Quality Assurance Study of Bacterial Antigen Testing of Cerebrospinal Fluid

Bacterial antigen testing (BAT) of cerebrospinalfluid (CSF) by latex agglutination is a low-yield procedure in patients whose CSF specimens have normal laboratory parameters. Between August 1992 and August 1994, we evaluated 287 bacterial antigen (BA) test requests to determine whether yields could be improved and whether patient costs could be reduced by canceling BAT for those patients with normal CSF parameters (cell count, protein, glucose) after consultation with physicians. A total of 171 (68%) BA tests were canceled by this approach. None of these CSF specimens was culture positive for an organism detectable by BAT. Of the remaining 116 CSF specimens tested, only 3 were positive by BAT, one each for Neisseria meningitidis, Streptococcus pneumoniae, and group B streptococcus. Only 43 of the CSF specimens tested had at least two abnormal parameters; the 3 positive CSF specimens were included in this group. In light of the low rate of positivity,thenumberofBAtestscanbefurtherreducedbyestablishingcriteriathatmustbemetbeforeaCSF specimen is accepted for BAT. After review of our data and the literature concerning this topic, we concluded that only specimens with leukocyte counts of $50 cells per mm 3 should be tested. Of 287 specimens evaluated in our study, only 36 met this criterion, including the 3 BA-positive specimens. Enacting such a restriction wouldhavereducedthetotalnumberofBAtestsby251(87%)withoutcompromisingpatientcare.Alaboratory cost savings of $6,500 per year would have been realized, with a substantial reduction in the cost per positive test. Patient charges would have been reduced by $12,500 per year. In the early 1970s, immunodiagnostic methods for the detectionofbacterialantigens(BAs)incerebrospinalfluid(CSF) emerged tofill the need for prompt identification of common agents of bacterial meningitis (5, 11, 31). Numerous assays and commercial kits, including counterimmunoelectrophoresis, coagglutination, and latex particle agglutination, were rapidly developed to serve as adjuncts to routine culture and Gram staining (3, 8, 9, 17). During this era, chloramphenicol was used as empiric therapy for meningitis to ensure adequate coverage against b-lactamase-producing Haemophilus influenzae type b. Because of the toxicity associated with chloramphenicol use in children, bacterial antigen testing (BAT) was deemed particularly important in this setting to detect H. influenzae meningitis. A negative BA test for H. influenzae in CSF would, in many cases, lead to the early discontinuation of chloramphenicol. In recent years, BAT has been dramatically affected by changes in infectious disease epidemiology and improvements in therapeutics. The value of BAT for detectingH. influenzae type b in CSF must now be called into question for these reasons. The extended-spectrum cephalosporins ceftriaxone and cefotaxime have been shown to be reliable therapeutic agents for empirically treating meningitis caused by H. influenzae, Streptococcus pneumoniae, and Neisseria meningitidis, thus making detection of these pathogens by BAT somewhat less important. More significantly, the widespread use of the conjugate H. influenzae type b vaccine has resulted in a substantial decline in the incidence of invasive H. influenzae disease. Over the past 22 months, no cases of invasive H. influenzaedisease have been seen at our institution. Although BAT has been used for more than a decade, much controversy has arisen over the proper use of this test. BAT was originally designed to be used in patients who demonstrated laboratory and clinicalfindings consistent with meningitis (3, 5, 8, 11). Despite these initial intentions, this test has been used much too often as a screening tool in cases of suspected meningitis in patients whose CSF specimens have normal chemistries and cell counts. The indiscriminate use of BAT without consideration of the chemical and cytological profiles of the CSF is a misuse of valuable resources. In view of the present emphasis in the health care community on the cost-effective use of laboratory services and the changing epidemiology of bacterial meningitis, we felt that it was prudent to formulate a policy concerning the judicious use of BAT. We prospectively evaluated 287 BA test requests over a 2-year period in an effort to determine whether unnecessary testing could be reduced by recommending to physicians that the test be canceled if the CSF analysis was normal. We also determined whether certain CSF parameters would be reliable predictors of BA test positivity and, thus, would allow us to limit our testing to such specimens.