Clearance Methods (OIH)

7.11.1 Radiopharmaceutical

• [¹²³I]Hippuran, also known as
  • [¹²³I]ortho-iodohippurate
  • [¹²³I]OIH

7.11.2 Uptake mechanism / biology of the tracer

[¹²³I]OIH is a radiopharmaceutical used for renal dynamic imaging. It has a renal extraction efficiency of 70-85%, slightly lower than the 90% extraction efficiency of para-amino hippuric acid, the standard for the measurement of effective renal plasma flow (ERPF) [136].

The measurement of ERPF with radiolabelled OIH is preferred to PAH, since compared with the latter, it is less time-consuming and does not require urethral catheterization with the associated risk of infection.

Nevertheless, in clinical practice [¹²³I]OIH is substituted with [^99mTc]Tc-MAG3, also a tubular radiopharmaceutical which is less expensive and more commercially available than [¹²³I]OIH. However, although [^99mTc]Tc-MAG3 might also be used for clearance studies with plasma sample techniques, its results should not be expressed in terms of ERPF, since its renal extraction is even lower than that of radiolabelled OIH.

7.11.3 Indications

• It is indicated mainly for the determination of single kidney absolute function, in conjunction with relative renal function measurement from dynamic kidney scintigraphy.

7.11.4 Contra-indications

• The measurement of ERPF with any tubular tracer, such as [¹²³I]OIH is only valid when the patient is adequately hydrated and should not be performed whenever a condition that alters proximal tubular function might be present, e.g. diseases such as renal artery stenosis, drugs that interfere with the renin-angiotensin-aldosterone system, or drugs such as cyclosporin or probenecid that interfere with tubular secretion.
• Breast feeding should be interrupted for 12 h following the examination [3].

7.11.5 Clinical performances

The use of radiolabelled OIH for the calculation of ERPF started being validated 50 years ago [137,138].

7.11.6 Activities to administer

The suggested activities to administer are
[¹²³I]OIH: 50-150 MBq

No recommendations are given for paediatric nuclear medicine.

7.11.7 Dosimetry

The effective dose for [¹²³I]OIH is assumed to be equal to that for ¹²³I-Hippuran with an ED of 12 µSv/MBq [109]. The organ with the highest absorbed dose is the urinary bladder wall: 190 µGy/MBq

The effective dose range for [¹²³I]OIH is: 0.6-1.8 mSv per procedure

Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."

7.11.8 Interpretation criteria/major pitfalls

In terms of clinical report, the following data should be included:

• date of the study;
• Patient ERPF (in mL/min);
• ERPF corrected for body surface area (mL/min/1.73m²);
• reference range appropriate for the patient’s age.

Radiolabelled OIH reference values according to age and gender have been reported [139].

The main precautions to be observed are related to the previously referred contraindications and precautions related to administration of activity and blood sampling which are detailed in the clearance method with other radiotracers, namely [⁵¹Cr]Cr-EDTA.

7.11.9 Patient preparation

Hydration and confirmation of the absence of disease conditions or medications as detailed under the contraindications.

Before initiating the examination, the presence of pregnancy should be ruled out and the possibility of breast-feeding should be questioned. It is necessary to interrupt breast feeding for 12 h after administration of [¹²³I]OIH (2).

7.11.10 Methods

A single-sample method, with sampling obtained at 44 min after injection of the radiolabelled OIH, can be used to measure ERPF with [¹²³I]OIH [114,140].