Discussion of Agglutination Tests
Bacteria commonly identified by commerical agglutination tests:
- Streptococcus pneumoniae
- Group B streptococcus (Streptococcus agalactiae)
- Haemophilus influenzae type b
- Neisseria meningitidis (serotypes A, B, C, Y, and W135)
- Neisseria meningitidis group B/Escherichia coli serotype K1
Agglutination tests are based on the reaction between latex particles coated with specific antibody and the target antigen (capsular polysaccharide)
Is routine bacterial antigen testing uselful?
Despite the early enthusiasm for rapid latex testing of CSF, their routine use is becoming controversial. Bacterial antigen detection tests were designed as adjuncts in the detection of meningitis in patients with clinical and laboratory findings consistent with meningitis. When CSF chemistry is normal antigen detection is of little value.
This test is ordered indiscriminantly as a panel without considering age or whether the meningitis is community or hospital acquired. The use of the H. influenzae b vaccine has changed the epidemiology of community acquired meningitis. In fact, false positives occured due to recent immunization with the Hib vaccine.
Gram-stained smears have proven to be fairly sensitive at detecting microorganisms at a much reduced cost. A latex agglutination should not be routinely done when the Gram-stained smear is positive. In addition, S. pneumoniae is the most common cause of meningitis in adults.
The use of third generation cephalosporins for empiric therapy has minimized the effect of bacterial antigen tests on patient therapy. These drugs have excellent activity against the most common bacterial etiologies of meningitis, including S. pneumoniae, N. meningitidis, H. influenzae and most enteric gram-negative bacilli. Consequently, an early diagnosis of the causative microorganism no longer necessitates a change in antimicrobial therapy in the majority of patients.
Latex agglutination test results can be false positive, providing misleading information. Most common reason is antigen cross-reactivity and recent immunization.
Guidelines for the judicious use of bacterial antigen detection tests
- Latex agglutination tests should not be used to screen CSF for abnormalities.
- Latex agglutination tests should not be performed on CSF specimens that have a positive Gram stain smear result.
- Latex agglutination tests should not be performed on patients who have nosocomially acquired meningitis
- Latex agglutination tests should not be offered as a panel
- Latex agglutination tests should not be performed on young children recently immunized.
- Latex agglutination tests should be restricted to situations in which a meaningful result can be expected (patients with prior antimicrobial therapy for community-aquired meningitis, patients with Gram-smear negative CSF and with a high cell count, low glucose and high protein).
