Bacterial Antigens By Latex Agglutination

Bacterial antigen detection by latex agglutination was first introduced in the early 1980’s for use in patients with laboratory and clinical evidence of meningitis. The reputed advantage was rapid and definitive detection of Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae and group B streptococcus. Although the tests were highly sensitive and specific for the detection of H. influenzae, sensitivity was lower for the other organisms and poor for N. meningitidis. The overall sensitivity ranged from 20-80%, which is lower than that of the cytocentrifuged Gram stain. Cross-reactivity with other organisms has been demonstrated with all antigens.

In recent years, several studies in pediatric and adult populations have addressed the clinical utility of bacterial antigen tests on the management of patients with suspected meningitis. In the largest study, which included over 5000 results, 54% percent of positive bacterial antigen detection tests were false positive. All true positive CSF specimens had positive Gram stains, and there were no positive antigen tests in specimens from patients whose cultures were presumed to be negative due to prior antimicrobial therapy. (J Clin Micro.1995; 33:1486-1491).

In our own study of 235 CSF bacterial antigen detection tests, there was only one true positive test, which also had a positive Gram stain and culture. There was one false positive test that resulted in increased length of hospital stay and unnecessary antibiotic use. There was one false negative test in a patient with a positive CSF Gram stain and culture. In conclusion, bacterial antigen detection tests on CSF are neither sensitive enough to rule out bacterial meningitis, nor specific enough to direct antibiotic therapy.

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