Hair analysis for monitoring adherence to inhaled respiratory medications: possibilities and limitations.

Purpose: Non-adherence to inhaled medication poses a significant clinical and economic burden on patients with respiratory diseases. This narrative review provides an overview of key aspects of hair analysis, in general and specific for inhaled medications, and explores the potential of hair analysis as a novel tool to monitor adherence to inhaled medications.

Methods: PubMed searches were conducted to explore four aspects: (1) mechanisms of (inhaled) drug's systemic absorption and deposition in hair; (2) quantification of drugs in hair; (3) factors impacting (inhaled) drug hair concentrations; and (4) clinical studies assessing inhaled medication adherence through hair analysis.

Results: Systemic absorption, deposition, quantification, and interpretation of drug concentrations in hair are complex phenomena and are influenced by various factors. Analysing drug concentrations in hair segments provides insights into adherence variability over up to 3 months. While studies suggest effective incorporation of several inhaled drugs into hair, inter-individual variability is influenced by external (e.g. UV-exposure), drug- (e.g. lipophilicity) and patient-specific (e.g. hair colour) factors, not just by adherence. The impact of these confounding factors on absolute hair concentrations is still unclear. Intra-individual variability unrelated to adherence appears, however, minimal.

Conclusion: Although hair analysis shows promise as a novel objective bioanalytical method for assessing long-term inhaled medication adherence, until further analytical refinement, clinical validation and a clearer understanding of confounding factors, it should not be relied upon as the sole measure of adherence.