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Background Collection of fluid from the lower respiratory tract ( LRT) plays an important role in both the pathophysiological investigation and diagnosis of respiratory tract disease. Enzymes such as ALP are, among others, indicators of cell damage or death, type II pneumocyte proliferation, and neutrophil invasion, and have been useful as biomarkers of respiratory disease in other species. Objectives The purpose of this study was to determine and compare tracheal wash ( TW) ALP activity in healthy horses and horses with LRT inflammation ( LRTI) determined by TW cytology profile. Methods Tracheal washes were collected from asymptomatic adult geldings to measure ALP activity. The horses were allocated to the healthy group or the group with LRT inflammation based on differential leukocyte counts of TW preparations. Horses with > 20% neutrophils and > 1% eosinophils were allocated to the LRTI group, the horses with < 20% neutrophils and < 1% eosinophils were the controls. Results Tracheal wash ALP activity, measured using a semiautomatic chemistry analyzer, was statistically significantly higher in 18 horses with LRTI (18.9 ± 11.2 × 103 U/L) than in healthy horses (10.3 ± 5.9 × 103 U/L) ( P = .021). Conclusions Determining tracheal wash ALP activity is a simple, inexpensive and safe technique that can be used to facilitate the early diagnosis of equine respiratory disease, since it is higher in asymptomatic adult horses with a TW cytology profile consistent with LRT inflammation than in healthy adult horses with a normal TW cytology profile. [ABSTRACT FROM AUTHOR]