Uses of stable isotopes in clinical diagnosis and research in the paediatric population.

The landmark experiments of Schoenheimer and Rittenberg in the 1930s provided the scientific foundation for the ensuing development and application of stable isotope techniques in clinical diagnosis and research. Stable isotopes are non-radioactive atoms of the same chemical element, which diVer only in their number of neutrons. Many elements also have radioactive (non-stable) isotopes. Aspects of macronutrient metabolism have been investigated employing molecules labelled with H (D or deuterium), C, N, and O. Extensive literature is available detailing the use of Mg, Mg, Ca, Ca, Ca, Fe, Fe, Zn, and Zn isotopes in studies of mineral metabolism. The most commonly used radioactive isotopes are C and H (tritium). More than 6000 stable isotope labelled compounds (tracers) are commercially available for use in metabolic studies. Examples of some of these tracers are [1-C] leucine, [1-C, N] leucine, [ring-D5] phenylalanine, and [6,6]-D2 glucose. It is currently accepted that these compounds have negligible biological side eVects, which renders them ethically acceptable for use in children. Following intravascular or oral administration, the tracer is metabolically indistinguishable from the equivalent unlabelled compound of interest (tracee). The metabolic fate of the compound can be assessed qualitatively and quantitatively by measuring the relative abundance of tracer and tracee and/or their respective metabolites with time. The detectable mass diVerence of tracer and tracee allows the analysis of compounds, extracted from plasma, by gas chromatography–mass spectrometry (GC– MS, picogram sample size, analytical range 0.1–100 mole %, precision ±0.2 mole %). 5 The detection limit is considerably less than 0.1 mole %, when tracers with multiple stable isotope labels (for example ring-D5 phenylalanine) are used. Stable isotopes in breath (12CO2 and 13CO2) are analysed using an isotope ratio mass spectrometer (IRMS, microgram sample size, analytical range 0.0001–0.01 atom % excess, precision ±0.0001 atom %. 7 Combustion IRMS has essentially the same analytical capabilities as IRMS but allows the combustion of tissue samples with subsequent analysis of gaseous isotope enrichment. Stable isotopes of minerals are typically analysed by thermal ionisation mass spectrometry (TIMS) or inductively coupled plasma mass spectrometry (ICP-MS) with high precision and sensitivity. The advantages of stable isotope labelled compounds compared with their radioactive counterparts are many. Most importantly, several diVerent stable isotope tracers can be safely administered simultaneously to the same subject without compromising future studies. The plasma volume which is needed from one sample to analyse the isotope enrichment is small, allowing even preterm infants to be studied. On average 0.5 ml of plasma is needed for one study. The intramolecular location of one or more label(s) is determined easily, which allows the mapping of metabolic pathways. In contrast, no realistic radioactive tracers exist for certain chemical elements (O2, N2) and the handling, application, and disposal of radioactive tracers makes such studies hazardous to all participants. This review is written for (general) paediatricians to provide a concise and up to date overview of the use of stable isotope labelled compounds in clinical diagnosis and research in the paediatric population. For more detailed information on stable isotope models and additional applications the interested reader is referred to recently published reviews. 9 10