LOW SPECIFICITY OF THE BACTERIAL INDEX FOR THE DIAGNOSIS OF BACTERIALPNEUMONIA BY BRONCHOALVEOLAR LAVAGE

The bacterial index (BI) as defined by the sum of log(10) colony-formi ng units (cfu) of microorganisms per milliliter of bronchoalveolar lav age (BAL) fluid, i.e., a multiplication of the single cfu/ml, has been used to distinguish between polymicrobial pneumonia (BI greater than or equal to 5) and colonization (BI<5). Since many false-positive resu lts are to be expected using this parameter, the diagnostic value of t he BI was studied prospectively by obtaining bacteriologic cultures of BAL fluid in 165 consecutive unselected patients. In 27 cases the dia gnosis of bacterial pneumonia was established on clinical criteria. In 133 patients pneumonia could be excluded, and in five patients the di agnosis remained unclear. Using a cut-off of greater than or equal to 10(5) cfu/ml BAL fluid, sensitivity and specificity for the diagnosis of pneumonia were 33% (9/27) and 99% (132/133), respectively. Sensitiv ity was mainly influenced by prior treatment with antibiotics, being 7 0% (7/10) in untreated and 12% (2/17) in treated patients. Applying th e BI methodology at a cut-off of greater than or equal to 5, however, resulted in an unacceptably high rate of 16 additional false-positive results, thus lowering the specificity to 87% (116/133, P<0.0001) whil e increasing the sensitivity to only 41% (11/27; P = 0.77). In conclus ion, given the high rate of false-positive results, the methodology of the BI is of doubtful value for the diagnosis of bacterial pneumonia by BAL in an unselected patient group. By applying the absolute number of cfu/ml BAL fluid, however, positive bacteriologic cultures of BAL fluid are highly specific for the diagnosis of pneumonia. Their sensit ivity is limited by previous antibiotic therapy.