We have read with great interest the comments by Khoubnasabjafari et al. elicited by our recent paper published in the Lung [1]. Malondialdehyde (MDA) is a frequently measured biomarker of oxidative stress in various pulmonary and non-pulmonary diseases. Nevertheless, Khoubnasabjafari et al. correctly states that as it stands now, MDA is not an ideal biomarker. Still, we believe that by employing a standardized protocol for collecting, storing, and processing clinical samples MDA may fulfill the requirements to serve as a biomarker for oxidative stress in the airways.

It appears that differences between MDA values reported in the literature can primarily be attributed to the different techniques used for MDA measurement (for review, see [2]). Analytic methods can be subdivided into derivatization-based and label-free methodologies. Among the label-free techniques, the simple ultraviolet absorbance-based method has poor sensitivity and specificity. The most frequently applied derivatization-based method is the thiobarbituric acid (TBA) assay. However, it is not specific for MDA. Other more advanced approaches include hydralazine-based derivatization methods that are coupled with separation techniques such as high-performance liquid chromatography (HPLC) or liquid chromatography–tandem mass spectrometry (LC–MS/MS). Finally, several hydralazine- and non-hydralazine-based derivatization procedures have also been described and used for the analysis of MDA by gas chromatography (GC)–MS/MS. Among these methods, HPLC, LC–, and GC–MS/MS are thought to be the most accurate for measuring MDA in different biological species.

In our previous study, we have established a precise method to measure MDA in respiratory samples such as exhaled breath condensate (EBC) and sputum using HPLC [3]. Based on our findings, MDA is a useful marker to assess oxidative stress both in patients with chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF). In our studies [1, 3], sample preparation was performed according to the protocol provided by the manufacturer of the MDA reagent kit (Chromsystems), and the MDA measurement was conducted using an isocratic HPLC system with fluorescence detector. While the intra- and inter-assay repeatability of the measurement was similar in plasma, EBC, and sputum, the between-visit variability of the assay was considerably higher in plasma. This is reasonable and may well be related to the more complex matrix of plasma compared to EBC or sputum. Of importance, when comparing MDA levels between respiratory samples, in both studies, the highest marker concentration was obtained in sputum. Moreover, exacerbation-associated increase in oxidative stress in patients with COPD could be detected only in sputum but not in EBC indicating that sputum may have superior clinical utility for monitoring oxidative processes in the lungs [3]. We think that until a more sensitive marker for oxidative stress is discovered, MDA determined by a standardized methodology in respiratory samples (particularly in sputum) should be the method of choice for estimating the degree of lipid peroxidation in the airways.