**8.2 Clinical data**

40 Basic Nephrology and Acute Kidney Injury

important because it is the most used marker in USA. For example, iothalamate has been

Iohexol is a non-ionic contrast product, mainly used for myelography. Its molecular weight is 821 Da (Olsson et al., 1983; Schwartz et al., 2006). Iohexol is chronologically the last marker proposed for measuring GFR. Actually, the first human was receiving iohexol in 1980 (Aakhus et al., 1980). In this study, it was shown that the substance was safe and fully excreted by the kidneys (this assertion will be criticized thereafter, see below). However, these authors also describe (but data are not available) a higher urinary clearance of iohexol than 51Cr-EDTA (Aakhus et al., 1980). The details of these comparison studies were published three years after (see clinical data)(Olsson et al., 1983). In the same study, the authors confirm that iohexol is distributed through the extracellular volume, which will be confirmed by other authors (including in CKD patients and in obese subjects) (Friedman et al., 2010; Nossen et al., 1995; Edelson et al., 1984; Back et al., 1988b; Olsson et al., 1983). Iohexol has not effect *per se* on GFR (Olofsson et al., 1996). Binding to protein seems very limited for iohexol. The first study described a binding to protein of only 1.5% (Mutzel et al., 1980). This will be thereafter confirmed (Back et al., 1988b; Krutzen et al., 1984). Physical properties of iohexol make it a good candidate to be used in simplified protocols like plasma clearance (Thomsen & Hvid-Jacobsen, 1991; Gaspari et al., 1995; Edelson et al., 1984). Contrary to the prior studies (Aakhus et al., 1980), several authors have shown that extrarenal clearance of iohexol is limited but not null (Arvidsson & Hedman, 1990; Krutzen et al., 1984). Back calculated at 6.2 mL/min the difference between total and urinary clearance of iohexol in healthy subjects (Back et al., 1988b). Frennby observed an extra-renal clearance lower than 2 mL/min in 6 anuric dialysis patients (Frennby et al., 1994; Frennby et al., 1995). These last very low results were also found by Nossen in 16 patients with severe CKD. Their mean measured GFR was 14 mL/min and the extra-renal clearance was estimated at 10% (Nossen et al., 1995). In 16 healthy subjects, Edelson estimated the extrarenal clearance of iohexol at 5% (Edelson et al., 1984). Contrary to iothalamate, there are very

used in trials having built the new creatinine-based equations (Levey et al., 1999).

few physiological studies on the renal tubular handling of iohexol.

As for iothalamate, iohexol can be measured by several different techniques. Among these, HPLC and XRF are the most used ones. HPLC was historically the first method used (Aakhus et al., 1980) and described (Krutzen et al., 1984). As we have shown, iohexol measurements by HPLC are sensitive, specific and reproducible (Back et al., 1988c; Farthing et al., 2005; Cavalier et al., 2008). The high performance of such dosage notably enables the use of iohexol low doses and the measurement on finger-prick samples (Krutzen et al., 1990; Niculescu-Duvaz et al., 2006; Mafham et al., 2007; Cavalier et al., 2008; Aurell, 1994). Iohexol measurement is also pretty stable at room temperature and at -20°C(Krutzen et al., 1984; O'Reilly et al., 1988). Measurement of iohexol by XRF method is less validated and probably less performing, especially in low plasma concentrations (O'Reilly et al., 1986; Back & Nilsson-Ehle, 1993; Effersoe et al., 1990; Brandstrom et al., 1998; Aurell, 1994). We will not discuss into details the other methods for measuring iohexol: capillary electrophoresis (Shihabi & Constantinescu, 1992) and mass spectrometry (Lee et al., 2006; Annesley & Clayton, 2009; Denis et al., 2008; Stolz et al., 2010). The safety of iohexol is now confirmed (Heron et al., 1984; Aurell, 1994), notably by the largest series of iohexol

**8. Iohexol** 

**8.1 Physiological and analytical data** 

The results of the first clinical study on iohexol as a reference GFR marker will be published in 1983 (Olsson et al., 1983). Actually, GFR was measured in 10 healthy subjects with urinary clearances of iohexol and 51Cr-EDTA. In this study, the iohexol clearance was significantly higher than the 51Cr-EDTA clearance (110 versus 96 mL/min). In this first study, large dose of iohexol was injected to the patient (from 375 to 500 mg I/kg)(Olsson et al., 1983). Thereafter, the doses of iohexol used will be drastically reduced but it has been well described that the physiologic handling of iohexol was identical if different dosages are used (Back et al., 1988a). In table 5, we resumed the study results having compared the performance of iohexol to inulin in adult subjects. To the best of our knowledge, only two studies have compared urinary clearances of iohexol and inulin. The results seem excellent but Bland and Altman analysis have not been realized (Brown & O'Reilly, 1991; Perrone et al., 1990). Contrary to other markers, iohexol plasmatic clearances have been the most studied. The relatively worst results obtained by Erley are explained by the patients included (Erley et al., 2001). Actually, the patients hospitalized in intensive care are prone to develop edema and, in this situation, plasmatic clearances are not accurate, whatever the marker (Skluzacek et al., 2003). The study published by Gaspari demonstrated a good performance of iohexol plasma clearance compared to inulin but the number of samples was high and these samples were drawn lately (after 10h)(Gaspari et al., 1995).


How Measuring Glomerular Filtration Rate? Comparison of Reference Methods 43

data). The limitations of iohexol are the lack of strong physiological data (notably regarding the tubular handling of the marker) and the relatively few studies having compared iohexol

In Table 6, we resumed the results of studies comparing reference markers (other than inulin). We selected studies in adults. We focused on studies having used the best statistical methods to analyze the results, i.e. the Bland and Altman analysis. It is difficult to interpret results from studies having compared different markers but also different methods (for example, plasmatic clearance of iothalamate with urinary clearance of 51Cr-EDTA) because it is impossible to affirm that potential differences are due to difference in markers or to difference in methods. Another limitation of several studies is the relatively small sample of subjects included. If we take into account these two limitations, we can stress on some interesting results showing good concordance (bias±SD) between plasma clearances of 51Cr-EDTA and 99Tc-DTPA (1.91±6.1 mL/min), and between plasma clearances of 51Cr-EDTA and iohexol (-0.16±6.17 mL/min in (Brandstrom et al., 1998), 4±7.9 mL/min in (Bird et al., 2009), 2±9.2 (Lundqvist et al., 1997), and -0.6±3.6 mL/min in

> **(mL/min/1.73 m²)**

16 NA 21 to 156 Cu 125It: Cu

15 urography 22 to 110 Cp of Io

**GFR methods** 

125It: samples at 180, 210 and 240 min + BM correction

Io (XRF): samples at 3 and 4 h after bolus

after bolus IV and infusion Cp of Dt: samples at 60 and 180 min+ BM correction

(XRF), Cr and Dt : samples at 0, 10, 20, 30, 120, 180, 240 and 300 min

37 to 137 Cp of Cr and

10 to 117 Cu of Dt and

**Statistics Results** 

It higher (p<0.001) 1.13 12±7.5

0.89 Io=0.89Dt+6.5 1.08±0.06 -0.7±14.8

> 0.99 3.2±6.1

Io=0.97Dt-11 0.96 Io=1.01Cr+8 0.95 -10.8±7.9 -9.4±6.9 -0.7±10.4

Wilcoxon

Ratio It/Cr BAr It-Cr

Correlation Regression Ratio BAr Dt-Io

Correlation BAr Io-Dt

Regression Correlation

BA : Cr-Io Dt-Io Cr-Dt

with inulin. More studies have actually compared iohexol with other GFR markers.

**9. Studies comparing reference methods** 

**References Sample Population GFR range** 

11 Nephrectomy

29 10 heart grafted 11 renal grafted 10 donors

and CKD

(Pucci et al., 2001)).

(Odlind et al., 1985)

(Lewis et al., 1989)

(Goates et al., 1990)

(Effersoe et al., 1990)


Table 5. Studies comparing iohexol with inulin. NA: not available, CKD: chronic kidney disease subjects, BA: Bland and Altman analysis, BAr: Bland and Altman analysis recalculated by us, BM: Brochner-Mortensen, HPLC: high pressure liquid chromatography, Io: iohexol, SC: subcutaneous, XRF: X ray fluorescence.
