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Abstract
Introduction The potential of exhaled breath condensate (EBC) as a non-invasive indicator of airways disease has been studied for three decades or more. 8-isoprostane is a product of lipid peroxidation which can be detected within EBC. Studies have reported this as a potential objective indicator of oxidative stress in asthma. We therefore aimed to assess the evidence for the use of 8-isoprostane in exhaled breath condensate (EBC) as a biomarker in adult asthma.
Design A systematic review and meta-analysis of EBC 8-isoprostane in asthma.
Methods We searched a number of online databases (including PubMed, Embase and Scopus) in January 2016. We included studies of adult non-smokers with EBC collection and asthma diagnosis conducted according to recognised guidelines. We aimed to pool data using random effects meta-analysis and assess heterogeneity using I2. Study quality and risk of bias was assessed using QUADAS-2 and GRADE.
Results We included twenty studies, the findings from which were inconsistent. Seven studies (n = 329) reported 8-isoprostane concentrations in asthma to be significantly higher than that of control groups, whilst six studies (n = 403) did not. Only four studies had results appropriate for inclusion in a random effects meta-analysis of mean difference between asthma and controls (see Figure 1). This found a statistically significant between-groups difference of +22 pg/ml in asthma.
Confidence in the result is limited by the small number of studies; by substantial methodological and statistical heterogeneity (I2 = 94); and by an inability to assess the risk of bias in key domains of the quality assessment tool.
Conclusion The clinical value of EBC 8-isoprostane as a quantitative assessment of oxidative stress in asthma remains unclear due to variability in results and methodological heterogeneity. It will be essential to develop accurate, reliable and standardised methods of both EBC collection and 8-isoprostane analysis if its use as a biomarker in asthma is to be evaluated.
Random effects meta-analysis of mean between-group difference (asthma vs controls)