To the Editors:
I read with interest the review of Wood et al. 1 on biomarkers of lipid peroxidation. I would like to congratulate the authors on this well-detailed overview of the topic. At the same time I would also like to point out that their statement saying “breath condensate samples…rely on the volatility of the substances being measured” is false. If, by this, the authors mean that only volatile substances can be captured in condensate samples they are misunderstanding this sampling method. The authors may not be familiar with this technique, which may be why they make this comment and also mention that exhaled ethane and penthane are materials being measured in condensate several times in their review. The latter two are present in the gas phase of exhaled breath and are measured directly in the breath and not in the cooled (condensed) sample 2.
The principle of exhaled breath condensate (EBC) collection is cooling the exhaled breath, resulting in a fluid sample that contains evaporated and condensed particles (water, ammonia, etc.) plus some droplets from the airway lining fluid. These droplets are released by turbulent airflow, and possibly by other currently not completely understood mechanisms, and can be added to the water vapour from anywhere between the alveoli and the mouth. Therefore, in EBC samples, not only volatiles, but also several other mediators with no volatile characteristics can be found and have been reported, including adenosine, different interleukins (−4, −5, −8), interferon-γ, etc. 3–5. Regarding markers of lipid peroxidation, EBC contains isoprostanes and thiobarbituric acid-reactive substances 2.
The authors are right in saying that there are methodological limitations to this type of sampling. However, this is mainly due to the limited understanding of solute formation and dilution of samples, and the accuracy of some of the currently available methods for measuring mediators in EBC. I agree that ambient air may influence the levels of exhaled biomarkers in EBC and this is shown for hydrogen peroxide 6. Volatility may be a problem when measuring mediators from EBC, not because the sampling relies on this characteristic, but because if a molecule is volatile it is very hard to figure out the result of its equilibration between the gas and the fluid phase while breath condensation is ongoing. A good example is the ammonia measurement 7.
Despite the misinterpretation of exhaled breath condensate, I believe that this review is a valuable source of knowledge and references on lipid peroxidation, with detailed information on the limitations and advantages of the current measuring methods.
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