The diagnostic utility of the antibody-coated bacteria test in intubated patients

Pilot study to determine if the presence of antibody-coated bacteria (ACB) in sputum specimens obtained from endotracheal tube suctioning would aid in the diagnosis of lower respiratory tract infection (LRTI).

All endotracheally intubated and mechanically ventilated patients for a two-month period were recruited for study. The diagnosis of LRTI was based on a clinical suspicion sufficient enough to start or change antibiotic therapy. Specimens were obtained by blind endotracheal tube suctioning. After processing, sputum smears were stained with fluorescein-labelled antibody to the Fc portion of IgG, IgM, and IgA. More than five fluorescein-labelled bacteria per oil immersion field were considered positive smears.

Seventy-one specimens were obtained from 36 patients. Eighteen specimens were positive in 12 patients, all of whom had LRTI. No specimen was positive in patients not diagnosed as having LRTI. The ACB test was positive in 12 of 25 patients with LRTI. Patients with LRTI but negative ACB were more likely to have received prior antibiotic therapy (p less than 0.001). ACB was positive prior to the clinical diagnosis of LRTI in seven of nine patients (av 4.1 days, range 2-6 days) and converted to negative in three specimens obtained seven or more days after starting appropriate antibiotics, while in three specimens it remained positive three-six days post treatment initiation.

The ACB test appears to be highly specific for the presence of LRTI in intubated patients. Sensitivity of the test may be adversely affected by prior antibiotic therapy. A positive ACB test may predict the subsequent development of LRTI. Further study is warranted.