the data from 48 patients were analysed. SCr and SCyC levels decreased (p < 0.001 after 48 h; p < 0.0001 after 72 h) and GFR increased (p < 0.001 after 24 h; p < 0.0001 after 72 h). When compared to a cohort of comparable patients with AKI from our historical series, the patients in the present study showed better SCr and SCyC levels. It was not necessary to discontinue the infusion of fenoldopam in any patient because of the occurrence of adverse events potentially attributable to it.

CONCLUSION

We showed that fenoldopam was capable of improving some renal function parameters in postoperative liver transplantation patients with on-going AKI. This preliminary study now sets the stage for a multicenter, randomized, placebo-controlled trial in order to provide definite evidence.

Iontophoretic transdermal sampling of iohexol as a non-invasive tool to assess glomerular filtration rate

PURPOSE

To explore the potential of non-invasive reverse iontophoresis transdermal extraction of iohexol as a marker of glomerular filtration rate.

METHODS

A series of in vitro experiments were undertaken to establish the feasibility of iohexol reverse iontophoresis and to determine the optimal conditions for the approach. Subsequently, a pilot study in paediatric patients was performed to provide proof-of-concept.

RESULTS

The iontophoretic extraction fluxes of iohexol in vitro were proportional to the marker subdermal concentration and the reverse iontophoretic technique was able to track changes dynamically in simulated pharmacokinetic profiles. Reverse iontophoresis sampling was well tolerated by the four paediatric participants. The deduced values of the iohexol terminal elimination rate constant from transdermal reverse iontophoresis sampling agreed with those estimated by conventional blood sampling.

CONCLUSIONS

Reverse iontophoretic transdermal extraction fluxes mirrored the subdermal concentration profiles of iohexol, a relatively large neutral marker of glomerular filtration both in vitro and in vivo. The efficiency of extraction in vivo was well predicted by the in vitro model used.

Measured GFR as a confirmatory test for estimated GFR

Clinical assessment of kidney function is central to the practice of medicine. GFR is widely accepted as the best index of kidney function in health and disease, and accurate values are required for optimal decision making. Estimated GFR based on serum creatinine is now widely reported by clinical laboratories, and in most circumstances, estimated GFR is sufficient for clinical decision making. GFR estimates may be inaccurate in the non-steady state and in people in whom non-GFR determinants differ greatly from those in whom the estimating equation was developed. If GFR estimates are likely inaccurate or if decisions based on inaccurate estimates may have adverse consequences, a measured GFR is an important confirmatory test. Endogenous creatinine clearance is the most common method used to measure GFR in clinical practice but may be difficult to obtain or fraught with error. We review methods for GFR measurement using urinary and plasma clearance of exogenous filtration markers and focus on urinary clearance of iothalamate and plasma clearance of iohexol compared with inulin clearance. We suggest plasma clearance of nonradioactive markers be more widely implemented in clinical settings. Further research is necessary on the impact of the use of measured GFR as a confirmatory test.

Evaluation of the Modification of Diet in Renal Disease equation (eGFR) against simultaneous, dual-marker multi-sample measurements of glomerular filtration rate

BACKGROUND

Estimation of glomerular filtration rate (GFR) using plasma creatinine remains controversial, especially when GFR approaches normal values. The aim was to re-examine estimated GFR (eGFR) using dual-reference GFR measurements.

METHODS

eGFR (simplified modified Modification of Diet in Renal Disease equation) was compared with GFR measured with iohexol for predicting GFR measured with (51)Cr-ethylenediaminetetraacetic acid (EDTA). Dual six-sample GFR (20-240 min postinjection) was measured in 60 patients and 20 normal volunteers with (51)Cr-EDTA (GFR(EDTA)) and iohexol (GFR(iohexol)) injected into separate arms and sampled contralaterally. This was repeated in the normal volunteers under fasting conditions (twice in nine). Percentage bias, imprecision (SD of bias) and disagreement (sign-less difference) between eGFR and GFR(EDTA) were compared with those between GFR(iohexol) and GFR(EDTA).

RESULTS

Changes between fasting and postprandial eGFR correlated significantly with corresponding changes in GFR(iohexol) and GFR(EDTA). eGFR predicted GFR(EDTA) less precisely (SD 19.9%) than GFR(iohexol) (10.5%; P < 0.01). Although eGFR showed a poorer correlation with GFR(EDTA) when GFR(EDTA) > 80 mL/min/1.73 m(2) compared with <80 mL/min/1.73 m(2), there was no significant difference with respect to imprecision or disagreement of >20 or 30%. However, eGFR was closer than GFR(iohexol) to GFR(EDTA) in a higher fraction of studies when GFR(EDTA) > 80 mL/min/1.73 m(2) (28/60) than when it was <80 mL/min/1.73 m(2) (9/37; P < 0.05).

CONCLUSION

eGFR is inferior to GFR(iohexol) for predicting GFR(EDTA). The disagreement between GFR(iohexol) and GFR(EDTA) illustrates the extent to which uncertainty in GFR(EDTA) contributes to the performance of eGFR. eGFR performs no better at lower, compared with higher levels of GFR.

Assessment of iothalamate plasma clearance: duration of study affects quality of GFR

BACKGROUND AND OBJECTIVES

Measurement of GFR is important for the management of chronic kidney disease (CKD). Although bolus administration of radiocontrast agents is commonly used to measure GFR, the optimal duration of sampling to assess their plasma clearance is unknown. The purpose of this study was to evaluate whether the duration of plasma sampling influences precision and estimation of GFR.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

GFR was measured by sampling plasma 12 times over 5 h in 56 patients with CKD (mean age 64 yr, 98% men, 79% Caucasian, 34% diabetics, estimated GFR 31.8 +/- 14.2 ml/min/1.73 m(2)). In a subset of 12 patients we measured GFR by sampling plasma 17 times over 10 h.

RESULTS

Short sampling intervals considerably overestimated GFR measured using total plasma iothalamate clearance, especially in larger patients. In the higher estimated GFR group (>30 ml/min/1.73 m(2)), the 5-h GFR was 17% higher and 2-h GFR 54% higher compared with the 10-h GFR, which averaged 40.3 ml/min/1.73 m(2). In the lower estimated GFR group (<30 ml/min/1.73 m(2)), the 5-h GFR was 36% higher and 2-h GFR 126% higher compared with the 10-h GFR, which averaged 22.2 ml/min/1.73 m(2). Short sampling duration also reduced the precision of the estimated GFR from 1.67% for 10-h GFR, to 3.48% for 5-h GFR, and to 7.07% for 2-h GFR.

CONCLUSIONS

GFR measured over a longer duration with multiple plasma samples spanning the distribution and elimination phases may improve precision and provide a better measure of renal function.

Comparison between slope–intercept and slope-only techniques for measuring glomerular filtration rate: Use of two independent markers and an independent arbiter

PURPOSE

Exclusive use of the half-time of clearance of a filtration marker in the bolus injection, single compartment technique for measuring glomerular filtration rate (GFR) is a convenient approach that gives GFR (GFR(SO)) already scaled for extracellular fluid volume (ECV). It has been criticized as less accurate than the conventional, slope-intercept technique (GFR(SI)). The aim of the study was to compare the respective levels of agreement of GFR(SI) and GFR(SO) with GFR based on plasma creatinine (eGFR) used as an independent arbiter.

METHODS

GFR was measured with both 51Cr-EDTA and iohexol, simultaneously injected into opposite arms. Plasma obtained bilaterally 20, 40, 60, 120, 180 and 240 min after injection was assayed for marker injected contra-laterally, 51Cr-EDTA by well-counting and iohexol by X-ray fluorescence. ECV and GFR, scaled to body surface area (BSA), were formally measured from six samples and GFR(SI) (scaled to BSA) and GFR(SO) from the last three.

RESULTS

Disagreement between GFR(SO) measured with 51Cr-EDTA and eGFR was not significantly higher than the corresponding disagreement between eGFR and GFR(SI). Disagreement between GFR(SO) measured with iohexol and eGFR was significantly higher than between eGFR and GFR(SI). GFR(SI) and GFR(SO) correlated more closely when measured with 51Cr-EDTA than with iohexol. Individual differences between GFR(SI) and GFR(SO) using one marker correlated significantly with ECV measured with the other.

CONCLUSIONS

GFR(SO) is critically dependent on the accuracy of half-time measurement and, measured with iohexol, appeared less reliable than GFR(SI). GFR(SI) and GFR(SO) measured with 51Cr-EDTA, however, have similar levels of reliability.

Cystatin C and Estimates of Renal Function: Searching for a Better Measure of Kidney Function in Diabetic Patients

Background: Early identification of impairment in renal function is crucial in diabetic patients. Serum cystatin C may be the most sensitive indicator of glomerular filtration rate (GFR) in the clinical setting.

Methods: We compared cystatin C with creatinine, the Cockcroft-Gault (C-G) formula, and the Modification of Diet in Renal Disease (MDRD) study equation for the assessment of early decreased renal function in 288 diabetic patients (125 type 1, 163 type 2) with renal impairment [GFR: 4–222 mL · min−1 · (1.73 m2)−1]. Relationships of cystatin C, creatinine, and iohexol clearance were linearized by plotting their reciprocals in a simple regression model. Diagnostic efficiency was calculated from ROC curves.

Results: In this study population, cystatin C (P = 0.0013) was better correlated with GFR (r = 0.857) than were creatinine (r = 0.772), C-G (r = 0.750), and MDRD (r = 0.806), a result replicated in patients with normal renal function (P = 0.023, type 1; P = 0.011, type 2), but not in those with decreased GFR. Mean cystatin C concentrations showed step-by-step statistically significant increases as GFR decreased, allowing very early detection of reduction in renal function. At 90 mL · min−1 · (1.73 m2)−1 and 75 mL · min−1 · (1.73 m2)−1 cut-points, diagnostic efficiencies of cystatin C (89% and 92%) were better than those of the other variables (79%–82% and 85%–86%, respectively; P = 0.01).

Conclusions: All data supported the value of serum cystatin C compared with conventional estimates based on serum creatinine measurement for detecting very early reduction of renal function. Use of cystatin C to measure renal function will optimize early detection, prevention, and treatment strategies for diabetic nephropathy.

Cystatin C as a marker of renal function immediately after liver transplantation

To verify whether cystatin C may be of some use as a renal function marker immediately after orthotopic liver transplantation (OLT), we compared serum cystatin C (S(Cyst)), serum creatinine (S(cr)), and creatinine clearance (C(cr)) levels with the glomerular filtration rate (GFR). On postoperative days 1, 3, 5, and 7, S(Cyst) and S(cr) was measured in simultaneously drawn blood samples, whereas C(cr) was calculated using a complete 24-hour urine collection. The GFR was determined on the same days by means of iohexol plasma clearance (I-GFR). The correlation between 1/S(Cyst) and I-GFR was stronger than that of 1/S(cr) or C(cr) (P< 0.01). In the case of moderate reductions in I-GFR (80-60 mL/minute/1.73 m), S(cr) remained within the normal range, whereas the increase in S(cyst) was beyond its upper limit; for I-GFR reductions to lower levels (59-40 mL/minute/1.73 m), S(cr) increased slightly, whereas S(cyst) was twice its upper normal limit. When we isolated all of the I-GFR values on days 3, 5, and 7 that were > or = 30% lower than that recorded on the first postoperative day, S(Cyst)(P< 0.0001) and S(cr) (P< 0.01) levels were increased, whereas C(cr) remained unchanged (P = 0.09). Receiver operating characteristic (ROC) area-under-the-curve analysis showed that the diagnostic accuracy of S(cyst) was better than that of S(cr) and C(cr). S(cyst) levels of 1.4, 1.7, and 2.2 mg/L respectively predicted I-GFR levels of 80, 60, and 40 mL/minute/1.73 m. In conclusion, cystatin C is a reliable marker of renal function during the immediate post-OLT period, especially when the goal is to identify moderate changes in GFR.