Comparison between direct smears and bacteriological cultures with respectto equine tracheal specimens

The aim of the study was to assess the relation between the direct smear ob servations and bacteriological results carried out on equine tracheal secre tions. A total of 304 cases comprising both the direct smear (Gram stain) a nd the bacteriological culture, over a period of 8 years, were submitted to this retrospective study. Most of the specimens (78%) were obtained using tracheal washings while the rest (22%) originated from tracheal brushings. A total of 96 specimens (32%) were negative for both direct smear and cultu re. The positive predictive value of direct smears appeared high (81-96%) w hile the negative predictive value was lower (48-79%). Sensitivity of direc t smear was considered as low and varied from 24 to 46% for tracheal brushi ngs and from 41 to 46% for tracheal washings. Specificity of direct smear w as high for both types of specimens. Results of this study indicate that th e use of Gram-stained smear examination, carried out on tracheal secretions , is a technique which is specific but not very sensitive for the predictio n of bacteriological culture results. Moreover, the use of tracheal washing , more than tracheal brushing, seems to increase the sensitivity of the tes t.