Summary Reasons for performing study: Endoscopic scoring of airway mucus quantity and quality has not been critically assessed. Objectives: To evaluate mucus scores for 1) observer‐ and horse‐related variance and 2) association with inflammation, mucus viscoelasticity and measured volume. Methods: Variance of scoring within and between observers and over time within horses were determined for airway mucus accumulation, apparent viscosity, localisation and colour, and correlations of mucus accumulation scores with neutrophil ratios in secretions. The relationship of accumulation score to measured volumes of ‘artificial mucus’ was investigated. Correlations of mucus accumulation, apparent viscosity and colour scores with measured viscoelasticity were tested. Viscoelasticity was compared between tracheal secretion samples collected ventrally and dorsally. Results: Mucus accumulation scoring showed excellent interobserver agreement and moderate horse‐related variance, was related to measured volumes of ‘artificial mucus’, and correlated well with neutrophilic airway inflammation. Scores of mucus viscosity, colour and localisation showed high observer‐related variance. Mucus accumulation, apparent viscosity and colour scores did not correlate with measured tracheal mucus viscoelasticity, but dorsally‐localised mucus showed 2‐fold higher measured viscoelasticity than ventrally‐localised samples. Conclusions: Mucus accumulation scores are a reproducible measure of mucus volumes in the trachea. Potential relevance: Endoscopic scoring of mucus accumulation is a reliable clinical and research tool. In contrast, apparent viscosity, localisation and colour scores should be interpreted with caution.

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